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Field Notes discusses alfalfa and early season insects

Angie Peltier, UMN Extension educator, Bruce Potter, Extension IPM specialist & Anthony Hanson, Extension educator

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.

Black cutworm

The same weather systems that carry moist air from the south to Minnesota bring along the lepidopteran pests that cannot survive the Minnesota winter, but spend their winters in southern states. At dusk, black cutworm or true armyworm will begin to fly and are carried north in low-level systems approximately 1,500 feet up, dropping out of clouds toward the tail-end of storms after having ridden north over a couple of days time. Monitoring for these pests is important because weather systems are unlikely to follow the same route to Minnesota each spring. High populations of these pests tend to coincide in an area when there have been heavy spring rains.

Fig. 1. Black cutworm moth captured in a pheromone
trap. Photo: Angie Peltier.
The earliest significant black cutworm flights in traps that were part of UMN Extension’s pheromone trapping network were detected in April in Martin and Swift Counties (Figure 1). More recently significant flights were detected after storms that came in May 11-15 in central Minnesota with some of the sugarbeet production areas hit pretty hard. 

Black cutworms have a relatively wide host range, affecting many of Minnesota’s economically important crops. At particular risk are those crops that have the combination of small cutworm larvae and small corn plants as there is a long window during which cutworms can feed and cut plants and a long period of time that crops are susceptible to cutting injury. Fields with soybean residue, with low-lying areas or with early-season weed growth are at particular risk as these are the favorite egg-laying haunts of the black cutworm.

Scouting for black cutworm 

In 2023, folks are going to have a more difficult time scouting for black cutworms as those same weather systems that brought cutworm moths north, dropped a lot of rain, causing stand issues unrelated to cutworm. Leaf feeding is typically the first sign of feeding injury caused by early stage cutworm larvae. If you observe leaf feeding in a field, dig down in the early morning or evening (cutworms are nocturnal), paying particular attention to the transition between dry and wet soil in the profile.

BCW management

Those that plant corn hybrids with above-ground Bt traits should have a pretty good level of black cutworm control. However, if very large larvae move over onto Bt corn, they are capable of cutting several plants before succumbing to death caused by Bt proteins. In 2023, cutworms hatching from moths that arrived during significant flights that took place earlier are expected to be large enough to begin cutting at the end of May or early June, while significant flights that occurred later may begin cutting toward mid to late-June.

Additional information about what black cutworm moths look like, scouting for larval feeding injury and management options can be found here and about the black cutworm trapping network catches can be found here.

True armyworm

Bruce Potter has monitored both pheromone and black light traps since the late 1970’s, catching the most armyworms he has ever caught over a single night – 193 moths. Of note, is that there was a pheromone trap with a true armyworm (TAW) lure located no more than 100 yards from the location of the black light trap that only caught 7. Caution is urged when relying solely upon pheromone traps to determine crop risk or time scouting.

Preferred TAW egg-laying habitat

Fig. 2. True armyworms caught in a pheromone trap. Photo: Angie Peltier
Armyworms (Figure 2) prefer dense vegetation in which to lay their eggs and tend to prefer grass crops including corn and small grains either grown for grain or as a cover crop.

Particularly when planting into a rye cover crop, it is recommended that one both use a sweep net and look at the ground for armyworm larvae as later stage larvae can decimate a new crop of corn seedlings (or sometimes soybean seedlings) overnight. Come back after termination to scout for true armyworm again to avoid surprises and treat promptly if risk is high.

TAW management

There is only one above-ground Bt trait labeled for true armyworm, the Viptera trait. Unfortunately, there is a limited number of hybrids in maturities adapted to Minnesota, although the number is increasing over time.

Additional information about true armyworms, how to scout for and manage them can be found here.

Alfalfa weevil

Each year, those that produce alfalfa battle ponded water, frost heaving and other weather-related perils over which they have little control. Alfalfa weevil has also become a perennial threat to alfalfa crops throughout much of the state. These pests feed on alfalfa leaves, leaving leaves in tatters with only veins remaining. As one can imagine, this can significantly impact alfalfa yield. With few effective insecticide classes available to manage alfalfa weevil, scouting and following other integrated pest management (IPM) tenants such as only treating with insecticides when warranted, will help to preserve the efficacy of the few products left in our ‘toolbox’.

