Order Homoptera include aphids, whitefly, scales, leafhoppers, and mealybugs. They are plant-sucking, and many excrete honeydew, a liquid high in sugar, which attracts ants and is used as a substrate for sooty mold fungus, which interferes with plant photosynthesis. Some are soft bodied, slow moving, or sedentary, forming colonies with wingless forms. Others are active. Adults have wings held roof-like over the body; the antennae are often short and bristle-like (as with leafhoppers). With sucking piercing mouthparts, many are vectors of plant viruses.Some secrete molted skins or a waxy, powdery substance that covers the body. Many are spread by the wind or carried by ants that feed on the honeydew and protect the insects from natural enemies.
Cabbage aphid (brevicoryne brassicae)
Common name; cabbage aphid
Scientific name: brevicoryne brassicae
Host crops: it is a serious pest of the cruciferae grup (cauliflower, kales, cabbage, broccoli, Brussels sprout)
Feeding prevalence is on young succulent growth. Heavy attack on young plants can check their growth beyond recovery. Attacked plants are characterized by contamination with caste skins, honey dew and fungal growth leading to loss in market value. It has a pest that sucks sap from the plant and a vector of viruses.
Cabbage aphids live in colonies thus forming clusters when feeding. Sexual forms are produced either by winged or wingless forms. Winged females are most commonly encountered. They are greenish in color covered by white mealy powder. Male cabbage aphids have not been encountered in the tropics and consequently one is not expected to see eggs in this species. Normally the aphids are wingless (apterous) but up on experiencing pressure due to large numbers and limited food resources due to competition, they develop wings (pterous) and fl to other crops where they are attached and develop new colonies.
A single wingless female produces 2-3 daughters a day. The daughters reach maturity after 8-10 days after which they remain productive for about 15 days. They have a post reproductive period of about 2-3 days. The rate of reproduction is high in aphids and intense colonization of host plants occur within a short period of time. The mortality rate of females increases after completion of reproduction.
Use of chemicals such as Dimethoate, Diazinon, Pyrethrin, Formothion, Ambush 10.3.2 Green peach aphid Myzus periscae
- Common name: Green peach aphid
- Scientific name: Myzus periscae Sulzer
- Host crops: it feeds on hundreds of host plants in over 40 plant families. Vegetables in the familiesâ€™ solanaceae, chenopodiaceae, compositae, cruciferae and curcubitaceae.
- Vegetables include; artichoke, asparagus, bean, beets, broccoli, Brussels sprout, cabbage, carrot, cauliflower, cantaloupe, celery, corn, cucumber, fennel, kale, kohlrabi, turnip, eggplant, lettuce, mustard, okra, parsley, parsnip, pea, pepper, potato, radish, spinach, squash, tomato, turnip, watercress and melon.
- Field crops such as tobacco, sugar beet and sunflower are also attacked.
- Numerous flower crops and other ornamental plants are suitable for green peach aphid development. Stone fruit crops such as peach are sometimes damaged before the aphids leave for summer hosts.
- High densities on young plant tissue, causes water stress, wilting, and reduced growth rate of the plant.
- Prolonged aphid infestation causes appreciable reduction in yield of root crops and foliage crops.
- Contamination of harvestable plant material with aphids, or with aphid honey dew, also causes loss.
- Blemishes to the plant tissue, usually in the form of yellow spots, may result from aphid feeding.
- Leaf distortions are not common except on the primary host.
Eggs are deposited on prunus spp trees. The eggs measure about 0.6mm long and 0.3mm wide, and are elliptical in shape. Eggs are initially yellow and green, but soon turn black. Mortality rate of the eggs sometimes is quite high.
Nymphs initially are greenish, but soon turn yellowish, greatly resembling viviparous (parthogenetic nymph-producing) adults. They undergo 4 instars with the duration of each averaging 8 days. Females give birth to offspring 6-17 days after birth. The length of reproduction varied considerably, but averaged 14.8 days. The average length of life was about 23 days, but this was under caged conditions where predators were excluded. The daily rate of reproduction averaged 1.6 nymphs per female. The aphids have a mean reproductive period of 20 days, mean total longevity of 41 days, and a mean fecundity of 75 offspring.
Winged aphids have black head and thorax, and a yellowish abdomen with a large dark part dorsally. They measure 1.8 to 2.1mm in length. Winged green peach aphids seemingly attempt to colonize nearly all plants available. They often deposit a few young and again take flight. This highly dispersive nature contributes significantly to their effectiveness as vectors of plant viruses.
Aphids can be controlled by use of;
- Natural enemies such as lady beetles (coleopteran: coccinellidae), flower flies (Diptera: Syrphidae), lacewings (Neuroptera: mainly Chrysopidae), parasitic wasps (Hymenoptera: braconidae) and entomopathogenic fungi (mainly Entomophthorales). Most are general predators, moving freely among green peach aphid, other aphids and even other insects.
