2.3 Fungal contamination and production of mycotoxins
Two major groups of insects harbour the mostly economically important post-harvest insect pests: Coleoptera (beetles) and Lepidoptera (moths and butterflies). Several Coleopteran and Lepidopteran species attack crops both in the field and in store. Crop damage by Lepidoptera is only done by the larvae. Several lepidopteran larvae entangle the feeding media through silky secretion which turns products into entwined lumps. In the case of Coleoptera, both larvae and adults often feed on the crop and the two stages are responsible for the damage.
Post-harvest insect pests may be primary, i.e. able to attack intact grains such as the genus Sitophilus, while others are secondary pests, attacking already damaged grains or grain products such as the genus Tribolium. The following is a list of the most common post-harvest and storage pests, their biology, distribution and common host plants.
The order Coleoptera is the largest order of insects and contains the most common and important stored product pests. Adults have their forewings modified as hard elytra. Beetles inhabit a wide variety of habitats and can be found almost everywhere. Those associated with stored products exhibit different behavioural types; some are primary and secondary pests feeding directly on the product, others are general scavengers, fungus feeders, wood borers or predators of other insects. arvae lack the presence of prolegs (abdominal legs) and only possess true legs on the three thoracic segments. Larvae of a few species may also lack true legs, e.g. Sitophilus spp.
2.1.1 Curculionidae (Snout Beetles)
This is a large group of beetles that contains some of the most serious crop and stored grain pests. Members of this family are characterised by the form of the snout (rostrum) which is elongated in most species. This family contains the most destructive stored grains pests in the world.
The Rice Weevil: Sitophilus oryzae (L.) (=Calandra oryzae L.)
The Maize Weevil: Sitophilus zeamais Motsch. (=Calandra zeamais Motsch.)
The Granary Weevil: Sitophilus granarius (L.)
The first two species are major primary pests that have a virtually cosmopolitan distribution throughout the warmer parts of the world. The rice weevil (S. oryzae) mainly attacks rice and wheat in stores, while S. zeamais is a serious primary pest of stored maize. However, both species are able to develop on all cereals, dried cassava and other processed food products. The two species are morphologically identical. In Europe, the two species are replaced by the granary weevil, S. granarius, which is wingless and can be distinguished by the sculpturing on the prothorax and elytera.
Natural history:
The life cycle and damage caused by both S. oryzae and S. zeamais are similar. However, S. zeamais is a little larger (5 mm in length) and a very active flier. Infestation usually starts in the field and later continues in the store. Both species are capable of inhibiting reserved breeding grounds near the threshing floors that are normally full of plant residues, where the population builds up in before moving to granaries. Adult females chew grains creating a small hole in which they lay eggs and then seal the hole with a secretion. The optimum temperature for oviposition is around 25oC and at grain moisture contents of over 10 percentage (Brich, 1944). Larvae tunnel in grains and are responsible for most of the damage. pupation takes place inside the grain and adults chew their way out through the outer layer of the grain. Adults live for 5-6 months depending on the temperature and humidity of grains (see Kuschel, 1961; Giles, 1969; Mound, 1989).
S. oryzae adult females can lay more than 500 eggs during their lifetime. The optimal temperature for development is 300C with maximum oviposition taking place at 18 percentage humidity. The rice weevil can live without food for 6-32 days depending on temperature. This species is highly affected by changes in temperature; all stages die in about a week at 00C. On the other hand, S. zeamais tolerates lower temperatures than S. oryzae and can live for 37 days at 00C (see Floyd, & Newsom, 1959; Stoyanova, 1984; Zewar,1993).
Natural history:
The granary weevil, S. granarius, lives for one full year at 20-250C and a relative humidity of about 15 percentage. Biology of this species is similar to the other two species, but it is unable to fly, thus restricted to the store (see Dobie & Kilminster,1978; Stein, 1994). This species prefers softer grains such as wheat, rye and barley, as food and habitat. In addition, S. granarius has a high resistance to low temperatures; adults can stay alive for up to two months at -50C. Insects can be controlled if exposed to 500C for 35 minutes which will kill all stages (see Pradzynska, 1995). Buchi (1989) showed that S. oryzae is displacing S. granarius in Switzerland.
2.1.2 Tenebrionidae (Darkling Beetles)
This is a large and varied group of insects that contains more than 10,000 species of which about 100 are associated with stored products. Most of the tenebrionids are black or dark brown in colour and mainly phytophagous. Adults are characterised by the tarsi of the hind leg with only four segments. Infestation by these beetles results in an unappealing smell due to the secretion of benzoquinones from abdominal glands. The following tenebrionids are serious secondary pests of stored grains and flour.
The Red-Rust Flour Beetle: Tribolium castaneum Herbst.
