The Effect Of Wet Or Flooded Soils On Insects That Feed On Crops

Very little research has been conducted on the effects of flooding of agricultural lands on insect species and the insect communities that live there. In some instances field flooding has purposely been used successfully to control soil insect pest larvae such as the cranberry girdler and sugarcane grubs. However, few studies have looked at the effects on flooding on insects of concern to crops in Manitoba. A few studies have looked at the effects of submersing grasshoppers and grasshopper eggs in water. And there has been some research on the effects of flooding on wireworms. In addition, later seeding as a possible consequence of flooding may make some crops more susceptible to certain insects.

Grasshoppers

Although the impact of flooded conditions on grasshoppers is not totally known, the eggs of some species can survive immersed in water for at least several days. One study from Montana concluded “even if grasshopper egg pods were submerged for weeks, some would still hatch”. Experiments in Lethbridge, Alberta concluded that “complete immersion in water for days did not kill the eggs or slow subsequent growth”. Other studies indicate that “spring floods can cause high egg mortality if they occur after diapause has been broken”. So a flooded area does not necessarily mean no grasshoppers will hatch from the area, especially early-season flooding.

Once grasshoppers hatch, how vulnerable they are to moisture depends on the stage of the grasshoppers when the moisture arrives, and the length and intensity of the event. Immediately after grasshoppers hatch they have few fat reserves and are vulnerable to cool, wet weather. If they are unable to feed readily during these early stages, high mortality will result.

Studies in Nebraska on 7 species of grasshoppers looked at how long grasshoppers needed to be immersed under water before they would be killed. The LT50 (time it takes for 50% of the population to be killed) ranged from 7.5 to 21 hours for adults, while nymphs had an LT50 of between 3 and 13 hours. These survival times indicate that mortality associated with immersion would rarely be caused through seasonal rainfall.

Grasshoppers thrive under hot, dry conditions and are very vulnerable to disease under prolonged periods of cool, wet conditions. If the cool wet weather persists for a couple of weeks the grasshopper population could be reduced significantly, particularly by a fungus that infects the them under these humid conditions. If periods of sunny and drier weather allow the grasshoppers to dry off then they will be less susceptible to disease. Also, cool weather means grasshoppers are less active and more susceptible to predators and parasites.

Wireworms

Studies in the United States have found that wireworms can be controlled by flooding, but the results are not always consistent and may depend on other environmental factors. A study in British Columbia looked at the effect of temperature and soil on the control of wireworms by flooding. They found that wireworms submerged at higher temperatures died quicker than those submerged at lower temperatures, and soil salinity may affect the effectiveness of flooding as a control strategy. Flooding at 5ºC required about 2 month (55 to 62 days depending on soil type) to kill 90% of the wireworms. In contrast, flooding between 10ºC and 20ºC required between 9 and 26 days to kill 90% of the wireworms, depending on soil type. So flooding in the spring, when temperatures are cooler, is less likely to reduce wireworm populations than flooding under warmer temperatures.

Wheat Midge

Only between heading and flowering is wheat susceptible to damage by wheat midge. Early seeding often gets the wheat past the stage where it is susceptible to midge prior to midge emergence. Later seeding can mean that more wheat will be in the susceptible stage while the midge is emerging. Peak emergence of wheat midge can be expected between about the first and second week in July, with emergence continuing until late July. Thus late seeded fields of wheat should be scouted carefully for wheat midge.

Flea Beetles In Canola

Later-seeded canola as a result of excess moisture, may mean the crop is emerging when flea beetle populations are at or near their peak. Canola is very sensitive to flea beetle feeding until the plants get to the 3 to 4 leaf stages. Flea beetle populations prior to mid-May are often low enough that early seeded canola that is growing vigorously is little affected by flea beetles. Plants that are in the cotyledon and first true leaf stages in early June may be very susceptible to flea beetle pressures, particularly if we get a warm, dry spell just as the crop is emerging. Decisions on flea beetle management are based on perceived risks. High populations last year, combined with late seeding, or canola not growing vigorously enough to outgrow flea beetle feeding may make purchasing canola seed treated with an insecticide (Helix, Prosper or Gaucho) an economical investment. Even when an insecticide has been applied to the seed, monitor fields on a regular basis until plants reach the 3 to 4 leaf stage, particularly on hot, dry days with little wind. Slow growing canola may not reach the more resistant stages before the seed treatment loses effectiveness. Foliar sprays can be used to control these flea beetle populations. At times spraying only the edge of the field is needed to provide good control, since the beetles will be moving in from shrubby areas outside the field.

For Further Information, See:

Effect of temperature and soil on the control of a wireworm, Agriotes obscurus L. (Coleoptera: Elateridae) by flooding. Crop Protection. Volume 25. 2006. pp. 1057-1061.

Immersion tolerance in rangeland grasshoppers (Orthoptera: Acrididae). Journal of Orthoptera Research. Vol. 16. 2007. pp. 135-138.