Issue 6, May 29, 2009

Weather Impacts on Insects

As we get more warm weather, insects will develop very quickly. Along with rainy weather, we have had somewhat cooler temperatures, which have slowed down the development of insects. That is, their progression through larval or nymphal stages. During this time, the plants have been able to get ahead of the insect pests which should result in less damage. Insects have a base temperature of around 50 degrees F, below which very little development occurs. The higher the temperature rises above that base temperature and the longer it stays above that temperature, the faster the insects develop. On the other hand, plants tend to develop above a threshold in the high 30's degrees F. During the last couple of weeks with lows in much of the state in the 40's to low 50's degrees F, and highs in the 60's, plant stems have continued to grow and leaves have continued to expand while their insect pests have been slowed by the cool temperatures. As a result, you are likely to see younger, smaller, insect stages for the size of the host leaves compared to that seen in other years.

Another factor affecting insect and plant interactions is rainfall. The abundance of rainfall this year, along with the cooler temperatures, promotes fungal growth and development. Although these conditions are conducive to leaf diseases, insects also get fungal diseases. This is noticeable particularly in soft-bodied insects, such as tree and shrub feeding caterpillars. Their numbers typically drop in cool, wet weather due to disease mortality. High soil moisture levels can also promote fungal disease and mortality among turf pests such as sod webworm and white grub larvae and pupae. Fungal disease can also be prevalent on adult May beetles, black turfgrass Ataenius, and billbugs. Dead, off-color insects are frequently signs of fungal attack. If moisture levels stay high, many of these fungi will produce sporangia that cover the insect with white or green fuzz.--Phil Nixon

Phil Nixon

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