Heat, drought help some pests succeed

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MANHATTAN, Kan. – Heat and drought can create a heyday for plant-feeding insects and mites.

First, more pests survive, according to Raymond Cloyd, K-State Research and Extension entomologist. Fungi are an important natural control for many insects. But, fungi function best in cool, moist weather.

“Excessive heat also speeds up insect and mite development. Some pests can increase populations rapidly,” Cloyd said.

The two-spotted spider mite is well-known for increasing populations during drought conditions. But, its population explosions are just part of the problem, he said. Warm-loving mites also tend to feed more when summer weather is at its worst. For them, dry air makes eating easier.

That kind of weather also defeats most plants’ ability to absorb and circulate enough water for good health, Cloyd added. So, when mites or insects eat plant fluids for dinner, they get concentrated nutrients – super food.

Perhaps that’s why insects with sucking mouthparts (aphids, scales) tend to thrive more than pests that chew (beetles, caterpillars) during hot, dry weather. Bark and wood-boring insects gain advantages, too.

Water-stressed plants simply can’t maintain their natural defenses – the allelochemicals that normally discourage pests. For example, the compound oleoresin deters wood-boring insects, Cloyd said. So, with less protection, plants become more susceptible to attack.

At the same time, water-deficient plants may emit such volatile chemicals as ethanol and alpha-pinene. These chemicals are signals that attract bark beetles and wood-boring insects.

In fact, droughty plants may actually call to bark beetles. As water-stressed plants lose moisture through transpiration (like sweating), their inner water- and nutrient-carrying tubes may break apart. That produces a sound bark beetles can detect, Cloyd said.

Any breakdown in the water-carrying tissues also will attract female wood borers, ready to lay eggs – more problems in the future. 

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