Late season grasshoppers and considerations for control
Grasshoppers have been damaging crops in many parts of Western Canada this year, especially with alarmingly dry weather in some regions. Pest grasshopper species have wide host ranges and can cause significant defoliation in crops when population numbers are high. High populations of grasshoppers will consume any aboveground plant tissues that they come upon during their searches for food. Controlling grasshoppers is important for current populations but, can control influence the next growing season?
There are four major economically important grasshopper species in Western Canada that are damaging to different crop types: Packard’s grasshopper, the clearwinged grasshopper, the migratory grasshopper and the most common pest species, the two-striped grasshopper.
Packard’s grasshopper (Melanoplus packardii) prefers lush habitats with a small amount of grass cover and can be damaging to legumes.
Clearwinged grasshopper (Camula pellucida) has a more selective host range of grasses and feed on cereal grains and other cultivated grasses. Migratory grasshopper (Melanoplus sanguinipes) has a broad host range and can be destructive in cereals, cultivated pastures and hay fields.
Two-striped grasshopper (Melanoplus bivittatus) prefers moist areas of lush vegetation and will feed on broad-leaf crops, cereals, and grasses.
Natural enemies can be effective at regulating grasshopper populations, especially when they occur in high numbers.
Some natural enemies attack the eggs in the soil. These include the larvae of bee flies and blister beetles and adult ground beetles, wasps, and crickets. Crickets feed directly on grasshopper eggs, while the flies and beetle larvae feed on eggs after hatching beside them. Scelio sp. are parasitic wasps that lay their eggs and develop inside grasshopper eggs.
Other natural enemies are predators of grasshopper nymphs and adults: spiders, robber flies and large wasps like the great golden sand digger. Several species of fly deposit eggs or larvae on the grasshopper’s body Larvae feed on the grasshopper from the inside, eventually killing it.
Vertebrates – birds, mice, rats, shrews, gophers, and badgers – will feed on grasshoppers and their eggs. Nosema locustae and Entomophaga grylli are pathogens that attack grasshoppers.
Nosema locustae is a protist (now considered a fungus) that reduces food consumption, restricts movement, and reduces the number of eggs laid. Entomophaga grylli is a fungus that causes ‘summit disease’ in grasshoppers under cool and humid conditions. Infected grasshoppers are found dead, clinging to the stems of plants.
Chemical control is a proven method for controlling grasshoppers. However, its use should be dictated by the economic threshold of grasshoppers in specific crops. Economic thresholds depend on the stage of the grasshoppers, type of crop, crop stage and growing conditions. Typically, the economic threshold for grasshoppers in cereal crops ranges from eight to 12 grasshoppers per square metre in the field and 25+ grasshoppers in roadside ditches.
Counting grasshoppers can be tricky, as they are very mobile, so using square metre counts or sweep nets is the easiest way to assess grasshopper populations. When using sweep nets, count the number of grasshoppers collected in four pendulous sweeps. This represents the number per square metre.
Sweeping is better used for young grasshoppers as adults and large nymphs have excellent vision and easily evade being sampled. Another way is to plant your foot in a square metre, cause a disturbance and count the grasshoppers that jump away. Always be sure to sample multiple points at each site and use the average of these assessments to determine population densities.
There are numerous control products registered for grasshoppers.
Products include sprays and baits. Baits include Eco Bran. This is a bran bait spiked with carbaryl. NOLO bait contains the protozoan Nosema locustae. The effectiveness of some chemical control products decreases as grasshoppers grow. These include Group 3 (synthetic pyrethroid) products. These are excellent for controlling juveniles, but adults are tolerant. Pest grasshoppers are primarily adults late-season, so Coragen, malathion and dimethoate are better choices for control. Information on insecticides for grasshoppers on specific crops can be found in the Guide to Crop Protection.
Migratory grasshopper
Will chemical control late- season influence next year’s population? There are reasons it may not. Adult grasshoppers begin laying eggs in latejuly and August.
Since eggs are laid beneath the soil surface, they are left untouched by insecticide sprays, meaning emerging spring nymph populations can still be high. Chemical control can be used to reduce adult populations, but agronomic techniques may be more effective at limiting next year’s grasshoppers.
Tillage works to eliminate weeds – which grasshoppers feed on – and can expose eggs to harsh environments. Fall and early spring tillage have both been reported as effective.
Fall tillage discourages female grasshoppers from depositing eggs in the fields because it loosens the soil and kills weeds, which are an ample food source. Spring tillage also eliminates weeds, so emerging nymphs have limited food and starve.
Deep plowing involves burying the eggs deeply into the soil, so emerging nymphs have trouble getting to the soil surface. The lower temperature deeper into the soil surface also slows embryonic development, which can delay emergence.
Discing involves disturbing the surface layer of soil, which exposes eggs to predators and parasitoids. It can also cause eggs to dry.
Two-striped grasshopper
Early seeding promotes earliermaturing plants that can withstand more feeding. In areas with heavy grasshopper infestations non-host or less preferred crops like chickpeas, peas and oats should be grown in the following growing season.
Trap strips concentrate grasshoppers into smaller areas, minimizing the amount of insecticide needed. Cultivated strips on the field edge also help concentrate grasshopper nymphs into the trap crop (wheat or spring rye.) This is most effective when grasshoppers have developed to second instar because they are mobile enough to move to the trap and susceptible to a broader range of insecticides.
Grasshopper control is challenging and requires incorporation of a variety of chemical, biological and cultural techniques to be effective. However, using an integrated approach, grasshopper populations can be reduced for current and next growing seasons.