Sounds Can Draw Natural Predators
MANHATTAN, Kan. – Trees aren’t silent when they’re suffering from drought – which can easily happen in the High Plains during a typical hot, dry summer.
Like a dog whistle’s sound, trees’ distress signals are too high for humans to hear. For many species of bark beetle, however, trees’ ultrasonic noises are a sirens’ call to come and feast.
“These distress signals are just one example of how plant and insect interactions can change during periods of drought – sometimes in rather curious ways,” said Raymond Cloyd, entomologist with Kansas State University Research and Extension. “Homeowners need to be aware of the possibilities, so they can recognize when they need to step in to protect their landscape.”
Warm, dry weather spurs a decline in the fungi that help keep plant pests in check during spring, he explained. At the same time, summer-type weather actually shortens the time pests need to progress from egg to adult. In turn, the faster development can expand both the number(s) and the size(s) of the generations pests are able to produce during the growing season .
“In some cases, the expansion can quickly become an explosion,” Cloyd said. “The two-spotted spider mite is well known for staging that kind of rapid increase.”
Oddly, though, drought-stressed woody plants sometimes change in ways that help or even foster the well-being of their enemies – the plant-feeding insects and mites.
“That’s one reason why supplying the extra water plants need can be so important when drought hits your landscape,” Cloyd said. “The changes that drought can cause may be how nature gets rid of weak plants. I doubt that will be much comfort, though, if your 60-year-old oak is drought-stressed and under attack.”
Trees in that situation don’t actually call out or even whisper to invite bark beetles to visit, he said.
Instead, they act a lot like a pan of boiling water. Their internal water system (xylem) starts to produce bubbles that quickly pop, creating little shock waves of sound and lid-rattling force. Rather than being a reaction to heat, however, the trees’ “boiling” liquid is the result of a big, drought-caused drop in internal water pressure.
Cloyd said extreme dry weather can alter the plant-insect interface in other ways, too:
* Drought stress cuts plants’ production of such compounds as oleoresin (a mix of oil and resin), which normally deters insects.
* Some water-deficient trees and shrubs will emit volatile chemicals (e.g., ethanol, alpha-pinene) that to many types of wood-boring insects are as attractive and attracting as perfume.
* Lack of moisture in a tree’s canopy can cause upper stem tissues to degrade in places, thereby turning those locales into prime egg-laying sites for such pests as the female bronze birch borer.
* As plants become water-deficient, the nutrients they contain become more concentrated. This translates into a nutrition boost that helps the plants’ attackers improve -- in survival rate, reproductive success and even larval weight.
Fortunately for the plants, however, insects and mites don’t all benefit equally from such situations, Cloyd said.
“Their benefits have a lot to do with their feeding behavior. The insects and mites with piercing-sucking mouthparts do fairly well. Their kind encompasses the broad range of aphids, whiteflies and scales. They also include the plant bugs that in Kansas can range from the tarnished plant bug to the chinch bug,” the entomologist said.
The insects that don’t benefit all that much are those with chewing mouthparts – e.g., caterpillars, beetles and sawflies.
“On the other hand, woody plant species vary a lot, too. For example, they vary in how much dry weather they can take before their leaves begin to wilt or brown,” Cloyd said. “They also vary in how soon drought can make their xylem start cavitating – which is the word for breaking apart inside and creating ultrasonic popping sounds.
“Trees’ range of drought tolerance is why you usually have to supply more water to a river birch than to a bur oak when you want to protect it from the effects of droughty weather. It’s also why limiting yourself to planting native trees can be a pretty good idea in the High Plains.”