Insects to Watch in Cereal Crops in 2020
Several insects got to levels of concern in cereal crops in Manitoba in 2019. The two most widespread and concerning were cutworms and grasshoppers. In some areas, armyworms were also of concern. On a positive note, aphid levels were low in cereals in 2019 and wheat midge was generally not a major concern. Cereal leaf beetle was found in some eastern areas of Manitoba for the first time, but generally is being contained to uneconomical levels by biological controls. This article will focus on some of the insects to watch for in 2020.
Cutworms: Cutworm levels were very high in many field crops in 2019. The two dominant species were redbacked and dingy cutworm. Both of these species overwinter in Manitoba. Redbacked cutworm overwinters as an egg, while dingy cutworm overwinters as a partially grown larva. Given the high levels of last year, and that they can successfully overwinter in Manitoba, be scouting for these cutworms as soon as the crop emerges. Cutworm populations can be quite patchy in a field. If high levels are found, determine if they are patchy or more widespread over a field. Cutworms are nocturnal feeders, and will be hidden during the day. You have to look in the soil or under debris to find them during the day. Insecticide applications are best done late in the day or at night. A new seed treatment called Lumivia has recently been registered for cutworms in cereals.
Redbacked cutworm larvae Dingy cutworm larvae
Grasshoppers: Grasshopper levels have been increasing over the last couple of years. The generally dry summers over the last few years have likely contributed to their increase. Each year, Manitoba Agriculture and Resource Development does a grasshopper survey in August to forecast levels for the next year. The forecast map for 2020 shows more areas at moderate risk than previous years, and some counts that were in the severe risk category. Twostriped grasshopper was the dominant species in many areas, followed by migratory grasshopper. One of the things that can reduce grasshopper levels in heavy rainfall, but timing of this is important. We had a lot of rain in September in Manitoba, and a heavy snowfall in early-October. The rain in September may have reduced egg laying somewhat, but there was likely a lot of eggs laid prior to this rain. The egg stage is very resilient to excess moisture. So don’t expect that the eggs would be killed by these late-summer rains. Heavy rains in early spring would also not be a major mortality factor for grasshopper eggs. The same heavy rains in early or mid-June, as the nymphs are emerging from the eggs could be quite detrimental. On a positive note, I noticed a lot of some grasshopper egg predators last year, particularly bee flies and black blister beetles. Start scouting for grasshoppers in late-may or early-June, and focus this early scouting on areas that would have had lush green vegetation late last year, such as field edges, pastures, late crops, etc.
Twostriped grasshopper adult (right) and nymphs
Armyworms: Armyworms do not overwinter in Manitoba. The adult moths move in from the south, so populations can be very different in successive years. In 2019 there were some higher levels of armyworms in the Eastern, Interlake and Central regions of Manitoba. Natural enemies also seemed to be at work on the armyworm population. In some fields that had armyworms, pupal clusters from a parasitic wasp called Cotesia were very visible at the top of the plants. In some cases, people may mistake these for insect eggs. Cotesia larvae live inside the armyworms, and once mature dozens of Cotesia larvae can emerge within minutes from a parasitized armyworm.
Armyworm Cotesia pupal cluster Cotesia emerging from armyworm
Summary: Start scouting for cutworms as soon as your cereals emerge. May and June is when most damage from cutworms occurs. Starting in late-May or early-June also start scouting for grasshoppers. Also watch for insects that may move in from the south, such as armyworms and aphids. Weather and natural enemies can have a major effect on levels of crop feeding insects, and no two years are alike.
Article and photos provided by John Gavloski