Volume 6 Issue 9
Beverly Gerdeman
WSU NWREC
Growers in Western Washington have been blindsided by large armyworm outbreaks in the past few weeks. British Columbia growers report similar outbreaks. While there are multiple species of armyworm, specimens collected in Skagit and Whatcom Counties in Washington and in Canada are the true armyworm, Mythimna (Pseudaletia) unipuncta, a member of the moth family, Noctuidae.
How do I identify the caterpillars?
• Larvae vary in color from greenish brown to black.
• Larvae pass through 6 instars and mature larvae reach 1.5 to 2 inches in length.
• Off-white, orange and dark brown stripes run along each side (Fig. 1).
• There is an inverted “V” in the center of their heads with dark netting on either side (Fig. 2).
• Each of the 4 pairs of fleshy prolegs have a dark black band at their base.
These markings will help identify this armyworm from other species of armyworms but they are not unique to this species.
Fig. 1. Each proleg has a black band at the base.
Fig. 2. A dark netting pattern occurs on each side of the head.
What to look for in suspect fields
• Flocks of birds feeding in the fields can indicate an insect infestation.
• Presence of caterpillars feeding in large groups often moving in mass into adjacent fields (Fig. 3).
• Most active at night but can be found hiding in plant debris during the day (Fig. 4).
• Tons of fecal pellets resembling round seeds on the ground or at base of corn leaves/roots (Fig. 5).
• Potential for extensive and sudden damage (Fig. 6)
Fig. 3. Armyworm outbreak can result in thousands/ acre. (photo – Lance Honcoop.)
Fig. 4. Best time to find larvae is at night. (photo – Lance Honcoop).
Fig. 5. Armyworm will feed on all parts of the corn plant leaving tons of fecal pellets collecting in leaf crotches and on the ground. (photo courtesy of Lance Honcoop).
Fig. 6. Armyworm movement across grass field. (photo courtesy of Lance Honcoop.)
What do the adult moths look like?
The wingspan of the adult moths is approximately 1.5 inches. The adults are unimpressive with forewings varying from beige to reddish brown with dark specks and light streak and spot on each wing (Fig. 7). The hindwings are gray with white edges. Like other moths, the adults fly at night and are attracted to lights.
Fig. 7. Adult armyworm moth. (Photo courtesy of Tracy Hueppelsheuser, B.C. Ministry of Agriculture).
Fig. 8. Armyworm pupa.
Fig 9. A blackened and limp larva indicates presence of a viral or fungal pathogen sweeping through the population.
What will they attack?
Armyworms prefer cereals, forage and corn and so far this year damage has been confined to grass and corn in both Washington and British Columbia. When populations are high, worms can swarm out of infested fields and attack nearby crops and their exact host range is unknown. Border sprays may be enough to protect adjacent fields from the hordes of migrating larvae.
What is their life cycle in Western Washington?
Armyworms reportedly do not overwinter in Washington State or in BC, however the potential for overwintering may exist if winters are moderate. Moths are believed to disperse into Washington each year around April, from areas to our south including Oregon, California and Mexico but their exact origin remains unclear. There are generally 2 and possibly 3 overlapping generations each year usually with two distinct flights, April – May and again July – August dropping off by September. During flights females lay clusters of eggs on lush grass which hatch into caterpillars or larvae, the damaging stage. The larvae feed on foliage but not roots. The first larval generation occurs late June and early July and the second larval generation occurs in August – September with the potential for some to feed until frost. Larvae undergo metamorphosis just beneath the soil surface or on the surface in a hard, reddish-brown pupa (Fig 8).
How do you monitor for true armyworms?
Adult monitoring – Flights can be monitored using universal moth traps or canister traps (with a pheromone placed inside) such as Unitraps from April – until September to best learn their flight periods in Western Washington. Canister traps such as the Unitrap are more useful for high density pests like armyworms that would quickly cover sticky cards rendering them useless.
Larval monitoring – Beginning in early to mid-June scout for larvae, especially at dusk and later or at dawn, by crawling to search for jagged feeding damage on grass blades and presence of larvae, pupae, or abundant fecal pellets on the ground. Damage can usually be expected in a grass crop at 5 larvae/foot2.
Why were they so bad this year?
A wet winter and spring followed by droughty conditions promote armyworm outbreaks. The winter/spring 2016-2017 was the wettest recorded period on record for Washington State and set up perfect conditions for an armyworm outbreak! Growers were blindsided likely because the resultant lush grass could have hidden damage by the first generation. The extensive drought then concentrated the second generation into irrigated crops, acting like green islands and exacerbating the damage which suddenly seemed to appear from nowhere.
Will natural enemies control armyworms?
Some natural controls are already working. Presence of large flocks of birds feeding in fields are a sign that there are large numbers of insects. Flocks of starlings have been reported feeding on larvae in Skagit County fields but not so much in Whatcom County. Presence of blackened, limp larvae in a Skagit County field indicate natural pathogens such as fungi and viruses are sweeping through the population (Fig. 9). Beneficial insects such as ground beetles and damsel bugs are also present as are parasitic wasps and tachinid flies. Despite their presence, they can reduce but not impact populations during an outbreak.
How can armyworms be chemically controlled?
Armyworm larvae 1.5 to 2 inches are nearing pupation and will stop feeding soon however the mixed populations include smaller larvae which will feed longer and even a short feeding period during outbreaks may require an insecticide treatment. There are several materials registered for hay/forage crops. Products with active ingredients including various pyrethroids, chlorantraniliprole, spinosad and malathion, among others, can control the larvae but control is more successful when the larvae are small. Controlling small larvae allows more time to use slower but longlasting products such as IGR’s and microbials like Bacillus thuringiensis Kurstaki. The larger the larva the harder it is to control. In Whatcom County, some pyrethroids were not performing as expected but likely that was due to the large size of the larvae. A malathion treatment however was reported successful. Organic growers can use Entrust® which is also effective against armyworm larvae. For a more exhaustive list refer to the WSU PICOL website and follow all label instructions.
Additional information:
True Armyworm: information for B.C. growers July 2017, Tracy Hueppelsheuser, BC Ministry of Agriculture http://www.acrd.bc.ca/cms/wpattachments/wpID254atID2500.pdf
True Armyworm (Pseudaletia unipuncta Haworth). University of Illinois https://ipm.illinois.edu/vegetables/insects/armyworm.pdf
Acknowledgments:
Lance Honcoop, Elenbaas, Lynden, WA
Tracy Hueppelsheuser, BC Ministry of Agriculture, Abbotsford, BC
Doug Walsh, WSU, IAREC, Prosser, WA