Natural enemies can be integrated into farming systems at several levels. The most basic
level is to understand and utilize the benefits of natural control to your advantage. This requires
no specific actions by the producer beyond an appreciation for what Mother Nature is doing. An
important example in the Midwest is the impact of the potato leafhopper disease, Zoophthora
radicans. This fungus is present throughout the upper Midwest. When a leafhopper becomes
infected it will die in 2-3 days. Under the right conditions (cool and moist) the fungus then goes
on to produce thousands of spores capable of infecting other leafhoppers. As the disease spreads
through the population, leafhopper numbers can drop rapidly. In Michigan, outbreaks of this
disease (epizootics) have occurred every year since 1989. Typically they occur in late July or
August in conjunction with a cool, wet period. Producers watch for these epizootics in both dry
beans and alfalfa, and frequently find no need for further insecticide applications after epizootics
occurs.
- Doug Landis, Michigan State University
Alternate Hosts for Aphid Parasite Might Improve Biological Control
The parasitic wasp, Diaeretiella rapae, is distributed worldwide and is known to attack over
30 species of aphids. It has been suggested that increasing the diversity of crop and noncrop
plants would increase biological control by allowing reservoirs where the wasp can buildup on
aphids and then move to other sites. However, little is known on the ability of this wasp to
rapidly shift to different aphid species. Wasps that had been reared for 12 generations on cabbage
aphids feeding on cabbageAphis helianthi (a
sunflower aphid). The percentage of aphids attacked ranged from 94% for cabbage aphid to 21%
for cotton aphid. There was no difference in survival of D. rapae from mummy stage to adult
emergence for different aphid host species, but there were significant differences in wasp adult
weights and developmental periods (ranging from 12.3 - 19.5 days). These results suggest that a
diversified cropping system with several potential aphid hosts would allow D. rapae to maintain
itself in an area during periods of low levels of aphid abundance on one crop.
Source: N. C. Elliot, D. K. Reed, B. W. French & S. D. Kindler. 1994. Aphid host effects on the biology of
Diaeretiella rapae. Southwestern Entomologist 19:279-284.
