Saturday, June 30, 2012

How Gross Are the Grubs?

By Lois Tilton (LTilton)
June 25, 2012

While there are several kinds of grubs that damage turfgrass, most are relatively harmless as adults. The most notable exception is the destructive import, the Japanese Beetle. Once it emerges from the ground where it has been gnawing the roots of your plants, its life of destruction goes into high gear. 

A typical white grub is a whitish color with a hard brown head capsule and a darker terminal segment.  Entomologists can identify the species of grub by inspecting the pattern of hairs on this anal segment.  White grubs are legged and their bodies curved in a characteristic C-shape.
While the life cycle of the different species varies, typically the adult female deposits her eggs in the soil, where they hatch into grubs.  These larvae then feed on the roots of plants, doing damage to crops and to turfgrass.  In the winter, they burrow deeper into the earth to avoid freezing, then tunnel upward again in spring.  Some species may spend more than a single year as larvae, others emerge in the next spring or summer as mature beetles.
In the lawn, a large infestation of grubs can destroy substantial sections of grass by eating away the roots so that the sod can be easily lifted from the soil.  When the grass is rolled away, the grubs can clearly be seen just below the sod.  Often, additional damage is done to the lawn when animals such as skunks and raccoons dig holes to hunt for the yummy and nutritious grubs.  This may reduce the grub population, but it doesn't help the grass!
It is this damage to lawns that has been the greatest motivation for the typical homeowner to make war on the grubs. It was once common to apply insecticides such as diazinon to the lawn, but such poisons kill indiscriminately and are now discouraged;  diazinon is now largely prohibited in the United States for non-agricultural use, but newer insecticides such as imidacloprid are still available.  Less toxic biological controls are becoming more widely used.  Spores of the bacterium Bacillus popilliae, often sold under the name of Milky Spore, can inoculate a lawn with bacterial milky disease, which effects the grubs of Japanese Beetles.  Another method of control is the introduction of several species of parasitic nematodes such as Steinernema carpocapsae applied to the lawn.

The problem with most these methods (Milky Spore is said to be specific to the Japanese Beetle) is their lack of target specificity.  A wide range of insects and other arthropods, many beneficial, live in the soil, particularly in their larval stages. Widespread application of toxic substances can do serious ecological damage.  This is also true of the use of nematodes.  Even if a species of nematodes only attacks grubs of the Scarabaeidae family, there are thousands of beetle species that might be affected, most of them harmless.
Few prevalent species of grubs grow up to be serious problems to gardeners.  The common June beetle, for example, is usually little more than an annoyance as it flies against window screens at night.  But in North America, a foreign invader has now changed the stakes.  Early in the last century, the Japanese Beetle Popillia japonica was accidentally introduced into an environment where it was free from natural enemies.  It has since spread westward, establishing itself in most areas east of the Mississippi.

This insect, while destructive as a grub, becomes an even worse pest as an adult.  The beetles can accumulate in vast numbers to feed on their preferred species of vegetation, and they are capable of serious damage, sometimes defoliating entire plants.  Their method of feeding on leaves is skeletonization, consuming the tissue between the veins.  They will also feed on flowers and fruit.  A few of their preferred targets are roses, grapes, plums and corn, as well as birch and maple trees.
As the Japanese Beetle enters a region, the use of both grub controls and insecticides rises in response as homeowners, gardeners and farmers combat the infestation.  Nontoxic methods have been tried.  Pheromone traps, which attract the beetles into a trap from which they are unable to escape, have had mixed results.  Many people insist that the traps attract more beetles to a location than were originally present.  This has been my own experience.  While I trapped thousands of beetles, the number of them feeding on my plants only seemed to increase.
So far, alas, the beetles seem to be winning, and causing unfortunate collateral damage among related species as humans attempt to control them.

No comments: