SW MN IPM Stuff 2016 Issue 8

Volume 16, Issue 8 - June 10, 2016

This newsletter is also available in a print-friendly pdf format: SW MN IPM Stuff 2016-08

This newsletter and the advice herin are free. You usually get what you pay for.

Crop Weather

Rainfall, air and soil temperatures, degree-days, soil moistures, and other current and historical weather data for the University Southwest Research and Outreach Center (SWROC), a little spot about two miles west of Lamberton, MN can be found at the SWROC weather webpage.

We have received 544 degree-days (base 50°F) and 6.26 inches of rainfall May 1 - June 9. There were 105 degree-days (base 50°F) accumulated from April 11-30.

The largest corn on the SWROC is averaging V7. The largest soybeans planted April 13 are at V6. Spring wheat and oats are boot - early heading stage.

Soybean early maturity

Corey Sinn and Stephan Nelson reported damping off of seedling soybeans in several fields, including treated seed treatment fungicides. Based on the photos, I suspect Pythium and Rhizoctonia or Fusarium. The University of Minnesota Plant Disease Clinic can help identify crop diseases. With the wet weather in parts of SW MN, I expect more problems will show up with the heat. Seed treatments do not last season long and are not effective against all species of fungi.

Rootworm egg hatch

For over a week, folks have been reporting firefly sightings. Last night, I was able to stay awake long enough to see some for myself, so it must be true that the corn rootworm egg hatch has started. For the more technology dependent agriculturalists, degree-day models are also predicting rootworm egg hatch should have started.

Soybean aphid watch 2016

I am starting to see small soybean aphid populations in more areas. You might want to start looking at older soybeans to find a few fields to track population growth. Look for ants and lady beetles to help find early season aphids. I am seeing good predator numbers. It is too early to treat. Leave the insecticide out of your soybean herbicide if you want to reduce costs and minimize problems. 

True armyworm

Bruce Potter, Revised 6/10/2016

The mere mention of armyworms can cause angst in those who have experienced outbreaks and the news of armyworms in the area causes can trigger unnecessary insecticide applications. Fortunately, other than taking some time, scouting for armyworms is fairly straightforward and they are easily controlled with insecticides.

armyworm larvae with distinct bands

Armyworm larvae. Note the banding on the abdominal prolegs.


The true armyworm (Mythimna unipuncta, formerly known as Pseudaletia unipuncta, Haworth) are relatives of cutworms and are in the Noctuidae moth family.

These are not the tent that feed on broadleaf trees and shrubs that are called armyworms by some.

Armyworms are native to eastern North America but they cannot overwinter in MN. Each spring, they migrate north like their black cutworm cousins. Armyworm tend to be more abundant in cool, wet years. Heat and dry weather are hard on armyworm eggs and small larvae. Because migrant moths drop out of low level jet streams with thunderstorms, armyworm infestations are sometime found in areas that have received hail.

Armyworm moths target specific environments in which to lay eggs. Areas of dense grasses are favored egg laying sites. Field edges near lodged grassy areas, lodged small grains and corn that had earlier areas of heavy grass weed problems should be checked. Armyworm infestations have also been associated with corn planted into rye cover crops.

Armyworm larvae have their share of problems. They are often heavily parasitized by flies and wasps and they can be infected by fungal and virus diseases. Eggs of fly parasites can sometimes be seen behind the heads of larvae and cocoons parasites cover some infested larvae.

The true armyworm prefers to feed on grasses. In previous infestations, I have seen armyworms clean out the weedy grasses in a sunflower or bean field and ignore the broadleaf crop. However, they have occasionally been reported as a pest on some broadleaves. This may be a result of larvae migrating when depleting their food. Hungry larvae will move a fair distance to find a new food source. The “armies” can easily cross a road and feed well into a field on the other side in a single night.

Armyworms have multiple but distinct, generations in Minnesota. There are six larval instars (stages), and most of the vegetation is consumed during the last week of larval life. Larvae are approximately 1½ inches long when mature. When these larvae begin to move underground to pupate, the year’s risk is over.


The larvae can range from tan and olive to nearly black in color. The pattern of a dark band flanked by white bordered pink to orange bands on along the side is a distinguishing character; as is the net-like pattern on the head and a dark band at the base of the abdominal prolegs.


true armyworm

Early instar true armyworm. Note net-like reticulations on head and the pattern of bands on the body. This moth larva has five pairs of prolegs. Color can vary from tan-olive to nearly black.

Scouting and management

The presence of a large migration flight of true armyworm into Minnesota can be detected with black light traps. The capture of moths can predict when a problem is likely and when it will occur but because immigrant moths can re-migrate, not where the problem will occur. Pheromone traps for true armyworm are available but what the captures mean in relation to crop damage is not clear.

Chewing damage on crop leaves and the presence of frass (insect fecal pellets) on plants and on the ground indicate that an insect was present. The presence of live larvae indicates the potential for future damage exists.

