Rodrigo Diaz, Ring, Dennis R.
Aphids are major pests of row crops and ornamental plants. Aphids are soft bodied insects that suck the sap and cause injury to plants including the injection of toxins and transmission of plant pathogens. Aphids secrete honeydew, and sooty mold grows on the honeydew. Aphid feeding may result in stunted plant growth, and yellowing, shriveling and deformation of leaves. In Louisiana, common aphid species include the green peach aphid, cotton aphid, pea aphid, cowpea aphid, black bean aphid, potato aphid, glasshouse-potato aphid, milkweed aphid and ornate aphid.
Parasitoids are small insects that play an important role in the natural control of aphids. Parasitoids lay their eggs inside or outside of the body of another insect (host). The parasitoids in the subfamily Aphidiinae are small wasps (0.08 - 0.12 inches) that lay their eggs only in aphids. Some species attack a limited number of aphid species and sometimes only a single species. The immature parasitoid develops by feeding on the aphid. Such parasitoids can be imported from other countries, reared under laboratory conditions and purchased from commercial insectaries to control target pests.
Aphid parasitoids will only attack and kill aphids. Despite their presence in gardens, aphid parasitoids will not cause damage to plants, others insects, pets or humans.
Aphid parasitoids in the genera Aphidius (Hymenoptera: Braconidae) and Aphelinus (Hymenoptera: Braconidae) are very effective in controlling aphids when aphid numbers are low. These parasitoids can be purchased from companies that deliver “aphid mummies” which are parasitized aphids. Aphidius is usually recommended for the control of melon aphid, cotton aphid, green peach aphid, tobacco aphid, cherry oat aphid, potato aphid and greenhouse potato aphid. However, parasitoids of this genus may be used against other aphids to reduce their numbers as well. The genus Aphelinus is suggested for management of the potato aphid.
Adult aphid parasitoids are small dark-colored wasps (0.08 - 0.12 inches) and have long antennae. The presence of the parasitoid can be determined by the presence of mummies (parasitized aphids) on the leaves. Mummies appear as dark, “bloated” aphids and are a clear sign of parasitism.
Parasitoids require one host to complete their lifecycles. The egg and larval stages develop in the aphid. In about 48 hours, the small larva emerges from the egg and starts consuming the aphid tissues. In the late stages of development, the larva will consume all the host’s tissues including guts and muscles causing death of the aphid. The larva makes a small hole in the aphid and fixes the aphid to the leaf.
The larvae remain inside of the dead aphid and pupate. At this stage, only the aphid’s skin is remaining resulting in a “mummy”. After the completion of development, the tiny wasp will make a hole in the mummy to exit the skin of the aphid and become a free-living adult. The development from egg to adult can range from nine to 23 days. The speed of development depends on the temperature; at low temperatures development is slower and at high temperatures development is faster. Adults live from seven to 15 days. Others factors such as rain, humidity and wind also can affect the development and length of life of the parasitoids.
Levels of control by aphid parasitoids vary. For example, parasitism of cotton aphid and green peach aphid by Aphidius sp. was 48 percent and 62 percent, respectively (Sampaio et al. 2001; Soglia et al. 2006), while the same species inflicted 70 percent control of glasshouse-potato aphid, blue alfalfa aphid and potato aphid (Sidney et al., 2010). To enhance the impact of aphid parasitoids, gardeners can use flowering cover crops and reduce the use of insecticides. Remember, parasitoids can be great allies in the battle against aphids.
Monique Ferreira de Souza, Dennis Ring, Rodrigo Diaz; Department of Entomology, LSU AgCenter.
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