Figure 9:
Red-Rust Flour Beetle
The Confused Flour Beetle: Tribolium confusum J. du Val
These two species are probably the most common secondary pests of all plant commodities in store throughout the world. Several other species of Tribolium are occasional minor pests and can be found in almost every store containing infested cereals or cereal products, specially in tropical and sub-tropical climates. Both species attack maize, wheat, flour and other foodstuffs, but T. confusum does not seem to be as common as T. castaneum in tropical climates (see Hill, 1987; Mills & White, 1994). Members of genus Tribolium are known to produce toxic quinones which contaminate flour and flour products (Gorham, 1989). Damage is done by both larvae and adults specially to broken or damaged grains.
Natural history:
T. castaneum adult females lay small, cylindrical, white eggs scattered in the product. At an optimum temperature of 32.50C, females lay up to 11 eggs daily. Larvae are yellowish with a pale brown head, and they live inside grains until pupation. Adults are about 3-4 mm long and can live for a year or more. Females are highly fecund and able to lay a maximum of 1000 eggs during a lifetime, with 400C and 220C as upper and lower limits for development. This species is also highly tolerable to humidity as low as 11 percentage. Adults are highly adapted to feed on a very wide range of commodities and perfect colonizers of new habitats. In tropical conditions, this species is dominant to T. confusum (see Howe, 1962; Dawson, 1977).
The confused flour beetle, T. confusum, is often confused with T. castaneum but they can be separated using the last three segments of the antenna which are much larger than the rest in T. castaneum and forming a club, while the last five segments in T. confusum gradually enlarge towards the tip. Just like T. castaneum, the confused flour beetle develops in crushed grain products and a constant inhabitant of flour mills specially in the temperate regions of the world. In contrast to T. castaneum, this species is not able to fly, but has a long life span that can reach three years under moderate climatic conditions (25-300C) (see Sokoloff, 1972; 1974; 1977).
2.1.3 The Yellow Mealworm Beetle: Tenebrio molitor L.
Natural History:
Tenebrio beetles are black or dark brown and they feed as larvae and adults on grain products. T. molitor is an important post-harvest pest and occurs spread all over the world. Adults are elongate, 16 mm long, and active fliers. Females can lay up to 600 eggs during its lifetime. Larvae firstly eat the germs of stored grains and can feed on a wide variety of plant products such as ground grains, flour, tobacco and foodstuffs. Larvae are very voracious and highly resistant to low temperature; they can remain alive for 80 days at -50C.
Other tenebrionids are less common polyphagous pests around the globe such as T. destructor, T. madens and Palorus depressus.
2.1.4 Bostrichidae (Branch and Twig Borers)
Members of this family are elongate with the head bent down ventrally to the thorax. Adults are characterised by rasp-like hooks on the pronotum. Most of the species are borers in wood or roots. Wood boring activities of these beetles may weaken timbers or wooden walls of the stores. This family contains two serious stored grain pests:
The Lesser Grain Borer: Rhizopertha dominica (Fabricius)
The lesser grain borer (R. dominica) attacks a wide range of stored cereals. It can be found attacking cassava, flour and other cereal products and is also able to attack rough rice grains. The pest originated from South America, but is now found in all the warmer parts of the world. This species is a serious pest in Australia, from where it was carried to the USA and other parts of the world during World War I. Adults of this species are tiny dark beetles, 2-3 mm in length, and are very voracious with a long life span. Females may continue to lay eggs for four months and are able to lay up to 500 eggs at 340C. They feed externally on grains and lay eggs on their surface. Larvae feed either externally or inside the grain and pupation takes place within the eaten grain. Larval development is relatively faster when fed on whole grains than on flour. Both adults and larvae eat the endosperm leaving powdered grains. This dust can accumulate on the walls of the warehouses and it is a sign of high infestation. Though are not common on pulses, adults are able to breed in grains that are too dry for fast development of Sitophilus. At 340C, development is possible on grains with moisture contents as low as 9 percentage, and they can daily destroy grains equal to their body weight (see Birch, 1945; Fisher, 1950; Aitken, 1975).
2.1.5 The Larger Grain Borer: Prostephanus truncatus (Horn)
The larger grain borer is a primary pest, often attacking maize in the field towards the end of the season, then continuing in the store. P. truncatus is a serious pest of maize in Central America and many parts of Africa. It was first reported in East Africa in 1981 and in 1984 in West Africa. Since then, it has spread rapidly in the African continent where it has become a major pest of stored maize and dried cassava. In Togo, soon after the discovery of P. truncatus, mean losses of 30.2 percentage were reported on stored maize six months after storage (Pantenius, 1988). Stored dried cassava is also known to become heavily infested by P. truncatus, which may lead to cross infestation of maize. Hodges et al. (1985) reported 70 percentage loss in dried cassava roots after four months of storage due to this species.
