Parasitized grasshoppers die in a characteristic "death embrace" - Photo: Marlin E. Rice, Iowa State University
Effect of climate and weather
Normal levels of rainfall are important to control grasshopper populations. If soil moisture content is high, many of the eggs will not hatch. Dry springs favor grasshopper populations, which is why grasshopper outbreaks generally occur during dry years.
Grasshoppers have many natural enemies
Natural enemies are the reason why we generally see only localized outbreaks of grasshoppers. Many natural enemies are specialists on grasshopper-type insects, while others are generalists, using grasshoppers as one of many hosts.
Many beneficial insects feed on grasshoppers. These include the larvae of blister beetles and ground beetles (which feed on the eggs), bee flies (parasites of eggs), robber flies, Scelionid wasps, flesh flies (Sarcophaga) and tangle-veined flies. The parasitic flies deposit eggs on the nymph or adult, and the emerging larvae eat their way into the body of the grasshopper. Generally, parasitized grasshoppers die earlier and do not reproduce.
A fungus, Entomophthora grylli, often causes locally high mortality in grasshopper populations. Once infected, grasshoppers can be seen in a characteristic pose at the top of a plant, in which the grasshopper grasps the plant in a "death embrace" with front and middle legs while the hind legs are extended. It dies in this position. Fungal spores develop in and on the body of the infected grasshopper. These spores become airborne and infect other grasshoppers. Under warm, humid conditions, great numbers of grasshoppers are destroyed by this fungus.
The red locust mite, Trombidium locustarum, is an important natural enemy that feeds on the eggs, nymphs, and adults. The mite uses its mouthparts to suck up the fluid from its host. A mite-infested grasshopper moves slowly, eats little, and dies early.
Several types of parasitic nematodes, such as the horsehair worm, are notable natural enemies of grasshoppers. Infested grasshoppers rarely produce young, and die early of dessication.
Finally, predation of nymphs and adults by larger animals such as toads, snakes, birds, skunks, shrews and moles have an impact on grasshopper populations during the summer months.
Grasshopper populations can be reduced by eliminating potential egg-laying sites. Since grasshoppers tend to select undisturbed areas for egg-laying, tilling these sites in mid- to late summer discourages females from laying eggs in these areas.
Eliminate tall grass and weeds around crops, trees and gardens you want to protect. This reduces food sources so grasshoppers are not attracted to these areas, exposes grasshoppers to greater predation from birds and mammals, and also makes these areas less attractive for egg-laying.
Similarly, summer weed control in fallow fields eliminates food sources so there will be nothing for small nymphs to feed on when eggs hatch, and fields will not be attractive to egg-laying adults.
Chemical control and biological control products
Adult grasshoppers are difficult to control with insecticides due to their size, mobility, and decreased susceptibility to the insecticides. The best time to control grasshoppers with insecticides is during the 3rd and 4th instars when they are 1/2 to 3/4 inch long. These stages will occur in early to mid-summer, depending on the species and area of the region. At this time most eggs will have hatched and the nymphs will be more susceptible to insecticides. Also, they will still be concentrated in their hatching areas where they can be controlled more effectively than when dispersed later in the summer.
The biological control agent, Nosema locustae, is a naturally-occurring microsporidian protozoan that is now being placed on various baits and marketed for grasshopper control under such names as NOLO Bait, Grasshopper Attack, Hopper Stopper and others. Grasshoppers must eat the Nosema-treated bait as second or third instar hoppers. This requires both early season scouting and treatment of grasshopper populations in border areas of the field. Flaky wheat bran treated with Nosema locustae is particularly recommended when control is needed near water or near threatened and/or endangered wildlife.