Wednesday, July 22, 2009

Pests of Stressed Plants

The following are some common pests of stressed plants. We are seeing drought stress in many parts of the state now.

Aphids and whiteflies are apparently drawnt o plants with yellowing foliage. Plantsunder stress often lose some of their chlorophyll and become yellowish in color. Yellow sticky cards are often hung in greenhouses to monitor for some of these pests.

Some species of caterpillars (not gypsy moth) are “samplers” when they feed. These caterpillars will sample individual leaves to determine if they contain plant defensive chemicals such as tannins or alkaloid toxins. If these toxins are contained within the leaves, the caterpillar may find it distasteful and move on to sample other leaves. A weakened plant unders tress may not have enough energy to produce these defensive chemicals and therefore, be more attractive for these caterpillars.

The greatest concern with stressed plants in the landscape is from borer infestations. The best examples of borer-prone plants are when drought conditions and high temperatures stress non-native plants. Pine bark beetles, bronze birch borers, and the two-lined chestnut borers are all encouraged when stressed tress are forced to “shutdown”their vascular systems (xylem& phloem tissues). When trees have low sap pressure, the borers can easily penetrate and cut through vascular tissue. Furthermore, trees with compromised vascular systems cannot readily transport defensive chemicals to areas invaded by borers.

Adapted from a presentation delivered by Dr. David Shetlar (The Ohio State University Extension)@ the RCE IPM Symposium, Nov. 1995

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