Figure 11:
Adults of P. truncatus bore in maize grains and produce large quantities of dust, in which their larvae seem to feed and pupate. This species proved to be highly tolerable to low moisture contents in grains. Field studies in Tanzania recorded heavy infestation in maize at a moisture content as low as 9 percentage. The introduction of this pest in Africa has influenced the economy of several countries, specially those depending on exporting of maize. Many countries now refuse to import maize from areas infested with the larger grain borer (see Boeye et al., 1992).
2.1.6 Bruchidae (Seed Beetles)
Most bruchids are short, stout-bodied beetles with a short forewing not reaching the tip of the abdomen. Adults are characterised by their compact hairy bodies and relatively long antennae. Larvae of most species feed inside seeds and some develop in stored dry grains or legumes. All bruchids are phytophagous with most species able to avoid feeding on seed covers that contain toxins. This family contains several important field and stored crop pests.
2.1.7 The Cowpea Weevil: Callosobruchus maculatus (Fabricius)
This is an important pest that mainly attacks beans of various species, and can alternatively attack other pulse crops (see Lienard & Seck, 1994). This species originated in Africa but is now found all over the tropics and sub-tropics. Adults are 2-4 mm, brownish with black markings. They have a short life span of about 12 days and do not feed. Two forms of this species have been identified; the active (flying) form and the flightless form. The flying form disperses and colonises cowpea fields. Adult females lay about 100 eggs glued to the seed surface or to pods. Larvae tunnel inside the seed where the entire development takes place. In the store, the normal form continues to reproduce until the end of the storage season. The flying form appears again in response to disperses to new locations. This species causes major problems in Nigeria and Niger, where most of Africa's cowpeas are produced (see Alebeek, 1996). Other species such as C. rhodesianus and C. subinnotatus may also be important in some parts of Eastern and Central Africa (see Gillon et al., 1992; Giga et al.,1993).
2.1.8 The American Bean Weevil: Acanthoscelides obtectus Say (Bruchus obtectus Say).
Figure 7:
American Bean Weevil
This species is widely distributed in Africa, Central and South America, New Zealand, USA and Southern Europe. A. obtectus exhibits high tolerance to varied degrees of temperature, thus, it is found in cool highland areas as well as the warmer parts of the tropics. It mainly attacks beans of various types and other pulse crops. Adults are grey and oblong in shape, with the body covered by yellowish green hairs. Females are almost twice as large as males. Infestation starts in the field when females lay eggs on the mature beans in plant pods. Larvae are tiny with strong mandibles and feed inside the seeds where life cycle is completed. Adults exit the seed through round holes about 2 mm in diameter (see Wendt, 1992).
2.1.9 The Groundnut Borer (Seed Beetle): Caryedon serratus (Olivier) = (C. gonagra (F)).
Natural history:
This species is common in West Africa and parts of South Eastern Asia where it probably originated. Adults are 4-7 mm in length with distinct serrate antennae. C. serratus attacks mainly groundnuts and other legumes, pods and seeds of Acacia tress and tamarind. Adult females glue their eggs on groundnut seeds soon after harvest. Larvae bore inside seeds making a large hole in the cotyledon. Pupation may take place inside or outside the kernel in paper-like cocoon attached to the pod. C. serratus is a serious pest of stored groundnuts in West Africa (see Delobel 1995; Satya et al, 1996).
Several other bruchids are known as post-harvest pests in different geographical areas of the world:
|
Species |
Remarks |
|
Callosobruchus chinensis (Linnaeus) |
Originated in tropical Asia, but is currently distributed all over the tropics and sub-tropics. It attacks chickpeas, cowpeas and green grams. Life cycle and damage is very similar to C. maculatus (see Parajulee et al., 1989). |
|
Callosobruchus subinnotatus (Pic) |
Formerly described as a strain of C. maculatus. It is found in West Africa where it attacks "Bambarra groundnuts" (see Mbata, 1994). |
|
Callosobruchus theobromae (Linnaeus) |
A pest of pigeon pea in India and was recorded in a groundnut field in Nigeria. |
|
Bruchidius atrolineatus (Pic) |
Mainly a field pest of cowpeas but eggs and larvae are taken to storage after harvest (see Monge et al., 1988). (Huignard et al., 1985) recorded 90% pods infestation from Niger in West Africa. |
|
Bruchus atomarius L. |
Distributed in Europe and parts of Asia. Attacks beans, peas and lentils. |
|
Bruchus lentis Frol |
A monophagous species that occurs in some warmer parts of the world. This species infests lentil seeds in stores (see Mozos, 1992). |
|
Bruchus pisorum L. |
Reported from Europe, Canada, South East Asia and former USSR. A monophagous species that attacks ripe plant pod and can only develop on peas (see Almasi, 1990). |
|
Bruchus rufipes Herbst |
Distributed in central and southern Europe, Asia and south Africa. Attacks vetch seeds in which they develop (see Bakoyannis, 1988). |
|
Bruchus dentipes Baudi |
This species occurs in bean cultivating area. Infests seeds of broad beans and other species of the genus Vicia (see Bakoyannis, 1988; Wendt, 1992). |
2.1.10 Cucujidae (Flat Bark Beetles)
Members of this family are small flattened beetles, mostly found under the bark of trees or in tunnels made by other beetles. This family contains one common pest of stored grains.
