Cereal leaf beetle: a new insect pest in Northwest Minnesota

Ian MacRae, UMN Extension Entomologist, Angie Peltier UMN Extension Educator - Field Crops, and Anthony Hanson, UMN Extension Educator - Integrated Pest Management (hans4022@umn.edu)

A new insect pest of wheat has reared its bright orange thorax in Northern Minnesota. The cereal leaf beetle (Oulema melanopus), was found by Extension IPM (Integrated Pest Management) Survey scouts the week of June 5-9 in both Mahnomen and Norman counties (Fig. 1 & 2).  One field had 25% of wheat stems infested and likely would have benefited from an insecticide application if the field had been found earlier. Further scouting this week found feeding damage and larvae of the insect in a number of fields within 15 miles of the original findings as well as additional finds in Red Lake and Clay county Fields examined had varying population levels.  However, none had larval numbers that were above treatment thresholds at this time. Many of the larvae found this week were still immature, so populations likely need to be monitored in these areas to see if they reach economically damaging levels.

Figure 1. Cereal leaf beetle larva. Larvae carry their frass (i.e., feces) on their back to protect against predators. This gives larvae a slimy black and sometimes striped appearance. Photo: Chris DiFonzo, Michigan State University, Bugwood.org.

Figure 2. Cereal leaf beetle larva on wheat. Larvae can have a slug-like appearance, but are actually beetles. Larval feeding causes window-paning or a frosted appearance when there is heavy leaf feeding. Photo: Amelia Landsverk - UMN IPM Scout

 

 

The cereal leaf beetle is native to Europe and was first detected in the U.S in Michigan in the early 1960’s.  Since then, the insect has spread through the eastern states, west to Montana, south to Missouri and east to Virginia.  It has been reported in southern MN, primarily east of the Minnesota River Valley but we believe this is the first reporting of cereal leaf beetle from northern Minnesota (Fig. 3). Economically damaging infestations haven't been widely reported in Minnesota yet, but finding these beetles in northwestern counties so far from previously known finds does raise concern that it may be present in other areas of the state. It is also possible these beetles moved in across the border from North Dakota where they also are documenting more finds of this insect.

Figure 3.  The Minnesota Department of Agriculture determined counties where cereal leaf beetle had been historically found prior to 2010 in the state prior to a statewide survey That year, beetles were only found in Goodhue county, but have historically been found in the southeastern portion of the state. Larvae were found in Clay, Norman, Mahnomen, and Red Lake counties in 2023.

Each summer, UMN Extension hires IPM scouts to survey wheat and soybeans fields in western Minnesota with support from the Minnesota Wheat Research and Promotion Council and the Minnesota Soybean Research and Promotion Council. This is also part of a collaboration with North Dakota State University to map pest presence throughout the growing season, and to find new areas where pests may be showing up. NDSU also has good existing guidance and pictures of the cereal leaf beetle's life cycle.

Adult cereal leaf beetles are about 0.25 inches long, have metallic dark blue wing covers, a bright orange thorax, orange-yellow legs, and black head with well-developed antennae.  They lay bright orange eggs singly or in twos on the leaf surface of grasses and small grains.  These eggs hatch into slug-like larvae with light yellow bodies and 6 small legs on their lower side, directly behind the head.  These larvae have the interesting habit of covering themselves with a black slimy mix of mucus and their own feces.  This discourages predation and probably camouflages its yellow coloring.  When walking through infested fields, this material will rub off on your clothing providing a reliable, but disgusting, way of discovering their presence. 

Cereal leaf beetles have one generation per year; adult beetles overwinter in shelterbelts and wooded areas in leaf litter.  They emerge and mate when temperatures rise above 50F and feed on winter/spring grains or other grass species for about 10 days, usually May through June.  Eggs are laid and hatch in up to 23 days, depending on ambient temperature.  Larvae feed and grow for 12-20 days, going through 4 molts.  When fully grown, larvae will drop and burrow into the ground to pupate.  After 10-21 days, pupation will be complete and the adults emerge from the soil.  The adults will feed for 2-3 weeks on grains, oats, ryegrass and other grassy species before finding overwintering habitat.

Both adults and larvae feed on the upper surface of leaves and consume leaf material down to the lower leaf cuticle, resulting in window panes on the leaf.  Adult feeding damage tends to be in long narrow strips along the leaf.  Larval feeding is more clustered and results in windowpanes that extend across the width of the leaf.  These window panes (lower leaf cuticle) will eventually dry and break off, causing even more damage to the leaf.  

When scouting for cereal leaf beetle, check entire fields; a “W” pattern of scouting through the field works well with this insect.  Early in the season look first for adult feeding, the long slits on the upper leaf surface.  As time progresses, look for eggs and larvae on individual stems and plants (depending on plant size).  Feeding tends to impact younger plants more than older plants.  Prior to boot stage, thresholds are 3 eggs or larvae/plant.  When flag leaves emerge, larvae will tend to concentrate there.  More than 3 larvae per flag leaf can cause significant impact on plants growth and vigor, resulting in decreased yield and grain quality.  After boot stage, the threshold lowers to one larva per flag leaf.

Cereal leaf beetles face significant mortality from natural sources.  Generalist predators, such as lady beetles, consume eggs and young larvae and several parasitic flies and wasps target cereal leaf beetle eggs.  If insecticide is required, check registered insecticides labeled for wheat; many will be effective including synthetic pyrethroids.  There is a possibility of insecticide resistance if the field or area has a long and regular historic use pattern of a specific insecticide mode of action.

Cereal leaf beetle is a new insect pest to northern Minnesota but exists in neighboring states.  While it can cause economic loss, it usually is not an economically important insect in wheat in neighboring states.  That said, the extent of this insect’s distribution in northern MN is not yet known.  It is recommended to scout fields and evaluate the situation in individual operations across the state, especially in northwestern Minnesota, in case its distribution is wider than what scouts have found so far this year. 

If you see this insect in your fields outside of any known counties, contact your local Extension office to help us gauge how widespread its distribution may be.