- The ephemeral nature of aphid infestation in many crops is believed to prevent the beneficial organisms from consistently locating the aphids and reproducing in a timely manner. Nevertheless, there is a strong association between the high aphid densities and sudden population decrease following the appearance of lady beetles, wasp parasitoids, or entomopathogenic fungi. For example, green peach aphid infesting spring- harvested spinach crops in Arkansas and Oklahoma is suppressed late in the growing season by Erynia neoaphidis fungus. Unfortunately, the disease is epizootic often occurs too late to keep aphids from attaining high numbers, and fungus-infected aphids remain attached to foliage, providing a serious contaminant of spinach foliage (McLeod et al. 1998). Various studies that selectively excluded or killed beneficial organisms have demonstrated the explosive reproductive potential of these aphids in the absence of biological control agents, thus demonstrating their value in reducing damage potential. In greenhouse crops, where environmental conditions and predator, parasitoid and pathogen densities can be manipulated, biological suppression is more effective and consistent.
- Use of parasitoids such as aphidoletes semiflavus Howard (Hymenoptera: Encyrtidae) and Diaeretiella rapae (mcintosh) (Hymenoptera: Braconidae)
- Use of pesticides
- Predatory midge aphidoletes aphidimyza (diptra: Cecidomyiidae) for greenhouse grown vegetables, especially in Europe (Gilkeson and Hill 1987, Milner and Lutton 1986)
- Cultural manipulations may benefit predators and parasitoids
- Cultural practices
- Among the natural enemies of the M. periscae areboth predators and parasitoids, including;
- Beetles such as the ladybirds the two spotted ladybird (Adalia bipunctata)
- Seven spotted ladybird (Coccinella septempunctata)
- Ten spotted ladybird (Adalia decempunctata)
- True bugs such as the anthocorids
- Pirate bugs of the genera Orius and Anthocoris
- Neuropterans such as the green lacewings of the genera Chrysopa and Chrysoperla, hoverflies such as Syrphus, Scaeva, Episyrphus.
- Gall midgets as Aphidolestes
- Aphid parasitoids such as Aphidius, and parasitic wasps of the family Braconidae
- They are also colonized and killed by insect pathogenic fungi of the order Entomopthrolrales.
Bean aphids: Aphis fabae
Common name: bean aphids
Scientific name: Aphis fabae
Host crop: beans
Give birth to live nymphs and complete life cycle in 7 days. Found on the lower leaf surface, petioles, flowers and pods. The aphids are usually wingless but when they are under pressure due to large numbers in a colony, they develop wings and fly to other plants where they continue with their life cycle. They live in colonies and have a very short life cycle.
It is a serious pest of beans especially during the dry season. Aphids live in colonies.
They cause plants to get stunted, leaf curling and puffy leaves.
They transmit viral disease such as the common bean mosaic virus.
The aphids excrete sugars that lead to the development of the sooty mould that reduces the photosynthetic potential of the beans.
- Uproot infested plants and bury them
- Overhead irrigation assists on reducing aphid populations
- Use pesticides such as dimethoate, formothiom, diazinon, ambush, pyrethrum, endosulfan.
Potato aphid ( Aulocorthum solani)
- Common name:Potato aphid
- Scientific name: Aulocorthum solani (Kaltenbach)
- Host crop: potato
The female gives rise to young ones. They live in colonies mainly on the underside of the leaves. They are pale green in colour and have conscipicuous cornicles on the abdomen. They may be winged or wingless. Both forms produce pale green nymphs. One generation takes about 2 weeks under favourable conditions.
- Infested leaves maybe distorted and may have a yellowish appearance.
- Necrotic spots,veined necrosis along the vein on the underside spreading of numerous necrotic to petioles and reaches the main stem.Leaves ma
- It is a sporadically serious pest of potatoes in the field.Like other aphids they feed by piercing and sucking sap from the young shoot and on the underside of the leaves.
- become complexly necrotic and die prematurel and remain hanging to the stem
- Top most leaves remain green in appearance. Infested plants are greatlystunted with short internodes,have bristles leaves
- The pest transmits 14 varieties of virus which causes leaf roll.
