Scouting for potential insect problems early in the year is important, because plants are small and more vulnerable. But although field scouting for insects and other invertebrates is necessary because of the problems they entail in crop production, it can be difficult.
Scott Hartley is Saskatchewan Agriculture and Food’s Provincial Insect and Vertebrate Pest Specialist.
“In the spring, insects are smaller and often below the soil surface, where methods for determining population levels tend to be more labour intensive,” Hartley says. “First on my list would be the wireworm. These insects can be persistent pests in Saskatchewan crops, due to the considerable time required for wireworm larva or click beetles — the adults — to complete their life cycle.”
The young larvae are white, changing to a yellowish and tan or brown colour with age. The larval bodies are more “leathery,” compared to the thinner skin of moth or butterfly larvae like cutworms.
“Wireworms prefer cooler and moist soils and temperatures. They move up and down in the soil in response to temperature. They’re more destructive in the spring, when they are feeding near the soil surface and plants have less vegetative growth. They tend to be more abundant in lower, damper areas of a field. Damage usually consists of shredded plant tissue below the soil surface. Initially, this may show up as mortality to the main central leaves, but eventually it can kill the entire plant.”
Unfortunately, the most accurate method to determine wireworm populations is to sieve soil samples, a time consuming process. The presence of wilted plants is often the first indication of a problem. Damage is often not noted early enough to reseed and can be misdiagnosed. The only viable control option is the use of a seed treatment the following year.
Another problem pest is the cutworm. Reports of severe infestations have increased in the last few years, according to Hartley.
“Large patches — some in excess of 20 acres — have been reported in a number of areas of the province. Weedy areas are more prone to problems, as the adult moths look for green growth to lay their eggs. Hilltops and drier areas of the field also tend to have more cutworm problems.”
Damage is typically in the form of a cut stem, explains Hartley:
“When an affected plant is pulled, the root is seldom attached, and occasionally, damage can be seen on leaves that were chewed prior to emergence from the soil. Early in the growing season, the larvae are tiny and difficult to see. As they grow, damage will generally increase, since they are capable of consuming more vegetative tissue.”
In spring cereals, three to four cutworms per square metre is considered to be the economic threshold.
Hartley recommends applying an insecticide to the soil surface in the evening. When cutworms come to the surface to feed at that time, they come in contact with the chemical. It can take up to 10 days for best results, as not all larvae come to the surface on any given night. Some species of cutworms, such as the glassy cutworm, do not surface and are less affected by insecticides.
Hartley also has some news about flea beetles:
“Overall, flea beetle populations were lower last fall compared to previous years. This is the same generation that will be affecting canola and mustard this spring. If higher rates of seed treatments are used at seeding time, flea beetles are less likely to be a problem. However, if there is high flea beetle pressure and a low rate or no insecticide is used at seeding, monitoring is critical in case a foliar insecticide is required.”
The action threshold for initiating insecticide application is when 25 per cent of the leaf tissue is consumed.
“Cool temperatures result in less flea beetle activity and slower plant growth. Therefore, the seedlings remain vulnerable for a longer period. In addition, the beetles remain closer to the ground surface and in cracks in the soil. In 2004, it was noted that this habit resulted in girdling of the stem and feeding on the underside of cotyledons. When environmental conditions are more favourable, the upper surface of the leaf tends to be most affected and damage is more easily noted.”
When it comes to diamondback moths, Hartley notes that adult moths were observed and collected in late April in a broad region of the Prairies. The cool weather may have helped to reduce some of the adult moths, but it would still be prudent to monitor for larvae. Early season feeding on the growing point of canola or mustard will result in stunted growth or even plant death.
Adult moths continue to be found in significant numbers in some areas, suggesting that there is the potential for egg-laying and serious infestations in the upcoming weeks if environmental conditions are more favourable.
To find out more about the use of registered pesticides for various crops, please consult the 2005 Guide to Crop Protection available online at: http://www.agr.gov.sk.ca/docs/crops/cropguide00.asp or call the Agriculture Knowledge Centre at 1-866-457-2377.
For more information, contact:
Scott Hartley
Provincial Insect and Vertebrate Pest Specialist
Saskatchewan Agriculture and Food
(306) 787-4669
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