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Scout for Early Season Insects: Cutworms, Cereal Aphids & Flea Beetles (4/26/12)

Since insects are temperature-dependent for their development, it is not surprising that insects are emerging earlier than normal with the warm winter and early spring. Some insects are already starting to appear in our field crops. So, scouting is essential to look for any early season insect infestations. Here are some insect pests to watch out for and scouting tips.

CUTWORMS, probably Army Armyworm cutworm larvacutworms or Pale western cutworms, are being observed in row crops, such as canola, in southwest North Dakota. Cutworms belong to the moth family called Noctuidae. Larvae (worms) are dark colors (brown to gray) with various markings, and up to a pencil in width and 1½ inches in length when mature. Minnesota and Nebraska are reporting large numbers of adult cutworm moths at house lights during evenings. These moths will be moving our way with southerly wind flows. Cutworm larvae cause damage by cutting the seedling off at the ground or below the ground. Larvae can be found by digging in the soil around the base of freshly cut plants. When disturbed, larvae curl up into a ball. As damage continues, fields will have areas of bare soil where the crops have disappeared. Row crops are often more susceptible to cutworm damage than small grains, because cut plants do not grow back (grains compensate by tillering). Rescue foliar treatments are warranted when cutworms exceed the following action thresholds by crop:

ENT Economic Threshold table

Spraying timing is the most important aspect of controlling cutworms. Insecticides are ideally targeted at the young larvae, which are easier to kill than the larger larvae (>1 inch). Use the higher labeled rate of an insecticide for longer residual and apply insecticide at night when cutworms are actively feeding. There are questions about tank mixing insecticides with herbicides for early season weed control. When tank mixing early season herbicide burn-down with an insecticide, you will only get partial control, because cutworms will continue to emerge over a three week period and insecticide residual will not last long enough to kill the late-emerging cutworms. Assuming that the field is at action threshold for cutworms, the best insecticide timing is to apply insecticides right at crop emergence, so you get optimal residual and crop protection against cutworms. There also have been some questions about applying an at-plant insecticide, such as in-furrow or T-band application for control of cutworms. An in-furrow insecticide application is generally not as effective in controlling cutworms as a T-band application. Since cutworms crawl on the soil surface, a 5-7 inch T-band application over the seed furrow would provide better cutworm control.

For insecticides registered in North Dakota for cutworm control, consult the 2012 Field Crop Insect Management Guide at:  http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm

CEREAL APHIDS have already been observed on winter wheat near St. Paul in Minnesota (J. Wiersma, UMN Northwest ROC, Crookston). Incidences of aphids and barley yellow dwarf virus have been high in the southern states. So, it is not surprising that cereal aphids have already been blown up with our southerly wind flows and detected in winter wheat. As winter wheat is still in the susceptible stage (prior to completion of heading) for yield loss and virus transmission from aphids, it is a good idea to monitor any winter wheat for the presence of cereal aphids. Action threshold is 85% stems with more than one aphids present or 12-15 aphids per stem. As spring wheat starts to emerge, we will need to be more aggressive on scouting for cereal aphids earlier. Stay tuned for more updates.

Adult crucifer flea beetleFLEA BEETLES have been observed in volunteer canola and wild mustards since early April. Adults start to emerge in the spring as the temperatures warm up to 58F. Adults feed on the cotyledons and first true leaves of seedlings causing pitting and holes in the leaves. Damage is most serious to seedling plants and can cause seedling death and significant stand loss. Currently, commercially applied insecticide seed treatments in the neonicotinoid insecticide class have provided effective control of flea beetles. Seed treatments will provide at least 3-4 weeks of protection against flea beetles. 

Adult flea beetles will emerge for a 3-4 week period in the spring.  As a result, scouting is critical for any untreated fields,  areas with a history of high populations of flea beetles, or any areas with delayed crop emergence due to cool wet weather (even if an insecticide seed treatment is used). Hot, sunny weather increases feeding activity and movement; while cool, damp weather slows feeding and favors crop growth.

If a rescue foliar insecticide application is necessary, the economic threshold is 25% defoliation on the cotyledons and true leaves. When flea beetle populations are high, more than one application may be required due to the short residual of insecticides, and the threat of re-infestation from surrounding areas.

Janet J. Knodel

Extension Entomologist

janet.knodel@ndsu.edu

 

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