- Control oxychloride (ridomil) for the virus diseases
Cotton aphid (Aphis gossypii)
- Common name: cotton aphid
- Scientific name: Aphis gossypii Glover
- Host crop: Cotton
- Alternative hosts:
Biology and life cycle
Outbreaks are common on young plant in spells of dry weather which clears up rapidly with the onset of rain. Plants may be badly damaged during aphid attack. The aphids vary in size from 2-5mm. Their reproduce very fast pathenogenetically and a single female reproduce 2 or 3young ones every day up to a total of 100 or more. The aphids mature in 7 days and may live for 2-3weeks
Their feeding mechanism (sap sucking) has an effect n the growth of the plants. They have the ability to transmit viral diseases (virus mosaic and leaf curl virus). The aphids secrete wax to keep the insect dry and repel enemies. They produce honey dew fro which saprophytic fungi develop.Symptoms of the viral disease include
- Twisted elongated barren plants
- Dwarfed and curled leaves
- Vein clearing and necrosis
- Progressively smaller curled leaves and flowers
The pests can be controlled by using the following insecticides
- Malathion and Dimethoate
Maize Aphids (Rhopalosiphum maidis)
Common name: Maize aphid
Scientific name: Rhopalosiphum maidis Fitch
Host crop: Maize
Alternative host: Sorghum,millet, sugar cane, wheat and on numerous associated wild hosts
The aphids are common and serious pest of maize. Young plants are most at risk. Aphids build up large numbers in colonies, on leaves and tassels. The plant become distorted, chlorotic and stuted. Heavily infested tassels may become sterile. Honey dew secreted by the aphids encourages growth of sooty moulds and cover the seeds in astrictly residue which makes processing difficult. These aphids transmit virus diseases of maize, such as leaf fleck and sugar cane mosaic
Aphids reproduce asexually (pathogenesis). Pathenogenic females give birth to living young and a generation can be completed in 8 days. They reproduce continuously throughout the year. The aphids varying colour from yellow green to dark blue_green. They may also be covered with a thin layer of white wax or shed skins. Both winged and wingless forms may be found on the same plant
- Vigorous plants are usually tolerant of aphids attack
- Natural enemies may provide sufficient control and should be encouraged
- Very heavy infestations may be controlled by applying an aphicides, such as pirimcarb or a systemic general insecticide such as dimethoat
White fly (Bemisia tabaci)
- Common name: White flies
- Scientific name: Bemisia tabaci
- Host plants: The pest has been recorded from more than 600 plants species. Crops that support large numbers of whitefly include cotton, okra, cabbage and other Brassica crops, cucumber, sesame, beans, peanuts, sweet potato and cassava.
- It attacks cut-flowers such as Poinsettia, African daisy, roses carnations and morbydisk. It also infests ornamentals plants such as euphorbia, hibiscus, lantana and chrysanthemum.
- Fruits such as grape, citrus, taro, pawpaw, lettuce and frangipani also harbor whiteflies.
- Weeds such as mustards (Brassica), Ipomoea spp. And nightshade.
- Large number of alternative host plants makes it difficult to control the pest.
Whiteflies are tiny, the adult resemble white moths and the adults have white wings and yellow body. The immature stages look like scale insects, adultsâ€™ wings are covered with a white, waxy powder, making them difficult to wet. The adults are about 1mm long and appear as narrow white wedge-shaped insects. When an infested crop is tapped these tiny insects can be seen to flutter out and rapidly resettle. The common species is Bemisia tabaci.
The females lay eggs on the undersides of young leaves. The eggs are white at first but turn brown before hatching. The larval or immature stages are greenish white, scale like and oval in outline. The pupal or resting stage is yellow, slightly pointed at one end and 1-2mm long. The life cycle takes 18days depending in temperatures. Upon emergence, the adult whiteflies remain on the leaf for several hours as they coat themselves with wax. The adults can live for about 60 days. Whiteflies can fly for several hours and wind assistance can traverse long distances. A female can lay up to 160 eggs during its life cycle.
- Adult stages are the most damaging stages. Whitefly adults and immature stages suck sap, causing plants to collapse. With high populations plants may die.
- Presence of sooty mould and infestation of high value crops affects their marketability.
- The pests produce copious honeydew that leads to growth of sooty mould. He produce may look unsightly and the mould can reduce rate of photosynthesis of infested plants.
- The damage can results from toxins produced by the feeding activities of feeding of immature insects.
- Cucurbit species, squash, zucchini and butternut can develop a silvering of the leaves along the upper surface known as squash silver leaf and also a whitening of the leaf stalks and stems. The silver leaf starts as a lightening of the veins of new foliage growth.
- Infestation on tomatoes causes blotchinesss or irregular ripening rendering them unmarketable. Feeding activities of the immature stages of broccoli and other brassicas result in development of a phytoxic disorder called â€œwhite streakingâ€.
- The pests also transmit several viruses including strains of tomato yellow leaf curl virus and tomato mottle virus. Worldwide, whitefly is capable of transmitting viruses that cause more than 40 crop diseases.
- Feeding of whiteflies causes yellowing of infested leaves.