2.1.11 The Red Rust Grain Beetle: Cryptolestes ferrugineus (Stephens)
Adults of this species are oblong flattened small beetles (1.5-2 mm long), with the head and prothorax relatively big and conspicuous. C. ferrugineus is a widespread secondary pest of stored grains, specially in the humid tropics. The genus Cryptolestes was reported to be of economic importance towards the end of the maize storage season in Togo (Pantenius, 1988). However, it might not be as serious as other pests in stores, often following an infestation by other insects. It usually attacks the germs of broken or cracked grains thus reducing germination. Other species such as C. pusillus (Schonherr) and C. pusilloides (Steel and Howe) are common in humid areas of the tropics (see Banks, 1979).
Silvanidae
This family was formerly included in Cucujidae. It includes two important species:
The Saw-toothed Grain Beetle: Oryzaephilus surinamensis (L), recognized by the toothed lateral margins of the pronotum.
The Merchant Grain Beetle: Oryzaephilus mercator (Fauvel), which is found in association with O. surinamensis.
Both species are virtually cosmopolitan and they infest a wide variety of stored grains, processed foodstuff and other food products. They are mainly secondary on stored products following more destructive primary pests. However, O. surinamensis prefers cereal products while O. mercator is more frequent on oil-seed products and more temperature sensitive. They enter damaged grains and feed specially on the germ. Optimum conditions for development are between 30- 350C and 70-90 percentage relative humidity.
Natural history:
Adults are 3 mm flattened narrow winged beetles but they rarely fly. Females lay their eggs loosely within the stored products. Larvae are free living and start by feeding on the embryo and the endosperm. They require 60-90 percentage humidity for optimal development, and neither species cannot develop or breed at temperatures less than 190C. All stages die in ten minutes if exposed to 550C (see Howe, 1956; Halstead, 1980).
2.1.12 Dermestidae (Skin Beetles)
Members of this family are ovoid in shape with hairy or sometimes scaly bodies. Larvae are very hairy. When stores are infested, these setae may be seriously hazardous if inhaled by workers. This family contains a number of very destructive and economically important species. One of the most serious stored product pests that belongs to this family is the Khapra Beetle: Trogoderma granarium Everts. Apparently the only phytophagous species in the genus Trogoderma. A native of India, the Khapra beetle is now found in most parts of the world specially hot and dry areas. Adults are oval, red brown insects with a dark thorax. Adult females may lay up to 120 eggs within the stored products. Larvae are considered primary pests as they attack undamaged grains and seeds and bore into stored pulses. They are highly mobile, and in the absence of food they enter a diapause that might last for more than two years, in which they can be highly resistant to the application of pesticides or fumigation. Adults are 3-4 mm long, dark wingless beetles that do not feed. Populations of this pest build up rapidly, specially in the hot humid tropics. This species was apparently eradicated in the United States and the former Soviet Union. It also seems to be absent from East and southern Africa (see Banks, 1977; Rebolledo & Arroyo, 1995; Sudesh et al., 1996 b).
2.1.13 Anobiidae
Anobiids are cylindrical pubescent beetles, 1-9 mm in length. The head is usually concealed from above by the hoodlike pronotum. Most anobiids live in dry vegetable materials or bore in wood, while others are fungus feeders. About 1000 species of Anobiidae are known, most of which are found in the tropics. The following are two widespread storage pests belonging to this family.
The Cigarette Beetle: Lasioderma serricorne (Fabricius) is a common pest of stored cereals, cocoa beans, tobacco, ground nut, peas, beans, flours and other foodstuffs. Originally from South America, it is now found in most of the warmer parts of the world. This species is notorious for attacking a wide range of intact cereal grains, pulse seeds and food stuffs.
Natural History:
Adults can breed anywhere at optimum temperatures of around 28-320C and a relative humidity of 75 percentage. Newly hatched larvae are very active and responsible for most of the damage. Adults are small brown beetles and the only damage they cause is due to their emergence holes. This pest can be controlled if exposed to temperatures below 180C. At 550C, all stages die in two hours (see Howe, 1957; Lefkovitch & Currie, 1967).
The Drug Store Beetle: Stegobium paniceum (Linnaeus) Another widespread pest that infests several cereals, but less common than L. serricorne in the tropics.
