Late Infestation of Armyworms Created Large Problems in Intermountain Pasture and Alfalfa Fields this year.

This fall there were seriousarmyworm outbreaks in California\&\#39\;sIntermountain Region. Many pastures, andhayfields were overtaken by this pest, especially inSiskiyou, Shasta,Modoc, andLassen counties.

While armyworms are only occasionally a problem in the Intermountain Area, when the numbers are high the amount of damage can be devastating. Certain fields were wiped out this fall with many growers losing their third cutting or fall grazing ground to armyworms. Injury can be dramatic, where entire fields can be eaten down to the ground seemingly overnight.

Armyworm damage (above), and True Armyworm Larvae (Mythimna unipuncta), found in an intermountain alfalfa grass mixed field underneath the windrow after cutting (2017).

Not a Common Occurrence. Armyworms are not a pest that plagues the Intermountain Region each year. The climate is not conducive to their lifecycle, as freezing temperatures will kill most worms. As such, adult armyworm moths need to migrate in from warmer lower elevations to lay eggs and establish populations, which is why the pest rears its ugly head later in the growing season. There are multiple species of armyworms which can be problematic: True armyworm (Mythimna unipuncta), western yellowstriped armyworm (Spodoptera praefica), beet armyworm (Spodoptera exigua), and the fall armyworm (Spodoptera frugiperda). While other species do exist, these are typically the most prevalent in the Intermountain Region.

They Multiply! Multiple generations of armyworms can occur in a single growing season. Fall damage in the intermountain area is typically from one of the later armyworm generations as the population size builds. Eggs deposited hatch within a few days and larvae mature in 2-3 weeks.The worms can grow quite large, typically up to 1-1.5 inches. During maturation extensive feeding occurs, but most of the foliage consumed is in the last couple days before they pupate. Armyworms can consume an estimated 80 percent of the total plant matter within the last 4 days of feeding. This is why crops seem to be eaten “overnight” as the worms march through the fields trying to quell their insatiable appetite. On grasses, armyworm typically eat the leaf, only leaving the spindly midvein behind. Fields can even appear to be dead if the worm population is high enough.

Understand Beneficial Insects and Natural Cycles. While these insects can be devastating in certain years, their populations are typically cyclical. There are many natural enemies of the larvae from spiders and lacewings, to parasitoids such as the caterpillar parasite wasp (Hyposoter exiguae). Viral diseases can also affect armyworms under certain conditions associated with moisture, turning the caterpillar bodies limp. Most years in the Intermountain Region these natural predators and pathogens help keep populations in check. Unfortunately, this wasn\&\#39\;t one of those years, with large outbreaks spanning the region. Possibly the wet spring produced an abundance of weedy vegetation that the armyworms built up on, faster than the natural enemies could keep them in check. When the weeds dried down, the worms (adults and larvae) moved into nearby crops where they continued to thrive.

True Armyworm adult and pupa (Mythimna unipuncta). Specimens collected form an irrigated meadow a few weeks after extensive feeding from the worms occurred, Fall, 2017.

Monitoring is the name of the game when dealing with insect pests, including armyworms. Monitor early and often. Typically, these worms do not like light and actively feed at night or on cloudy days, which can be good times to look for them. Be sure to look in the soil, under leaf litter and dirt clods where they often hide during the day. While there is no economic threshold for pasture and grasses in California, other states recommend treatment after worm populations exceed a certain population (2-4 larvae/square foot for any species of armyworm present). This type of monitoring requires getting down on your knees and really looking for the worms on the foliage of the plant, but also around the base of the crown and on or just under the soil surface! Threshold levels will vary depending on the stage of the stand; stubble or younger stands will have a much lower threshold or tolerance to armyworms because these pests can do a lot of damage to smaller plants, so be sure to watch for crop damage.

What is the Threshold for Damage? In California economic thresholds for armyworm infestations in alfalfa have been set using a sweep net for monitoring. Below is a link to a UC IPM page which has a great video detailing armyworm and alfalfa caterpillar monitoring with details regarding detection of parasitized worms! It is important to determine if worms have been parasitized during monitoring, so money isn\&\#39\;t wasted on an unnecessary treatment. Sweeps with counts greater than 15 non-parasitized armyworms per sweep in alfalfa justifies a treatment.

Treatments – Conventional and Organic. Treatment options vary by crop and organic treatments are limited. In alfalfa and grass hay fields, one cultural method which can reduce damage is cutting the field early. Often you will find armyworms under the windrows hiding from the light, but in general cutting will diminish their populations. However, cutting early isn\&\#39\;t always the right option as yields can be reduced, and sometimes treatment with insecticides is justified, especially when the damage exceeds the cost of control.

XenTari® or Agree WG® (Bacillus thuringiensis BT) are labeled for organic production but are mainly effective on the 1st to 2nd instars of the armyworm larvae. BT products typically do not harm beneficial insects. Applications can be made to the younger armyworm instars, without impacting beneficial insect populations, which can be a concern with many conventional insecticides. Both crop group 17 (grass forage, fodder, hay, range/pasture, excluding cereals) and 18 (non-grass animal feeds, forage fodder straw and hay) are on the XenTari label.

In conventional production, Intrepid 2f® (methoxyfenozide) and Coragen® (chlorrantraniliprole) are effective armyworm products. Both products are also labeled for grass and non-grass forage crops (both crop group 17 and 18). Steward EC® (indoxacarb) is an effective insecticide choice for alfalfa, but is not labeled for grass and other forage crops. While smaller worms are more susceptible to insecticide control, applications of these conventional products to armyworms in their early instars could be counterproductive, as they can negatively impact beneficial insect populations before they have time to do their work. Deciding when to treat is a balancing act between the number of armyworms in the field, not treating too early before beneficial insects can control the population, and not treating too late before the worms grow too large and cause significant crop damage.

Be Prepared for Next Year. While the armyworms are done marching through the intermountain region this year, next year monitor early and often so the pest doesn\&\#39\;t “appear” and eat your fields overnight.

NOTE: Any mention or use of pesticide products does not mean a recommendation or endorsement by the University of California. Do not use any pesticide off label, and always refer to the pesticide label for proper use. Trade names of pesticides are used throughout this blog for informational purposes only and are not an endorsement of those pesticides.

True Armyworm Adult (Mythimna unipuncta). Notice the small white dot on the center of the wing.

Relevant Extension Publications

Product Labels

Xentari DF®

Agree WG®

Intrepid 2f®


Steward EC®