Armyworm larvae are most active at night and cloudy days. During the heat and bright sunlight, larvae often hide under leaf litter on the ground. Scouting and insecticide applications are often more effective near dawn and dusk and on cloudy days. When disturbed, armyworms drop to the ground and curl into a C-shape to “play possum”. 

Preliminary scouting for armyworms in small grains, field edges and even grassy areas within row crops can be done with a sweep net. Once armyworm are found, switch to a crop specific scouting method.

corn damaged by armyworm

2013 Armyworm damage to corn. Photo: Tyson Kaldenberg

Wheat, barley, oats

Pay close attention to areas that are lodged, near lodged grass borders, or have grassy weeds when trying to detect larval populations. When an economic armyworm infestation is suspected in a small grain field, populations per square foot should be estimated. Shake the plants and look for larvae on the ground in a square foot area. In small grains the treatment threshold is 4-5 larvae/square foot. Check under debris and soil clumps. Do this in at least 5 locations within the field.

The larvae occasionally clip heads and when this damage is significant it can require treating at lower populations. Head clipping is a behavioral change and usually occurs after leaves have been defoliated or senesce. Scouting at dusk will often find the larvae at the top of the plant.

In spite of the preference for broadleaves, anyone, including an armyworm, can make a mistake. I’d be a little nervous with an alfalfa under-seed being undamaged. A barley (or wheat) crop may have more armyworm pressure than oats but all are hosts.


Grassy weeds are attractive to egg-laying moths. When scouting, pay close attention to field borders and within-field or areas with current or past high grass weed pressure. Grass cover crops may also be attractive egg laying sites if they are not killed before moths arrive.

Examine plants for feeding damage and larvae. Larvae can often be found in the whorl and the nighttime feeding often occurs in the whorl.

Treat whorl stage corn when 25% of plants have 2 larvae/plant or 75% of plants have one larva or more. On tassel stage corn, minimize defoliation at or above the ear leaf.

Bt hybrids

This handy Handy Bt trait table from Chris DiFonzo at Michigan State University shows which Bt proteins control on various insect species including. While several Bt traits control fall armyworm (FAW) only the Viptera trait is promoted for control of true armyworm control. All Bt traits can have difficulty controlling large populations of large armyworm larvae. We are typically dealing with larger, less susceptible larvae moving from weeds and field borders into corn. Secondly, insects must eat the Bt to be affected. As a result, damage could occur with very high armyworm populations on the move even with an effective Bt protein.

As a result, damage could occur with very high armyworm populations on the move even with an effective Bt protein.


Do not base treatment decisions solely on field edge populations. The presence of live armyworm larvae should be confirmed before an insecticide is applied. Insecticide treatment of populations that are starting to pupate or are heavily parasitized is not recommended.

Partial field or border insecticide treatments for armyworm are often sufficient when infestations are well identified by scouting and armyworms populations are found early are the armyworms are migrating. Treat several boom widths ahead of the infestation.

Long insecticide residuals are not needed because of the short time a larval generation is damaging. Many insecticide products are labeled and effective. Refer to the insecticide label for rates. It is important to check the pre-harvest interval of any small grain pesticide. Take precautions to protect pollinators, particularly as corn nears tassel stage.

True armyworm lookalikes in MN spring corn and cereal crops

sawfly larva

Sawfly larva. Note three pairs of true legs and 8 pairs of abdominal rear prolegs and the single eye. Photo: Aaron Hass

Be aware that there can be an armyworm imposter lurking on field edges. Grass sawfly larvae range from tan to green. They are in the order Hymenoptera (bees and wasps) rather than Lepidoptera (butterflies and moths).

A giveaway is the fleshy prolegs, which number more than five. In the Lepidoptera, the prolegs number 5 or less. Lepidoptera caterpillar prolegs have minute hooks (crochets) and while those of sawflies do not.

bright green sawfly larva

Some sawfly larvae are green

Sawflies can clip small grain heads but I cannot remember a Minnesota infestation heavy enough to require treating.

Wheat head armyworm - Are common insects but rarely a pest of MN cereal crops. There are a number of related species that are not easy to differentiate. The larva tend to feed on the heads of cereals.

Cutworms - Several species of cutworms may be found in corn and small grain crops. These will have five proleg pairs like armyworm.

Self-inflicted wounds

stunted cornThe potential for herbicide carryover is increased with late applications. As shown in the fomesafen example in the photo, sprayer laps frequently show injury because of the high application rate. 

There is likely to be some more expressive than typical crop responses to herbicides applied during this hot humid period. The sense of urgency to get fields treated is understandable. Weeds and the crop are both taking advantage of the weather with rapid growth. Watch your label restrictions for heights and crop stage.

When you apply a pesticide, remember to check to see if you obtained good results. You can do this and still have time to go fishing or play golf.

We saw examples of soybean aphid resistant to pyrethroid insecticide and waterhemp resistant to post-emerge fomesafen applications in 2015.

Stuff happens! This week, "Frenchie" Bellicot sent some photos of waterhemp looking remarkably healthy after an application of dicamba.

Happy trails,

Bruce Potter