Natural history:
Adults are 3.5-4 mm in length, brown hairy beetles, and they do not feed. Females lay about 75 eggs and optimum conditions are 300C with 60-90 percentage relative humidity. Larvae are active feeders and they can be indiscriminate in their food choice, biscuits, macaroni, dry fruits and other products. This species is commonly found in the temperate areas of the world (see Lefovitch, 1967; Haines, 1991).
2.1.14 Trogossitidae (Bark Gnawing Beetles)
Trogosittids are brownish beetles. The Caddle (Tenebroides mauritanicus (L.)) is a common pest in granaries. Observed for the first time in Mauritius, it is now considered a cosmopolitan pest associated with a wide variety of commodities. T. mauritanicus attacks mainly cereals, oilseeds and their products. Both adults and larvae are highly tolerant to very cold conditions. Though larvae are known to predate upon other insects, both adults and larvae feed directly on stored food and larvae are able to tunnel in wooden walls of the store to create a pupation chamber (see Girish & Pingale, 1968; Aitken, 1975).
Lepidoptera is the second most important order of insects pests of stored products. Adults are active flyers with two pairs of scaly wings. Mouthparts of the adults are modified to suck plant nectar or other fluids and are not able to chew, while those of the larvae possess well-developed mandibles. Larvae are distinguished from beetle larvae by their pseudopods (false legs) on some of the abdominal segments. Lepidoptera larvae occur frequently in a wide range of habitats and are known for their silk-spinning activities that result in the additional loss of quality of stored products. Some species attack the product in both the field and store. Several moths are pests of the ripening crop and their larvae can be found in recently harvested stored grains. They either continue their attack for a short time in the store or form an entry point for further attack by true storage pests. The following families contain the most economically important lepidoptera post-harvest pests.
2.2.1 Pyralidae
Pyralidae is a large family, of which only a few species are stored product pests. Most pyralids are small and delicate moths. Members of this family exhibit a great deal of variation in appearance and habits. Larvae of all species possess glands which secrete silk with which they interlink food products as they move. This family is divided into a number of subfamilies, with the subfamily Phycitinae containing some of the most important stored grain pests. The best-known species in this subfamily are the following:
The Tropical Warehouse Moth: Ephestia cautella (Walker) = Cadra cautella Hb.
A very serious cosmopolitan stored product pest infesting a wide variety of hosts such as maize, wheat, and other grains in stores. It can also feed on dried fruits, beans, nuts, bananas and groundnuts.
Natural history:
Adult females lay up to 300 small round sticky eggs within the substrate and through holes in bags. Optimum conditions for larval development are 32-330C and 70 percentage relative humidity. Larvae feed on the seed germ and are fairly mobile within the produce. A considerable amount of damage results from webbing in the grain and on the surface of bags forming large lumps, therefore food is no longer fit for consumption once infested. Pupation takes place in crevices or between bags. Adult moths spread the infestation in the warehouse through egg laying. This pest is cosmopolitan in tropical and sub-tropical parts of the world (see Burges & Haskins, 1965; Hill, 1987; Mound, 1989; Bowditch & Madden, 1996).
The Warehouse Moth: Ephestia elutella (Hub.)
This species is a polyphagous pest that feeds on a vast variety of stored products such as dried cocoa beans, dried grains, pulses, nuts, tobacco, coconut and dried fruits. Infestation is mainly post-harvest.
Natural history:
The whole life cycle takes about 30 days at 300C and 70 percentage relative humidity. Most of the damage is due to contamination of food with exuviae, dead bodies and frass. Silk produced by larvae may be extensive. The warehouse moth is a world wide pest, but more abundant in the sub-tropics and temperate areas of the world. This pest shows high levels of resistance to several groups of pesticides (see Kamali & Taheri, 1985; Meng et al., 1990; Ryan, 1995).
The Mediterranean Flour Moth: Ephestia kuehniella = Anagasta kuehniella (Zeller).
Adults are similar to E. cautella but the body is relatively longer. A major pest of flour mills, its main habitats are flour and grout mills, corn milling plants, bakeries and any other place used for processing grains or preparing flour products. E. kuehniella occurs in most of the temperate and sub-tropical parts of the world, where average temperatures are around 200C-250C. Complete development requires about 74 days at 250C and 75 percentage relative humidity. Larvae entwine all the material on which they feed resulting in solid lumps of food particles, faeces and larval exuviae (see Jacob & Cox, 1977; Locatelli & Biglia, 1995).
Indian Meal Moth: Plodia interpunctella (Hübner)
This insect feeds mainly on meals and flours but can attack raisins, nuts and some pulses and whole cereals. The Indian meal moth is distributed all over the tropics and sub-tropics and in some parts of the temperate regions, specially in heated buildings. In the hot tropics, it is more abundant in cooler highland areas. Most of the damage occurs due to larval feeding on the germinal part of the grains. Damage also occurs through the contamination of foodstuff with dead larvae, frass and silk webbing.
