The coleopteran (beetles and weevils) are the largest insects order, including pests and beneficial insects. The adults have a hardened, sometimes horny outer skeleton, usually with two pairs of wings, the outer pair thickened, leathery, or hard and brittle, usually meeting in a straight line down the middle, and the inner pair membraneous (mostly). Adults usually have a noticeable pair of antennae, variously shaped. Both adults and larvae have chewing mouthparts. Beetle larvae also known as grubs have a head capsule, 3 pairs of legs on the thorax, and no legs on the abdomen. Weevilsâ€™ larvae lack legs on the thorax. Foliage feeders, including Chinese rose beetles feed at night and heavy infestation causes lace-like appearance of leaves. Rose beetles are common and damage many different plants including; roses, grapes, beans, eggplant, corn, cucumber, ginger and ornamentals.
Tobacco flea beetles are tiny brown beetles whose feeding damage causes shot-hole appearance of leaves. They are found on eggplant and tobacco. Stem borers include long horned beetle, whose adults have long antennae and larvae bore into stems and wood; pinhole borers that leave pin-hole in branches, and wood; orchid weevils, whose larvae bore into orchid stem and tissue; black twig borers, whose adults bore through stems of coffee and other economical and ornamental plants and whose larvae feeds on fungus cultured by the adult female.
Root-borers include banana root borers, whose grubs bore into the banana corm causing damage and poor growth and sweet potato weevil, whose grubs feed inside the stems and tubers often followed by decay organisms. Fruit weevils include pepper weevils, the adults and grubs of which infest peppers and cause internal damage and premature drop, and mango seed weevil whose grubs bore into the seed, preventing fresh fruits to be exportable. Household pests include confused flour beetle, rice weevil, cigarette beetle and carpet beetles; they may infest stored grain products and other household belongings. Beneficial beetles include ladybird beetles, also called ladybugs which feed on homopteran insects such as aphids, scales, mealybugs, whiteflies, and psyllids, and scavenger beetles which help to remove carcasses from the environment
Cowpea weevils (Apion pullus)
Cowpea weevil Apion pullus is a major pest for cowpeas. The pest causes considerable damage on peas in coastal province on cowpea seeds and lower parts of machakos and kitui on green grams.
Actual damage by adults appears to be negligible except in the case of heavy infestation where the weevil can cause significant perforations in the young cowpea plants. The female cuts in the skin of the young developing pods and inserts her eggs through each cut. The cut normally heals over completely and become invisible to the naked eye. The incubation period lasts between 3-5 days.
Larvae are the primary source of damage to the seeds. Upon emergence, the larvae eat their way to the young developing cowpea seeds. Growth of larva is rapid except with the last two Instars where there is no increment in body size. There are 4 larval instars. The larval stage lasts 9-1 days. The larval instar is quiescent (dormant) for 2-3 days before pupation occurs. The larva spends its entire cycle in the seed. Up to 15of the seed are destroyed. Pupa period ranges from 4-7days. Pupationoccurs within the pod and by the time the adult emerges, the cowpeas are fully mature and exist holes are conscicous on dry pods. The entire development period ranges from 13-18 days.
Since the weevil lays its eggs in the young developing pods, it is very important that the application of insecticides should be made immediately the plant initiates flower buds. It may also be nessary to have a second application of insecticides when there is 50% flowering. Later application will not be effective in controlling the pest.
Mango weevil (Sternochetus mangiferae)
The major problem associated with mango production in Kenya is infestation by mango weevil. The weevils damage the crop considerably by burrowing into the seeds. The weevils frequently affect the appearance of the fruit by causing the decay of the fruit from the seed outwards. Weevilsâ€™ injury also hastens the ripening fruits to fall prematurely. The adult is dark brown measuring 6-8mm in length.
Adult usually emerges from the fruit after it has been harvested or fallen from the tree. The female makes a shallow depression with her ovipositor on the surface of the skin of the fruit in which she lays her eggs. She then produces brownish excrement from the ovipositor which completely covers the oviposition site. Using her mouth parts she punctures the skin of the fruit just above the oviposition site. The puncture results in a flow of sap which in time hardens or solidifies and covers the egg with a protective coating. NB. Only one egg is laid per oviposition site. There is a pre-oviposition period of 25 days and oviposition period of 75 days. One female adult may lay up to 190 eggs during her life cycle. The incubation period ranges from 4-8 days. The larva is a white grub with a brown head. The newly hatched larva bores throng the pulp of the fruit and into the developing seed where it feeds in the seed within the stone of the fruit.
The newly hatched larva feed by mining tunnels laterally in the soft testa of the seed. As the larva develops, it penetrates the testa and feeds on the cotyledon of the mango seeds. As it grow in size it feeds more vigorously and excrement accumulates and fills the parts of the seed which of the seed which the fruit which has been eaten. A fully developed larva constructs a pupal cell just before pupation. It then transforms to pupa in this cell. The minimum period required for the development of larva stage is 19-30 days. Pupal period lasts from 6-8days.
It is difficult to control this weevil due to the fact that mango trees are very tall thus making it difficult to apply insecticides should be applied during flowering and fruit setting. Later application will not be effective because already hatched larvae will have penetrated the testa and migrated to the seed.
Banana weevil Cosmopolites sordidus
Banana weevil or banana root borer is a major pest of bananas. The grubs bore into the banana corm causing damage and poor growth, often followed by decay organisms.
Tobacco flea beetles
They are tiny brown beetles whose feeding damage causes shot-hole appearance of leaves. They are found on eggplant and tobacco. Stem bores include long-horned beetles, whose adults have antennae and larvae bore into stems, and wood; pinhole borers that leave pin-holes in branches, and wood; orchid weevils, whose larvae bore into orchid stem and tissue; black twig borers, whose adults bore through stems of coffee and other economical and ornamental plants and whose larvae feed on fungus cultured by the adult female.
Möller, R. (2002). A Biorobotics Approach to the Study of Insect Visual Homing Strategies
Chapman, A. D. (2006). Numbers of living species in Australia and the World. Canberra: Australian Biological Resources Study. ISBN 978-0-642-56850-2