Field Crops News

Velvetbean Caterpillar Virus Persists on Soybean Foliage

The velvetbean caterpillar is a sporadic pest of soybeans in the midwest, but is more common further south. A naturally occurrring nuclear polyhedrosis virus (NPV) sometimes greatly reduces velvetbean caterpillar populations.

Bioassays of soybean leaves after a natural occurrence of NPV-killed velvetbean caterpillars in Arkansas showed that the virus persisted on foliage and killed up to 62.7% of caterpillars up to 7 weeks later. This is in contrast to other studies with prepared sprays of NPV on soybeans which showed a half-life of 1-3 days in the field. NPV activity was higher on leaves from the lower canopy than those from the upper canopy, regardless of whether the NPV-killed caterpillars were in the upper or lower canopy. This may be related to the movement of NPV into the lower canopy by the action of rainfall and wind; also NPV in the lower canopy is better shielded from ultraviolet light, which inactivates the virus particles. The longer activity of NPV from dead caterpillars compared with foliar sprays could be due to screening of the virus from uv light by the insect remains.

In another experiment, uninfected velvetbean caterpillars were caged on soybeans at weekly intervals after the death of NPV-infected caterpillars. Mortality of caterpillars during a 4 week period was greater in treatments where the NPV-infected caterpillars had died in the upper canopy. This may be due to the preference of velvetbean caterpillars to feed in the upper canopy.

Source:

S. Y. Young & W. C. Yearian. 1989. Persistence and movement of nuclear polyhedrosis virus on soybean plants after death of infected Anticarsia gemmatalis. Environ. Entomol. 18: 811-815.

Using Artificial Honeydew to Manipulate Lady Beetles

Adult lady beetles move around a lot looking for prey and tend to aggregate in areas of high prey density. When they find aphids or honeydew they tend to settle down and search in that restricted area. Simple sugar solutions can be used as artificial honeydew to promote aggregation of adult beetles. This responsiveness of beetles to artificial honeydew suggests that their dispersal and spatial distribution might be managed by local applications of artificial honeydew, thereby enhancing their impact on aphids or other pests.

In experiments in Utah, the responses of three species of lady beetles--that normally aggregate in alfalfa in response to pea aphid--were examined when artificial honeydew was applied to either centers of fields or field edges.

Application of sugar sprays to the center of alfalfa fields resulted in dramatic changes in the distribution of lady beetle adults throughout the field. Within 24-48 hours of making the sprays, lady beetle density in the sprayed areas was greatly increased; beetle densities at distances greater than 30 meters from the treated areas declined. This shows how quickly lady beetles can move around within a field in response to changing environmental conditions.

Applications around field edges had a slightly different effect. Lady beetle numbers increased not only in and near the sprayed areas, but also well into the unsprayed center of the field. This suggests that beetles were being lured into the field, rather than just moving around within the field.

It was suggested that sugar sprays might be used as a management technique to remove adult lady beetles from fields scheduled to be treated with insecticides, by spraying adjacent crops with artificial honeydew to attract beetles from the fields to be sprayed. Although all species of lady beetles in these experiments responded positively to artificial honeydew, there were subtle differences among the species that might affect the degree of effectiveness of manipulation.

Source:

Evans, E. W. and D. R. Richards. 1997. Managing the dispersal of ladybird beetles (Col.: Coccinellidae): Use of artificial honeydew to manipulate spatial distributions. Entomophaga 42 (1/2): 93-102.


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