Natural history:
Larvae feed in tubes they weave from silk secretions. Adult females stick about 200-400 eggs to the substrate or to the storage walls. Larvae develop and feed within the substrate and are sensitive to changes in temperature. The number of generations may be only two per year in Europe, but increases in the tropics to eight generations. Complete development takes about 27 days at 300C and 70 percentage relative humidity. Development ceases below 150C. All stages die at 550C in five hours (see Bell, 1975; Aitken, 1984; Locatelli & Biglia, 1995).
2.2.2 Gelechiidae
Gelechiidae is a large family of lepidoptera. All moths are small in size and several species are important plant pests. This family contains two serious post-harvest pests:
The Angoumois Grain Moth: Sitotroga cerealella (Olivier)
Figure 13:
Angoumois Grain Moth
This species is a serious primary pest that mainly attacks maize, wheat and sorghum, both in the field and in stores. A recent survey in southern Ethiopia revealed that this pest alone was responsible for 11.2 to 13.5 percentage weight loss in stored maize (Emana & Assefa, 1998). Infestation with S. cerealella starts in the field as females lay their eggs, singly or in groups, on grains. Larvae start feeding inside the grains, while still in the milk stage, and spend their entire life inside one grain. Thus, infestation is difficult to detect at this stage. Adults leave a conspicuous emergence hole at one end of the kernel. Infested grains are characterised by this circular window created by the larvae. Stored grains may be completely destroyed. Adults are active fliers, thus, they are able to infest neighbouring granaries, which is known as "cross-infestation". This pest is distributed throughout the warmer parts of the world (Africa, South and Latin America and southern Asia and Australia) (see Grewal & Atwal, 1969; Boldt, 1974)
The Potato Tuberworm: Phthorimaea operculella (Zeller) = (Gnorimoschema operculella (Zeller))
This species is a cosmopolitan pest of potatoes, tomatoes and eggplants. It attacks plants mainly in the field, but continues to feed on tubers in storage. Larvae mine in the leaves and stems and later bore into the tubers. Damage can be seen on leaves as silver spots due to the tunnelling larvae, or as tunnels in the plant stem.
Natural history:
Each female lays about 150-200 eggs and larvae tunnel through leaves and stem down to the tuber where pupation may take place. In the store, eggs are laid individually on the tubers near the eyes or on sprouts. P. operculella is an important pest in traditional potato stores in North Africa (Arx et al., 1987; Lagnaoui et al., 1996. See also Haines, 1977). High infestations of up to 50 percentage of tubers can take place in Yemen due to this pest (Kroschel, 1994).
2.2.3 Acaridae
The Flour Mite: Acarus siro
Mites are widely distributed tiny arthropods. They can live and develop on various plant in the field or indoors. Mites can be found in granaries, feed mixing plants, threshing floors, stacks of hay and straw, dead organic matter, soil or plant residues. Several species are predacious on other mites or insects. Mites are easily transmitted by virtue of their tiny size which allows them to be carried with dust, winds, insects, birds or rodents. About 30 mite species are known to be associated with stored products. Family Acaridae contains some damaging species, in which Acarus siro is probably the most important and commonly encountered mite in granaries. This mite is about 0.7 mm in length with an oval body. A. siro is a widely distributed polyphagous species that can be found on almost all products of plant or animal origin. It requires relatively high humidity (70 percentage), with humidities below 11 percentage being lethal to the mite. Temperatures below -150C for 24 hours kill all stages. At 600C, all stages die in 5 minutes.
Attacked grains lose nutrients and the ability to germinate due to feeding on the germ. Crushed bodies of Acarus cause coloration in flour that reduces the products value. Under normal conditions, this mite develops according to the following pattern: egg, larva, nymph I, nymph II, and adult. Some strains of A. siro may produce hypopus under favourable conditions. Hypopus is a diapause form that can be carried by rodents or insects to other storing places. However, this species does not seem to occur in most of the tropical lowlands, though it might sometimes infest grains in cooler upland areas (see Haines, 1991).
2.3 Fungal contamination and production of mycotoxins
Another important cause of grain deterioration is infection by fungal diseases. Just like infestation by insect pests, fungal infection mainly starts in the field and is later carried to the store. High relative humidity is a crucial factor for encouraging fungal infestation. Factors influencing the degree of humidity in the store can be a high moisture content in the product if it has not sufficiently dried after harvest, infestation with insect pests that results in hot spots and increased humidity, or improper storing technique that allows for contact with rain water or humidity condensation (see Ayertey & Ibitoye, 1987; Gwinner et al., 1996). Fungal infestation results in reduction of grain quality, change in colour, taste, smell, reduction in nutritional value, increase in free fatty acids (FFA) and reduction of germination ability (Dutta & Roy, 1987; Prasad et al., 1987; White & Jayas, 1993; Dharmaputra, 1997).
