Field Crops News

Parasite Can Keep Greenbug in Check

Studies were conducted in Nebraska during 1993 and 1994 to better understand the relationship between greenbugs on grain sorghum and the braconid wasp, Lysiphlebus testaceipes. Often L. testaceipes heavily parasitizes greenbugs on grain sorghum, but this may occur too late to prevent economic damage. One approach to using these wasps would be to encourage their growth earlier in the season, perhaps by planting field length strips of a greenbug susceptible sorghum hybrid in a field of greenbug resistant sorghum. Greenbugs would develop first in the susceptible strips, become parasitized by L. testaceipes and then the wasps would disperse into the rest of the field to begin controlling greenbugs.

Field cage studies showed that L. testaceipes is able to keep greenbugs below economically damaging levels when present early enough. Greenbug populations in cages where no wasps were released increased from an initial 20 greenbugs per plant on whorl stage sorghum to exceed an economic threshold of 1000 greenbugs per plant. In cages where wasps were released (at rates of 0.16, 0.33, 0.5, 1 or 2 wasps per plant) greenbugs stayed below economic levels. In 1993, 0.5 wasp per plant, and in 1994, 0.33 wasp per plant resulted in the greatest numbers of mummies. Greenbug population levels were related to the proportion of greenbugs that were mummies; as percentages of mummies exceeded 20%, greenbug growth rates declined and greenbug numbers declined sharply thereafter. This last result supports our current extension recommendation that if 20% or more of the greenbugs present are mummies, insecticide treatment is not needed, because populations are likely to decline naturally through the activity of the wasp.

In a separate field study, rubidium labelled L. testaceipes moved at least 200 ft from the release site in one day, demonstrating their ability to disperse throughout a field.

Based on Fernandes, O. 1995. Population dynamics and spatial distribution of Lysiphlebus testaceipes and its host, the greenbug, Schizaphis graminum: Studies for the development of an augmentation program. Ph.D. dissertation, Univ. of Nebraska-Lincoln.

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