By James Tansey, PhD, AA, Provincial Insect/Pest Management Specialist, Regina
With the localized heavy pressures seen in some parts of the province last year, there is some concern for the possibility of significant grasshopper issues this year. Conditions last year were very good for grasshopper development, with the warm, dry conditions allowing these them to speed relatively quickly though their development from egg to adult. Grasshoppers, like all insects, are ectothermic, which means that their development and behaviours are dictated in large part by temperature. Early onset of adulthood and egg-laying due to warm weather paired with an extended egg-laying period can contribute significantly to the next year’s populations.
Many parts of the province saw very warm temperatures throughout the growing season last year and a long warm period late in the summer. This, coupled with the fact that many parts of the province also received adequate moisture this spring to spur growth of plants as a food source for young nymphs, suggest the potential for localized problems this year.
Although there are approximately 85 species in the province, only about four of these are regularly pests. Three of the four pest species are spur-throated grasshoppers, which are identified by the presence of a tubercle or knob between their front legs. These include the two-striped grasshopper (Melanoplus bivittatus), the lesser migratory grasshopper (M. sanguinipes) and Packard’s grasshopper (M. packardii). The first two are often very numerous and important pests of several crop species; Packard’s is less common. Bruner’s spur-throated grasshopper can often be mistaken for the lesser migratory grasshopper and has been known to outbreak in Saskatchewan, though at more northern sites. The fourth species that is also an important pest in Saskatchewan is a banded-winged grasshopper species, the clearwing grasshopper (Camnula pellucida), which is primarily an issue in grass crops.
All pest species in Saskatchewan overwinter as eggs. If it has wings in the spring, it’s likely not going to cause problems. The pest species typically begin to hatch in late May and hatching can be extended over several weeks. They then proceed through a series of moults and become larger with each successive moult. The period between moults is called an instar, with the duration of each instar affected by temperature. For all the pest species, the warmer the temperature is, the shorter the time between moults.
There are various natural enemies of grasshoppers that help control populations. Nymphs are preyed upon by a host of predators. These include carabid ground beetles and wolf spiders. The larvae of Epicauta sp. blister beetles and field cricket adults also consume grasshopper eggs. There are also parasitoid wasps and flies that attack eggs and growing grasshoppers, as well as disease-causing fungi that attack juveniles and adults. These diseases are the most prevalent and therefore can have significant effects on pest populations when humidity is high, and populations are dense. Since there are many natural enemies of young grasshoppers, control of first and second instars is less commonly needed. Noticeable damage to crops usually begins with third instar nymphs and becomes more significant as they grow.
Third instar juveniles of each species will typically be seen late June to early July and can be identified by characteristic markings. Two-striped nymphs are a pale-yellow brown with dark stripe along the hind femur, Packard’s are pale yellow to bright green without the femoral stripe. Lesser migratory nymphs at this stage are light brown with a distinct white stripe below the eye, and clearwing nymphs are a mottled grey-brown.
If significant damage starts to occur, there are several products registered for control in most crops, including biological controls. Use the following guidelines: If 50 or more nymphs per square metre are seen in non-crop areas adjacent to crops, or 30 or more per square metre are seen in the crop, consider control. These are very high densities and should be assessed at multiple locations throughout and adjacent to the field. Keep in mind that grasshoppers tend to invade crops from field edges, and this can allow for spot spraying if needed. Again, damage accumulates more quickly with size. Damaging densities of 10 to 12 per square metre of mature nymphs or adults may require control in many crops. Some crops are more sensitive to grasshopper damage. Densities of two per square metre may require control for flax in the boll stage and lentil in the flowering and podding stages.
It can be difficult to estimate numbers per square metre when densities are high. The total grasshoppers caught in four 180 degree sweeps with a fifteen-inch net approximates a per metre count. Be sure not to sweep the same plants twice!
Your choice of control products will be influenced by grasshopper growth stage and temperature. Some synthetic pyrethroids are very effective for young grasshoppers but only if temperatures don’t exceed 25 C. Other products will control mature nymphs and adults. See the Saskatchewan Guide to Crop Protection for specific recommendations.