2.3.1 Fungal diseases
Fungal diseases may be highly hazardous as certain species of fungus produce mycotoxins (Christensen, 1975; Reddy & Nusrath, 1988; Latus et al., 1995; Miller, 1995), which are poisonous substances produced by moulds during their growth and development. Mycotoxins are highly stable compounds that cannot be destroyed through food processing, and the only way to avoid them is to prevent the fungal growth. The first recorded case of poisoning due to food contamination with fungal infestation was in the early 1930s, when 5000 farm horses died in Illinois, USA, due to a disease that was called the "mouldy corn disease". It occurred among farm animals that fed on maize left in the field after harvest (see Christensen & Kaufmann, 1969). Later in the mid 1930s, a plant pathologist in Minnesota, USA, isolated Fusarium sp. from maize infected by ear rot, and the extract gave similar disease symptoms on swine. A few years later in the former USSR, hundreds of people were affected by what was later described as "alimentary toxic aleukia" (see Taylor et al., 1996; Wild et al., 1996). People had eaten millet from plants overwintered in the field and gathered later in the spring. The grains were infected by different species of fungi, some of which produced potent toxins. In 1960, about 100,000 turkeys died in England of an unknown disease. Later a fungus identified as Aspergillus flavus was isolated from a suspected groundnut meal that was imported from Brazil. Extracts from this fungus confirmed the presence of a toxic substance that was given the name "aflatoxin" (see Christensen & Meronuck, 1986). This material has been extensively studied and proved to be highly toxic to man and farm animals. It is a liver toxin which can induce cancer in susceptible animals. Fungal growth can be very rapid as well as the production of aflatoxin, specially in tropical and sub-tropical countries, where environmental conditions are highly conducive (see Highley et al., 1994; Hennigen & Dick, 1995, Scudamore & Hetmanski,1995).
The fungus is widely distributed all over the world and has been found on all foodstuff and their products (Christensen & Kaufmann, 1969; Wareing, 1997). Several strains of A. flavus produce aflatoxin and can contaminate grains, pulses, cassava, oilseeds and other foodstuff. Factors during cereal storage can favour the development of the fungus and the production of aflatoxin (Cloud & Morey, 1980; Christensen & Sauer, 1982; Bhatti et al., 1990). A moisture content that is slightly above 9 percentage in groundnuts or around 16 percentage in cereals is enough to support the development of the fungus (Christensen & Meronuck, 1986; Paderes et al., 1997).
Aflatoxin has been found in sausages in Germany and other meat products. In the Philippines, aflatoxin was found in the majority of the samples of peanut butter in stores. Moreover, aflatoxin consumed by dairy cattle, though altered in their body, still remains toxic and shows up in the milk (see Christensen & Meronuck, 1986; Gwinner et al.,1996).
A. flavus can grow and produce aflatoxin in many kinds of plants and plant products. However, major agricultural crops in which aflatoxins can create a serious problem are groundnut, maize and cottonseeds, specially where crops are grown in warm and humid conditions (see Awuah & Kpodo, 1996; Bankole et al., 1996; Fufa & Urga, 1996). On the other hand, not all strains of A. flavus produce aflatoxins, some can even be used in the preparation of foods for human consumption. Several other Aspergillus species and other fungi in different genera are associated with stored products, some of which may produce other important mycotoxins (Jacobsen et al., 1995; Bottalico, 1997; Cvetnic & Pepeljnjak, 1997). The following is a list of the most common stored product fungus species.
Aspergillus candidus
This fungus is common in stored grains and their products where moisture content is at least 15-16 percentage. It is known to cause preliminary heating of stored grains. Its presence is an indication that a stored lot is contaminated with spoiled grains (see Jevtic et al., 1990; Bujari & Ershad, 1993; Awuah & Kpodo, 1996).
Aspergillus clavatus
This fungus is commonly found in soil and decaying plant materials. It requires a moisture contents of 23-25 percentage in cereal seeds and can grow at lower relative humidities on groundnut meal or copra (see Adisa, 1994; Famurewa et al., 1994; Lopez-Diaz & Flannigan, 1997).
Aspergillus fumigatus
This fungus occurs in decaying plant materials and requires relatively high temperature to develop (400C). It was reported to result in a high level of abortion in cattle feeding on contaminated food. A. fumigatus may also infect human lungs. However, this species requires a high relative humidity of 95-100 percentage to grow (see Darwish et al., 1991; Pandey & Prasad, 1993; Abdu et al., 1995).
Aspergillus parasiticus
An aflatoxin producing fungus which attacks maize, groundnuts and oilseeds (see Christensen & Meronuck, 1986; Le et al.,1995).
