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Insects often outbreak unexpectedly
Jun. 19, 2012
Insect outbreaks often appear unexpectedly. Whether it's bark beetles killing acres upon acres of pine trees, houses invaded by ladybird beetles, armyworms stripping wheat fields bare, or Gypsy moth caterpillars turning oak leaves into a steady rain of caterpillar droppings. The obvious question is "Why do insect outbreaks occur?"
Several insects are known for their abrupt increase in abundances. In the eastern United States, the Gypsy moth has a habit of appearing suddenly within a forest before disappearing again for many years. This insect voraciously feeds on the leaves of trees and shrubs, threatening residential trees, forests and agricultural tree crops. Gypsy moths have garnered more notice, frustration, research dollars and environmental concerns than any other forest or urban tree pest in the eastern United States. Most of the time, natural controls keep Gypsy moth populations in check. These include predators, parasites, disease organisms and even bad weather, all of which tend to maintain low populations of the moth.
However, if something happens to alter this balance and the survival rate of the insect increases, it can trigger a sudden increase in their population. Because one female moth can lay up to 1,000 eggs, Gypsy moths are capable of rapid population increases. All it takes to maintain the normal population level is for one male and one female out of that 1,000 to survive. That's only a 0.2 percent survival rate. To put it another way, 99.8 percent of the eggs and larvae can die without reducing the total population.
In northern Arizona, there is a native insect called the Pandora moth that also appears unexpectedly. Every twenty to thirty years, large populations of this insect overtake the ponderosa pine forests north of the Grand Canyon. Interestingly, this insect has a two-year life cycle with adult moths occurring during even years (2012, 2014, etc.) and caterpillars during odd years (2011, 2013, etc.) Because defoliation happens only every other year and trees are able to recover in alternate years, the damage caused by caterpillars does not kill pines. However, the defoliation can be unsightly and diminish the experience of visitors to the canyon.
Pandora moths started to outbreak near Jacob Lake last summer, and many caterpillars were seen feeding on ponderosa pine needles. This summer, expect to see thousands of moths near Jacob Lake in mid-August, as these caterpillars metamorphose into adults. Why there are periodic outbreaks of this insect is not known, but a virus is believed to cause populations to decline. The Paiute people in California used to celebrate these outbreaks and would harvest, prepare, and store the larvae (which they call piuga) as a preferred food.
Insect outbreaks appear to happen overnight because people generally don't notice the increasing numbers until large areas are affected. Fortunately, natural biological controls or harsh weather conditions can reduce insect abundances. Eventually the booming population crashes, and the insects that were so numerous seem to disappear as mysteriously as they arrived.
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