Wasp parasites contribute to greenbug suppression in two ways. There is direct mortality caused by the wasp parasitism, but also 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. Some wasp species attack L. testaceipes and other parasitic wasps. These parasites of parasites (hyperparasites) may reduce the effectiveness of parasitic wasps if they are abundant.
Parasite 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, because 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 65F, and adults are not active if temperatures are below 56F. However, greenbugs are much more tolerant of cool temperatures and continue to reproduce until temperatures drop to 40F. 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 that 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., E. P. Boring III, G. J. Michaels, Jr. and F. Gilstrap. 1993.
Biological Control of Insect Pests in Wheat. Texas Agric. Ext. Service Publication B-5044, 8 pp.
- Bob Wright, University of Nebraska
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