Aspergillus restrictus
This species is known to have a "restricted" growth. It is able to kill and discolour wheat germ at a narrow range of relative humidity of 13.8-14.3 percentage. A. restrictus is usually associated with rice weevils, but even when the insect pest is eliminated, the fungus will continue to grow. It is also associated with some grain infesting mites (see Jevtic et al.,1990; Silva et al.,1991; Udagawa, 1994).
Alternaria alternata
An important mycotoxin producing fungus that attacks rice, sorghum and soybeans (see Jevtic et al.,1990; Jacobsen et al., 1995; Hasan, 1996).
Fusarium graminearum
This species produces deoxynivalenol, which is a serious and acute human toxin. It also produces zearalenone. Both toxins are produced on maize, wheat and barley (see Wang et al., 1990; Sidorov et al., 1996; El-Sayed, 1997).
Fusarium moniliforme
This species commonly invades stems of maize plants and it is known to produce the mycotoxin, fumonisin. In high moisture conditions, F. moniliforme may be involved in rotting of the kernel. However, it requires a 22 percentage moisture contents to grow, thus, it does not cause serious problems in stores (see Lee et al., 1994; Tavares et al., 1995; Bacon & Hinton, 1996; Jin & Qiu, 1996).
Fusarium roseum
This species causes scab of wheat, barley and oats. Symptoms are the discoloration of seeds. It also causes "ear rot" in maize and may continue developing on maize left on the plants after harvest (see Biswal & Narain, 1991; Assemat et al., 1995; Adam, 1996).
Fusarium tricinctum
A mycotoxin producing fungus. Heavy infestations are common when maize is stored on the cobs in cribs (see Bao & Wang, 1991; Roinestad et al., 1994; Lin et al., 1994).
Helminthosporium spp.
Fungi belonging to this genus may cause seed infections in different cereals such as maize and rice (see Kedera et al., 1994).
Scopulariopsis spp.
A predominant fungus associated with black and white pepper, soybean flour and powdered milk (see Jevtic et al., 1990; El-kady & Youssef; 1993).
Penicillium verrucosum
This fungus infects barley and wheat. It produces ochratoxin, a mycotoxin that may lead to kidney damage in farm animals. Other certain species of Penicillium produce citrinin, an important mycotoxin that may lead to kidney damage in humans and farm animals. (see Skrinjar & Dimic, 1992; Mantle & McHugh, 1993).
2.3.2 Fungal infestation
Improper handling of crops during post-harvest processes can cause fungal infestation. Any damage to stored products increases their susceptibility to fungal contamination (see Tagliaferri et al., 1993). More importantly, insects activity can have a profound effect on the spread of fungal diseases through transmitting the spores and increasing the surface area susceptible to fungal infection, which eventually increases the production of mycotoxins. Dunkel (1988) indicated that some storage insect species are disseminators of storage fungi while others are exterminators; some storage fungi attract storage insects and promote their population increases while others repel and secrete toxins harmful to insects. Therefore, knowledge of basic biological relationships between insects and fungi in the stored grain ecosystem is crucial for their management. Several studies demonstrate the importance of insect pests as promoters or facilitators of fungal infection. In Nigeria, for example, Acholo et al. (1997) showed that the yam beetle, Heteroligus meles, which was the largest cause of damage to tubers, facilitated the spread of different Fusarium species and other less abundant fungi. None of the fungi was able to infect undamaged yams in the laboratory. In India, Pande & Mehrotra (1988) sampled wheat grains for Sitophilus oryzae and found that A. flavus was the most frequent species in their alimentary canals, followed by A. candidus, A. sojae, A. fumigatus, Penicillium rugulosum and Cladosporium cladosporioides. This indicates the possibility that S. oryzae transmits fungus spores from infested to healthy grains. In the USA, Beti et al. (1995) showed that maize kernels infested with A. flavus-contaminated Sitophilus zeamais weevils had higher levels of aflatoxin than A. flavus-inoculated maize without weevils. The presence of S. zeamais resulted in increased kernel moisture content which was positively correlated with aflatoxin contents. In addition, aflatoxin levels in infested maize increased with increasing numbers of A. flavus-contaminated S. zeamais, as S. zeamais carried spores both internally and externally on their exoskeleton.
In some crops, efforts to remove broken and discoloured seeds can effectively reduce the production of mycotoxins. However, this may not be practical for many products, especially when the fungal growth is internal and difficult to detect. In Thailand, an in-store drying system to control aflatoxin contamination in maize was developed. High moisture maize is dried to 18 or 19 percentage moisture content within 2 days and continuously dried to 14 percentage within 14 days. An airflow rate of 3.6-4.6 m3/min per m3 of maize is required to decrease moisture content from 19 to 12 percentage (Prachayawarakorn et al., 1996). In an experiment in India, cinnamon oil treatment of maize, in combination with sodium chloride, synergistically inhibited fungal infection, growth and aflatoxin production (Chatterjee, 1989).
