News update April 6, 2004, regarding the effects of rain and snow

Some commentators and news items have noted that recent rain and snow could reduce the grasshopper threat, or that recent lack of moisture could accentuate the threat. That dry conditions in the early spring will worsen the coming grasshopper problem, and that rain in March and April will suppress grasshoppers, are persistent beliefs that are not quite right, because of some quirks of grasshopper biology. I held this incorrect view myself in 1983, but found by experimentation that flooding does not harm eggs, and that grasshoppers are resistant to harm from moisture in most stages except the very young immatures. You can also do tests yourself to prove that conditions can be even too dry for grasshoppers!

Key points:
The pest grasshopper species overwinter as embryos in eggs that are somewhat breathable but nearly waterproof. For most of the winter and spring they are dormant and resistant to moisture, but at one point the embryos actually need water to complete a stage of development. In some years, it seems clear even before winter that sufficient water is available in the soil, so that it is unlikely that the spring soil will be so dry that grasshoppers would be retarded in their growth. In other years, spring moisture is needed for grasshopper development. The negative effect of moisture on grasshoppers comes later, when the grasshoppers are hatching around June 1.

Based on what I have seen during annual sampling and numerous lab experiments during 1983-2003 on the Canadian Prairies, the following rules apply to the eggs of the current common pest grasshopper species in northern grassland:

- extended warm, dry conditions in September prepare grasshopper eggs for early hatching the following year, as long as some moisture is available in fall and spring

- precipitation as snow in October to March benefits grasshoppers because it provides extra insulation from cold (although grasshopper eggs are rarely killed by cold)

- precipitation as rain in March to May benefits the embryonic growth of grasshoppers

- precipitation as snow in March to May does not hurt grasshopper eggs, but is accompanied by cold temperatures that halt embryonic growth of grasshoppers until the soil warms (significant growth occurs above about 14 C)

- temporary flooding (for example, under water for a week) does not 'drown' eggs. I've hatched thousands of eggs of several species, after simulated flooding and then drying. Not all species are this resistant in the dormant stage, but the pest species are.

- rain in June can greatly reduce the survival of young grasshoppers. They are killed directly, attacked by disease, and slowed in their growth and development. This is when rain will help reduce the problem. If the rain comes before hatching, it has no harmful effect on grasshoppers. Cool spring conditions in Canada result in grasshoppers hatching around June 5 to 15. Warm springs result in hatching between May 25 and June 5. (These dates are for western Canada; in Texas they hatch in March.)

- precipitation during July has little negative impact on grasshopper except to reduce feeding during the period in which they are cool or slightly bloated. In some years, moisture has increased the incidence of fungal diseases. Incidence of fungal disease has been high in some years and regions, but was low in 2000-2003. In 2002-03, low numbers of the two-striped grasshopper were infected, especially in Saskatchewan, and cadavers could be found late in the season.

- precipitation during August and early September will interfere with the ability of grasshoppers to find mates and produce egg pods. Moist soil can also encourage fungi that attack grasshoppers.

The technical models and details depend on species, weather (such as soil temperature and sun hours) and vegetation inputs, but the generalizations above normally hold true.

- Dan Johnson