Field Crops

Green Lacewings Not the Answer for Cotton Aphid Control

Green lacewings are often abundant in mid- and late-season cotton fields in the San Joaquin Valley of California, but don't suppress cotton aphid populations. A number of other generalist predators, such as big-eyed bugs, damsel bugs, assassin bugs and minute pirate bugs, are also common in cotton fields. Direct observations during two field seasons revealed that nymphal big-eyed bugs and nymphal and adult assassin bugs prey on lacewing larvae. When predators were excluded in cage studies, lacewing survival was greatly improved. In these studies damsel bugs and assassin bugs killed all the lacewings when caged together, while big-eyed bugs decreased survival from 47% without any predators to 20% with predators. Populations of cotton aphids were higher when predatory bugs were present, but none of the predatory bugs, when considered alone, had an impact on aphid population growth. Only lacewings were able to cause aphid populations to decrease rather than increase.

Densities of lacewing eggs are naturally high in San Joaquin Valley cotton fields, so augmentative releases should be considered only after scouting shows low natural populations. But because of the impact of the other generalist predators, extremely high release rates would be necessary to have any impact on aphid populations. Such high release rates would probably not be economically feasible. Destruction of the other predators is not warranted, since they  help control other cotton pests, so research continues to identify other biocontrol agents for the cotton aphid that are not susceptible to these predators (or develop other control tactics).

Sources:

Rosenheim, J. A. and L. R. Wilhoit. 1993. Predators that eat other predators disrupt cotton aphid control. Calif. Agric. 47(5): 7-9.


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