- Whiteflies excrete honeydew, a clear, sugary liquid. This honeydew covers the lower leaves and supports the growth of black sooty moulds, which may coat the entire plant.
- Where plant viruses are transmitted, plants show the typical symptoms of the virus diseases.
- Presence of whiteflies can also be recognized by a cloud of tiny whiteflies flying up when the plants are shaken. The whiteflies resettle soon on the plants.
- The affected stages include; seedling, vegetative growing and flowering stage where the leaves gets affected.
There are many ways that the insect may be detected;
- The pest can be detected either as the fluttering adults or as the immature scale-like insects on the underside of older leaves.
- When whitefly insects are observed building-up on alternative host plants.
- Another good indication of the presence of whitefly is the silver leaf appearance on the upper leaf surface. Farmers need to note that only a few immature stages are necessary to cause the silver leaf reaction.
- Any sudden build-up of small whitish fluttering insects on plants should be investigated as a possible infestation.
Management and control
There are very few insecticides registered which will give effective control of whitefly and every effort should be made to prevent the infestation of crops by whitefly. Scout for presence of adults or the scale-like immature stages on the underside of the leaves. Use insecticides such as insect growth regulators, insecticidal (potassium) soaps, petroleum and vegetable oils.
Monitoring and decision making
- For early detection, inspect for adults and eggs. They are usually found on young leaves. It is important to identify the whitefly and the type of damage caused, as well as the stage of the crop for making decision. Small numbers of whiteflies do not cause major direct plant damage to healthy, mature plants and therefore do not justify any chemical interventions.
- Control measures can be justified if large numbers of whiteflies are present during the early stages of the crop. However, where virus transmission is involved, as in the case of tobacco whitefly on tomatoes, sweet potato or cassava, even smaller numbers of whiteflies need to be controlled
- Yellow sticky traps can be used to monitor the presence of whiteflies for the timing of interventions
Providing conditions for growing healthy plants
- Ensure adequate growing conditions for the crop such as good soils, adequate water supply, proper feeding (avoid application of high dose of nitrogen fertilizer, since it favours development of the pest), proper spacing and good nursery management to start the crops with healthy, vigorous plants.
Mixed cropping systems
- Intercropping can be used to manage whitefly populations. For instance, intercropping tomatoes with capsicum or cucumber reduces whitefly numbers when compared to tomatoes alone or tomatoes planted with eggplant or okra.
- Planting of border rows with coriander, which is non host of B. tabaci, serves as windbreaks, and are favorable for natural enemies and also whitefly repellants.
- Growing African marigold discourages whiteflies.
Avoid the season when whiteflies are more likely to occur
Host plant resistance
- Growing resistant varieties is particularly useful for management of diseases caused by viruses transmitted by whiteflies.
- Tomato varieties resistant to tomato yellow leaf curl (TYLC) virus are available and can be bought in Tanzania and Kenya.
Weeds play an important role in harbouring whiteflies between crop plantings. They also often harbor whitefly transmitted viruses. Therefore, weeds should be removed in advance of planting. Fields should also be kept weed free.
Biogical pest control
- Use of natural enemies such as parasitic wasps Eretmocerus spp, predatory mites Amblyseius spp and Typhlodromus spp, predatory thrips, lacewings, rove beetles and ladybird beetles. The dusty lacewing Conwentzia Africana is considered to be one of the most important predators of B. tabaci in east and southern Africa.
- Parasitic wasps are very important for control of whiteflies. Encarsia Formosa in particular, has been widely used for control of whiteflies worldwide.
- Several fungi verticillim lecanii, Beauveria bassiana, paecilomyces fumosoroseus attack whiteflies and can be useful control agents in situations where the crop is grown in high humidity conditions. Commercial preparations are available.
- Natural enemies commercially available in Kenya include the parasitic wasp Encarsia Formosa, produced by Dudutech and pathogen Beauveria bassiana under the trade name BbplusÂ® by Juanco SPS Ltd.
Neem (Azadiracta indica)
Neem based pesticides are reported to control young nymphs, inhibit growth and development of older nymphs, and reduce egg laying b adult whiteflies. They also reduce significantly the risk of transmission of TYLC virus.
- Yellow sticky traps are usually used to monitor the presence of whiteflies for timing of interventions, have also been used as a control method for low density infestations in enclosed environments.
- Spraying with soap and water reportedly controls whiteflies, however, care should be taken, since the use of strong soaps at high concentrations can scorch the plants.
Mealy bugs (Pseudococcidae)
Mealybugs are cottony looking insects with piercing/sucking mouthparts. They can be found on almost any part of the host plant including leaves, stems, roots and fruits. The pest affects different crops.
Dudley R. (2002). The biomechanics of insect flight: Form, function, evolution. Princeton, NJ: Princeton University Press