Wednesday, July 22, 2009

Landscape - Hot, Dry, Weather Pests

In some parts of down state Delaware, we have not had rain for 3 weeks and drought stress is evident. Hot and dry weather will favor certain pests. The following is more information.

Hot-Dry Weather Pests

It is well known that lacebugs reach their highest populations on plants located in sunny-dry locations. There are few predators in these areas,and the shallow rooted azalea is often under drought stress. Lacebugs typically thrive under these conditions.

Scale insects also generally will do better in hotdryweather primarily because the immatures are notknocked off the plants by raindrop “bombs.” Recently hatched scale crawlers are unprotected for several days as they move about the plant looking for a place to settle down. If rains do cause the crawlers to fall to the ground, it is unlikely they will be able to climb back up the plant before they die (their short-stubby legs do notwork very well). The timing of the rainfall to suppress the scale crawlers is critical, because they typically are active for only a few days. Once they settle down and insert their mouthparts into the plant tissue, they are less vulnerable to the raindrops.

Various leaf feeding beetles and caterpillars usually prefer hot-dry conditions, not because they may get knocked off the plants (they will often simply climb back up), but since they can be infected by fungal diseases if a lot of free moisture is present. For example, the Entomophagafungus that has been decimating most of the Gypsy Moth populations for more than a decade does an excellent job of keeping these caterpillar populations in check in cool, moist springs. However, when warm and dry springs occur, this pest typically returns to become a problem again in certain areas.

One of the most common of the warm season pests is the two-spotted spider mite. These pests thrive in very dry conditions, and they will reproduce more rapidly when it is warmer. Mites are vulnerable to being dislodged by heavy rains unless they are under protective leaf covering. Invariably, the worst spider mite populations within the landscape occur when plants are sited under over-hanging structures. In moist conditions, they also can be attacked by diseases but in dry weather, these diseases are not present. In the landscape, twospotted spider mites can have between 10 to 15 generations per year.

Adapted from an article by Steven K. Rettke, Ornamental IPM Program Associate, Rutgers University

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