Grasshoppers are voracious eaters, but they are less likely to devour tomato or squash plants than more tender plants, such as beans, carrots, lettuce and onions. If food is scarce or there is a large population of grasshoppers, the damage to plants, even tomatoes, can be severe. Adult grasshoppers are difficult to eradicate, so methods of control should be applied when the nymphs -- the immature grasshoppers -- are less than ½ inch long.
If the grasshopper population is not large, removing them by hand and killing them is the easiest and fastest control method. To kill grasshoppers, step on them, smash them with a hard object, drop them into a bucket of soapy water or snip them in half with garden pruners. Poultry, such as guinea hens and chickens, eat some type of grasshoppers, but may damage the plants by pecking the foliage.
The most common biological control of grasshoppers is Nosema locustae, a fungal spore. Mix it with bait -- food that grasshoppers like -- and spread it in areas where they are located. When the grasshoppers eat the mixture, the spores infect them and they die. Nosema locustae is slow acting and not completely effective, with a kill rate between 30 and 40 percent of the treated grasshoppers.
Pyrethrins are botanical insecticides made from chrysanthemum flowers that are native to Africa and Australia. They affect the grasshopper’s nervous system, causing paralysis and death. Pyrethroids are natural pesticides adapted from pyrethrins and are more stable when exposed to sunlight. Both are fast acting and work best on small nymphs. Pyrethrins are not very effective, but pyrethroids are very effective.
Several chemical pesticides are approved for use on tomato plants to control grasshoppers but it is best to not use these unless the infestation is large and causes severe damage to the plants. Although you spray the pesticide directly onto tomato plants, the grasshoppers will continue to chew on the foliage and fruit, because they have to ingest the poison for it to work. Most insecticides last only 24 hours because they begin to break down when diluted with water. Direct sunlight and high temperatures also increase the speed of the breakdown process.
Use biological controls, natural insecticides and chemical insecticides according to the manufacturer's directions. Only use products labeled for use on grasshoppers on tomato plants.