Gardening Articles: Care :: Pests & Problems
Managing Grasshoppers (page 2 of 2)
by Whitney Cranshaw
Grasshopper populations can fluctuate greatly and unpredictably from season to season. Outbreaks typically follow cycles of 11 to 20 years, during which time the numbers of grasshoppers may build a hundredfold over a few years, then "crash." The reasons for periodic grasshopper outbreaks are little understood, but they seem to be the result of an interplay between natural predators, parasites, diseases and weather factors. For example, spring and summer rains affect the abundance and nutritional quality of spring grasses and other food plants important to the survival of young grasshoppers. In severe outbreaks there is little you can do to save plants except use screening to exclude the hoppers.
Several native insects eat grasshoppers. On prairies, many fall victim to predaceous robber flies. Praying mantids will snag a grasshopper or two during the summer. Most blister beetles (an occasional pest problem themselves) develop by fe on the egg pods of grasshoppers. Birds such as horned larks and kestrels rely on grasshoppers as a major part of their diets.
Grasshoppers also succumb to fungal diseases, which cause them to crawl to the tops of plants where they cling and stick as they die, producing an eery sight when an epidemic of the disease has occurred.
Perhaps the strangest natural enemy of grasshoppers is the large nematode, Mermis nigrescens, which reaches a length of more than four inches. The adults lay eggs on grasses and other plants on which grasshoppers feed. If these eggs are eaten, the nematodes develop within the grasshopper, sterilizing and prematurely killing it.
All grasshopper control by gardeners must be preventive, and it needs to happen at some distance from the area you want to protect. Never spray for grasshopppers within your vegetable garden, either with botanicals or synthetics. It won't work. Even with the most potent chemicals, grasshoppers cannot be controlled once migrations into yards accelerate and they are jumping over the fence. Stop them before they get to the garden.
The most effective way to reduce grasshopper numbers is to kill them on their breeding ground with either the biological control Nosema locustae. Fortunately, these breeding areas are often fairly limited. Grasshoppers seek out untilled, dry, weedy ground. (Tilling during fall or early spring exposes and destroys egg pods of grasshoppers.) Empty lots or roadsides are common breeding sites.
If the breeding sites provide favorable succulent plants through the summer, the migrations may be retarded or never occur. However, if these plants dry down--or if they are mowed--invasions to nearby gardens will rapidly increase. Lawns and gardens that are surrounded by pastures and prairies become a magnet for late-season grasshopper invasions when plants in the grassland dry up.
Keeping a ring of irrigated, green plants around the yard can defer the attack. A half-wild windbreak th mixes hardy trees, shrubs, perennials and grasses is ideal. These areas may be more safely treated with insecticides, providing immediate control yet excluding the poisons from the garden.
Using the protozoan disease Nosema locustae to consistently prevent or further lessen grasshopper problems can be more difficult. This organism occurs naturally in grasshoppers but also can be mass-produced at reasonable cost and applied (in baits) to areas where grasshoppers are developing.
Several different formulations of this disease are sold under such trade names as Grasshopper Spore, Semaspore and Nolo Bait. Nosema locustae works best when applied over large areas where grasshoppers are breeding. But in general, it is not highly effective. It's slow-acting and many grasshoppers are not very susceptible to it--it often kills only 50% of the population over a four-week period.
The benefits of poultry for grasshopper control are well known in the High Plains. Grasshoppers are a highly nutritious food, and chickens relish hunting for them. Turkeys and guinea hens will also consume them voraciously. By fencing off the garden area to prevent them from scratching, you can create a zone where the poultry patrol can range freely and intercept most hoppers.
When the grasshoppers get out of hand, or if they are serious plagues more years than not, you may want to screen individual plants or even an entire garden area. Floating row covers and window screening can work for a time. Metal screens are superior because grasshoppers possess strong mouthparts and can gnaw through most fabrics.
Although grasshoppers will eat almost anything, some garden plants are less preferred. Tomatoes and many squash family plants will almost always be around even after the zinnias, iris and lettuce are long gone.
Whitney Cranshaw is an associate professor and extension entomologist at Colorado State University.