There are two biotypes of alfalfa weevil with biotypes moving into some areas that may have historically had a different biotype that farmers are used to, leading to a bit of confusion regarding this pest that is not a new one in Minnesota. The eastern strain and western strain of alfalfa weevil differ in when they lay eggs. Scouting is important so one can determine when weevils are present in relation to when first and second cutting is to take place to make the best management decisions for them and their farm in a given growing season.

Alfalfa weevil scouting in 2023

Mid-May to June is the typical time-frame during which scouting for alfalfa weevil should commence. It is at this time that adult weevils are moving into fields from leaf litter where they survive the winter. These adults mate and lay their eggs in alfalfa, but it is their larvae that cause damage.


Fig. 3. Alfalfa weevil larva. Photo: Clemson
University - USDA Cooperative Extension Slide
Larval appearance tends to change over time with development, appearing yellow to olive in color in the first larval stage, transitioning to a bright green color with a distinct white strip running down the center of their backs (Figure 3) as they continue to molt and grow. While larval stages of other insect pests are green, alfalfa weevil larvae have black head capsules. When fully grown, larvae are approximately 3/8 of an inch long. Once larvae develop into pupae, the threat to the crop has passed for the year as weevils will no longer feed on the crop.

How to scout

To get an idea about the population density of weevils in a field, farmers are urged to visit a minimum of five sites in a field away from the field edge prior to first cutting with tools including a very sharp pair of shears, a white 5 gallon bucket, a hand lens and your phone. At each location, cut 30 or more stems, invert them into the white bucket and roughly beat the stems against the side of the bucket to dislodge any larvae. Double check that there aren’t first stage larvae (which are difficult to dislodge with beating) that are hanging out in curled leaf tips.

At each location, use your phone to record the number of stems cut, the number of weevil larvae and the height of the alfalfa. When you have visited at least 5 locations repeating this process at each location, calculate the average number of larvae per stem by dividing the total number of larvae by the total number of stems examined. Also calculate the field’s average plant height.

Alfalfa weevil management

One can then use the number of alfalfa weevils per stem, the average stem height and estimated treatment cost to determine whether it is best to either cut the crop early or use an insecticide that has a short, medium or long pre-harvest interval. If one is using early cutting to manage alfalfa weevil, be sure to not leave the crop sitting in windrows as prompt baling will expose those weevils that are using them for shelter, leading to less completing their life cycle to cause problems in future growing seasons.

For crops that are further along in growth and development when scouted (ex. 50% bud stage or further along), an early cutting might just do the trick. Active ingredients with a shorter pre-harvest interval (PHI) are required for crops that are in the early bud stage (greater than 20 inch tall), a.i.’s with short-to medium PHI are required for crops in the late vegetative growth stage and for crops in the mid-vegetative growth stage (10-15 inch tall) an a.i. with a long-residual is recommended. If you do end up needing to treat the crop for alfalfa weevils, rotate between insecticide groups to slow the speed of resistance developing in the population to any one group.

Alfalfa weevils undergo only one generation per year, meaning that if population densities of larvae are low enough that one can forego an insecticide application until the pest pupates, the danger to the crop has passed.

Pea aphids 

Pea aphids have also been observed in alfalfa in 2023. This insect tends to rarely cause economic injury except when weather has been dry and alfalfa is water-stressed. If one sprays for alfalfa weevil, scout to both determine whether the insecticide was effective for managing alfalfa weevil and for subsequent flares of pea aphids or potato leafhoppers.

Do you want to attend Field Notes next week? 

This program will run from 8:00-8:30 a.m. on Wednesdays through the 2023 growing season. Weed management  will be the topic on May 31 with Debalin Sarangi, UMN Extension weed specialist, and special guest Joe Ikley, NDSU Extension weed specialist. They will cover a wide range of topics including early season weed control, cover crop termination and cover crop interception of preemergence herbicides. 

Other topics during the growing season 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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