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Field Notes program talks cereal leaf beetle, armyworms, cutworms and grasshoppers

Angie Peltier, UMN Extension educator, Ian MacRae, UMN Extension entomologist and Bruce Potter, UMN Extension IPM specialist

The following information was provided during a 2023 Strategic Farming: Field Notes session. Read further to learn more about this free program that takes place each Wednesday morning throughout the growing season.

Cereal leaf beetle

A new insect pest of wheat has reared it’s bright orange thorax in Northern Minnesota. The cereal leaf beetle (CLB) (Oulema melanopus) was found by IPM Survey scouts in both Mahnomen and Norman counties. Further scouting found feeding damage and larvae of the insect in a number of fields within 15 miles of the original findings. Fields examined had varying population levels. However, none had larval numbers that were above treatment thresholds. 

The cereal leaf beetles are native to Europe and were 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 but we believe this is the first reporting of cereal leaf beetle from northern Minnesota.


Fig. 1. Adult cereal leaf beetle (Oulema melanopus)
Photo: Hania Berdys,
Adult cereal leaf beetles are about ¼” long, have metallic dark blue wing covers, a bright orange thorax, orange-yellow legs, and black head with well-developed antennae (Figure 1). 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 (Figure 2). 

Fig. 2. Cereal leaf beetle larva. Larvae carry their frass (ie. feces)
on the back to protect against predators. This gives larvae
a slimy black and sometimes striped appearance. Photo:
Chris DiFonzo, Michigan State University.
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. 

Life cycle

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.

Damage and treatment thresholds

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 larvae per flag leaf.

Cereal leaf beetles face significant mortality from natural sources. Generalist predators, such as lady beetles, consume eggs and young larvae and a number of parasitic flies and wasps target the eggs of cereal leaf beetle. 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.

True armyworm

Fig 3. Armyworm feeding on grass hay.
Photo: Simon Warmkagathje
True armyworms have been a worry for University of Minnesota IPM specialists throughout spring 2023 after several early-season rainstorms brought multiple waves of significant densities of adult moths into Minnesota that were subsequently captured in both blacklight and pheromone traps.
The 2023 armyworm infestations have led to crop injury in pastures, grass hay crops (Figure 3) and in corn, particularly corn that had been planted into winter rye fields. We have had a combination of factors coincide in 2023 to favor true armyworms: 1) weather systems that brought moths north from where they overwintered in the southern US, 2) dense grasses that proved irresistible to adults looking to mate and lay eggs, and 3) dry conditions that slowed plant growth and so when larvae ran out of food along field edges they had all the encouragement needed to march en mass to find a better source of food.

Keep scouting!

Growers and crop advisors are urged to keep scouting their crops for armyworms for at least the next few weeks and to treat their hay or pasture crops when the 4-5 larvae per square foot treatment threshold is reached. Treating where the infestation began is also an important strategy so that armyworms don’t simply just ‘march’ over to an adjacent row crop field.

Black cutworm and variegated cutworm in sugarbeet

In an effort to protect their sugarbeet seedlings from sandblasting and soil from eroding, sugarbeet producers plant oats with their sugarbeets. Unfortunately, migrating cutworm moths are attracted to the oats. Once the oats are killed off, cutworms will move over to sugarbeet plants and small sugarbeet seedlings are very susceptible to cutting. 
The North Dakota State University Insect Management Guide states that application of an insecticide labeled for use in sugarbeet is advisable in young beets when larval cutting of seedling stems reaches between 4% and 5%. Control may be justified for late-season infestations of three to five larvae per square foot if they are feeding near or above the soil surface. However, sugarbeet yield and quality is very stand dependent and this year's poor stands in many southern MN fields added difficulty to the decisions for black and variegated cutworms.


Fig 4. Abundant grasshoppers in an alfalfa field
immediately adjacent to an oats field in
southern Minnesota. Photo: Bruce Potter.
After two years of very dry weather and drought in southwest Minnesota, there are very high populations of red-legged grasshoppers (Figure 4) appearing in alfalfa fields. Differential and red legged grasshoppers prefer to lay their eggs in firm, bare ground. This is why alfalfa and no-till soybean fields tend to attract them.

As food quality in areas adjacent to row crop fields declines, grasshoppers will not stay put and starve, but rather move into the row crop field. The conditions under which we could expect grasshopper populations to tumble in spring (cool, wet weather after hatching that either favors entomopathogenic fungi that attack grasshoppers or keeps grasshoppers from moving to feed) have not happened in 2023 and so populations will reach treatment thresholds in some fields in some areas in 2023.

Blister beetles

Blister beetle larvae feed on grasshopper eggs, and so when there are high populations of grasshoppers in alfalfa hay fields, you may end up with a hay quality issue the following year, particularly when it comes to horses. When cutting alfalfa hay, don’t crimp or spray insecticide on the hay, and giving the blister beetles a bit of time to disperse before baling hay can reduce the chance of the irritant being present in the hay. Additionally, letting alfalfa go to flower will cause blister beetles to congregate in the crop.

Do you want to attend Field Notes next week? 

Join us June 28 when we discuss crop management considerations during this abnormally dry season. The program runs from 8:00-8:30 a.m. on Wednesdays through the 2023 growing season. Topics will be announced the week of the program and may include issues related to soil fertility, agronomics, pest management, equipment, and more.

Learn more and register

Can’t make the live session? No problem. The discussion-based series will be posted immediately following the webinar to your favorite podcast-streaming service to listen at your convenience. Listen here online.

Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.

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