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Scouting for insect pests is critical for effective pest management. Throughout the growing season there are many different insect pests that farmers should be on the lookout for. Japanese Beetle, Corn Rootworm and Western Bean Cutworm are examples of some of the pests that are consistently detected above threshold levels across the Midwest.

Some insect pests can be more difficult to identify and manage because they aren’t always a consistent threat to crop fields. Pests like True Armyworm and Fall Armyworm are good examples of insects that may not be detected in fields year after year because they depend on the migration of adult moths from southern states to infest fields in the Midwest. While similar in name, these pests differ in most other ways including appearance, feeding habits and damage to crops.

True Armyworm

True Armyworm

Photo credit: Iowa State Extension

True Armyworm can vary in color from a green-brown (as above) to almost black, but will typically have white, orange, and brown-black stripes that run the length of the body. The head capsule is orange-brown and the insect can be approximately 1.5 inches long at maturity.

Fall Armyworm

Fall Armyworm 

Photo credit: Purdue Extension

Fall Armyworm is tan (as above) to dark brown-black in color with three thin yellow stripes down the back. A thicker, dark brown-black stripe runs the length of the body along the side and a thick yellow stripe is below that. To distinguish this insect from other larval field pests, look for the yellow Y-shaped mark on the head capsule.

 

Life Cycle

Neither the True Armyworm nor the Fall Armyworm overwinter in the Midwest. Adult moths travel north from southern states in search of attractive fields to lay their eggs. Multiple flights to northern states are possible throughout the year.

Armyworm lifecycle chart

Figure 1. The life cycle for multiple generations of True Armyworm, which can overlap with the Fall Armyworm life cycle. These pests may overlap mid-season, which may make it more difficult to distinguish between the two species. True Armyworm has the potential to have multiple generations in one growing season, which could extend their damage through the end of the season in October, further overlapping with Fall Armyworm.

Scouting

Armyworms get their name from their behavior of migrating from field-to-field in large masses in search of new food sources. It’s important to continuously scout fields that may be targets for this marching army.

True Armyworm are most active at night or on cloudy days. During the day larvae may be found inside the whorl of corn plants or under residue at the soil surface, but damaged plants are often going to be the primary indicator of an infestation. Armyworms are attracted to grasses and grain crops. 

First, scout corn fields that were planted into grassy cover crops or that have grass weeds present; these will be most attractive to females for egg laying. Fields that are no-till or reduced-till will likely see larger infestations than tilled fields due to the number of grassy weeds present. 

Next, scout fields that are adjacent to fields with grass crops or cover crops – fields like rye, wheat, and oats. This pest is most likely to be detected along field edges as they move inward from grassy fencerows or harvested grass and small grains crops.

Fall Armyworm are daytime feeders and, unlike True Armyworm, can readily be found on corn plants when the sun is out. Scout for Fall Armyworms on late-maturing corn from mid-July through harvest. This pest can typically be found in patches throughout the field, which may make detection more difficult.

To scout for either Armyworm pest in corn: inspect 20 consecutive plants in 5 different areas of the field for a total of 100 corn plants. Determine the number of larvae per plant and the percentage of damaged plants per set. In small grains, inspect three feet of one row at 5 different areas of the field. Strongly shake the plants to dislodge insects and search under residue. Count the total number of Armyworms per area. 

Crop Injury

Feeding patterns differ between the two Armyworm species. In small grains like wheat, True Armyworm will defoliate the plant but will usually leave the head untouched. Depending on the growth stage, this may not affect the yield too drastically, but a severe infestation can reduce yield by 50% if not treated. In severe infestations, Armyworm may damage the heads or clip the stems just below the head. Once the small grain is harvested, True Armyworm will “march” to an adjacent field, such as corn, to complete their life cycle.

True Armyworm damage in corn is typically restricted to leaf margins moving in toward the midrib. Plants may look completely defoliated with just the midrib left untouched, but corn will often survive and recover if the damage is above the growing point.  

Fall Armyworm damage in corn is often found on leaves and in the whorl, but this pest is also known to damage corn tassels. 

Damage in corn by either Armyworm can sometimes be mistaken for hail damage, or for another pest such as black cutworm. Close inspection of the plants for the insect is important for a correct diagnosis. 

Management

Armyworm infestation is dependent on weather patterns, cropping practices, and field conditions and so infestation levels will vary from year to year. Management is not always necessary or economical. Insecticides should only be applied when Armyworm are detected above threshold levels. In most states, the threshold for True Armyworm in both corn and wheat is 25% of plants damaged with larvae present, or 2 larvae per plant. For Fall Armyworm, consider taking action when egg masses are present on 5% of plants, or when 25% of plants are damaged with larvae present. Insecticides applied after silking may not be effective in preventing ear damage. Check with your state’s Extension service for more information on local thresholds, products and rates.

In corn, there are several Bt traits available to control Fall Armyworm, and certain traits have been documented for True Armyworm. However, because Armyworm infestations are difficult to predict, and Bt traits are not always effective at controlling infestations when they do occur, other management strategies should be considered.  

Beneficial Insect “Army”

Not every insect spotted in your fields will be a pest; there’s a whole “army” of beneficial insects out there from pollinators to predators to parasites! Some common beneficial insects you may encounter are Spined Soldier Bugs (A), multiple species of ladybugs (B), various spiders (C), and, my personal favorite, the Green Lacewing (D, E). Lacewings are voracious predators as larvae, attacking pests such as aphids, spider mites, leafhopper nymphs, and various moth eggs, such as Armyworm. As adults, lacewings are a beautiful lime green color that you may see fluttering from plant to plant.

beneficial insects

 

Kristin Poley
Kristin Poley
Kristin Poley is the Michigan Research Manager for Soil Health Partnership.