Integration of biological control includes management to increase the potential impacts of predators and parasitoids. This can include breeding crop varieties on which these insects can do best. For example, cucumber varieties have been bred with trichome densities optimal for the whitefly parasitoid Encarsia formosa. We have been examining the potential of genetically altering crop surface waxes to improve the effectiveness of generalist predators. On Brassica, predators are more effective on plants with reduced wax bloom. Recently we have begun studying a similar phenomenon on peas.
In a field study in 1996, weekly aphid populations were consistently 50% lower on glossy peas with a reduced wax bloom versus a sister line with normal waxes. The reduction in wax bloom gives the pea surfaces a glossy sheen as compared with a normal pea leaf. To determine whether the reduction of pea aphids on the glossy peas was due to increased efficacy of the natural predators on these glossy surfaces, cage studies were performed. Individually caged glossy or normal wax peas were infested with a known number of aphids. After allowing the aphids to settle, four adult convergent lady beetles were introduced into each cage. The suppression of aphid population growth by these predators was significantly greater on glossy peas on day 4. After the fifth day, when aphids became very abundant, predators were equally effective on both pea types.
Observational studies were performed to determine the effect of pea wax surfaces on the behavior of the predator. These studies showed that on the normal peas, the ladybird beetles spent more time "grooming" and "falling" than in any other activity. However, on glossy peas, the beetles were able to walk and search more, and spent far less time in unproductive grooming and falling. In addition, the traction generated by beetles while walking was 30 times greater on the glossy peas, which can explain their greater mobility on these plants.
Our experiments show that, as in Brassica, a simple genetic trait reducing surface wax bloom in peas can increase predator mobility and effectiveness. These findings indicate a potential for breeding crop varieties without wax blooms that will facilitate generalist predators.
- Cynthia White and Sanford Eigenbrode, University of Idaho, Moscow
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