Aphidiid wasps attack only aphids. The conspicuous sign of aphidiid activity is the presence of aphid "mummies" - swollen, dead aphids that have been tanned and hardened to form a protective case for the developing wasp pupa.
L. testaceipes is a tiny (<3mm) black wasp and is not commonly seen. However, the distinctive aphid mummies which remain on leaves after the parasitoid has killed the greenbug can easily be detected. The mummy consists of the outer skin of the greenbug which becomes modified into a tough protective shell after the developing wasp kills the greenbug by its internal feeding. Greenbugs parasitized by L. testaceipes are beige or tan in color and are round and swollen compared to healthy greenbugs.
Wheat and grain sorghum.
L. testaceipes overwinters as a grub or pupa inside a parasitized aphid. The newly emerged wasp mates, and then begins to search for new aphids to attack. The female wasp inserts an egg into the greenbug and in about 2 days a tiny wasp grub hatches and feeds internally on the living aphid. The wasp grub completes feedings in about 6-8 days, resulting in the death of the aphid. Movement of the wasp grub inside the aphid expands the aphid, giving it a swollen appearance. The larva cuts a hole in the bottom of the aphid, attaches the aphid to a leaf with silk and a glue, and the dead greenbug changes color from green to a brown "mummy". Then the wasp grub molts to the pupal stage, and after 4-5 days a wasp emerges by cutting a circular hole in the top of the mummy.
At 70°F, development from egg to adult takes about 14 days. Wasps disperse by flying, or by being carried inside winged aphids which may undergo long migration flights.
Wasp parasites contribute to greenbug suppression in two ways. There is direct mortality caused by the wasp parasitism, but, in addition, parasitized aphids have reduced reproductive rates. Parasitized greenbugs stop reproducing within 1-5 days, while healthy greenbugs give birth to 3-4 live greenbugs a day for 25-30 days. Thus, the activity of these wasps can greatly reduce the rate of greenbug increase.
Hyperparasite attack may reduce the effectiveness of L. testaceipes if the hyperparasite is abundant.
Parasitoid activity in the field can be monitored by looking for greenbug mummies on crop leaves. As a general rule, a greenbug infestation usually declines rapidly after 20% of the greenbugs are mummies; at this point most of the living greenbugs have been parasitized but have not yet turned into mummies. Normally mummies appear 8-10 days after wasps lay their eggs in the greenbug.
Temperature is an important factor influencing the efficacy of wasps as biological controls of greenbugs. Wasps develop most rapidly when temperatures are above 65°F, and adults are not active if temperatures are below 56°F. However, greenbugs are much more tolerant of cool temperatures and continue to reproduce until temperatures drop to 40°F. Thus wasps may not be effective in controlling greenbugs in wheat in the fall and spring due to cool weather.
Pesticide use in wheat or grain sorghum may decrease activity of these parasitic wasps. Insecticides applied as sprays will kill adult wasps as well as immature wasps developing inside greenbugs killed by insecticides.
Research in Texas has shown the methyl parathion and chlorpyrifos are more toxic to adult wasps and to immature wasps inside greenbugs than systemic insecticides such as dimethoate or disulfoton, especially at lower rates. However, the shorter residual activity of methyl parathion allows parasites to recolonize a field sooner after treatment. A fungicide (triadimefon; Bayleton) used to control leaf rust in wheat is also very toxic to adult wasps.
Knutson, A., Boring III, E.P., Michaels, Jr., G. J., and Gilstrap, F. (1993) Biological Control of Insect Pests in Wheat. Texas Agric. Ext. Service Publ. B-5044, 8 pp.
Additional Reference:Hoffmann, M.P. and Frodsham, A.C. (1993) . Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.