Know Your Friends

Wasp Parasites of Greenbugs

There are several parasitic wasps which may attack greenbugs in wheat and grain sorghum, including Lysiphlebus testaceipes, Diaeretiella rapae and Aphelinus varipes. These tiny, black wasps are not commonly seen, but the distinctive aphid mummies which remain on leaves after the parasite has killed the greenbug can be easily 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 Lysiphlebus or D. rapae are beige or tan in color, and are round and swollen compared to healthy greenbugs. Greenbugs killed by A. varipes are black and similar in size and shape to live greenbugs.
L. testaceipes is the most common wasp attacking greenbugs in wheat and grain sorghum. Each female wasp can parasitize about 100 greenbugs during her 4-5 day lifespan. Female wasps insert an egg into the greenbug. In about 2 days a tiny wasp grub hatches and feeds internally on the living aphid. The wasp grub completes feeding 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 and attaches the aphid to the leaf with silk and a glue. The dead greenbug changes color from green to brown and is referred to as a 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. The newly emerged wasp mates, and then begins to search for new aphids to attack. At 70F, development from egg to adult takes about 14 days. L. testaceipes overwinters as a grub or pupa inside a parasitized aphid. 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 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.

Adapted from:

- Bob Wright, University of Nebraska

Drawing by Rhonda Kappler.
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