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  • TBO Staff

Outbreak Of Yellow-Striped Armyworms In Pennsylvania

An outbreak of yellow-striped armyworms has occurred in potatoes along the Susquehanna River, north of Harrisburg. High populations have defoliated fields up to 30 acres in size.

The yellow-striped armyworm, Spodoptera ornithogalli, overwinters in southern areas and migrate northward each year. Eggs, larva, and adults are killed by freezing temperatures. Pupa can withstand colder temperatures, and this species overwinters in North Carolina and Kentucky. In warm winters, migrants may reach our area earlier and complete more generations.

Yellow-striped armyworm on potato. Photo: Bob Leiby

It is not new to have yellow-striped armyworm (YSAW) in our area. What is new is to see it reaching high enough numbers to be considered a pest. The geographic distribution of this species reaching pest status has historically been limited to southern states, the Caribbean, Mexico, and South America.

The yellow-striped armyworm has an extensive host range. A shortlist includes asparagus, bean, beet, crucifers, cucurbits, tomato, alfalfa, small fruit, peach, wheat, and the literature includes many more. Important weed hosts include dock, horseweed, lambsquarter, plantain, and pigweed. In recent years, we have seen YSAW causing significant problems in tomatoes, and the text in this alert is from an alert about YSAW in tomatoes written with Steve Bogash in 2012. But it is relatively rare to see YSAW causing major problems in potatoes.

Yellow-striped armyworm causing field scale defoliation and damage in potatoes, August 28, 2020. Photo: Bob Leiby

Adults look very much like the adult form of fall armyworm. The wingspan is 1.3 to 1.6 inches. Front wings have a complex pattern of brown, tan, and grey areas. Adults are difficult to distinguish from their relatives (such as the fall armyworm), but the larvae are distinct. Larvae have a yellow stripe running longitudinally along their body. Less distinctive stripes are below that, including a pink stripe above the prologs. Black triangles are positioned above the yellow stripe on most abdominal segments.

The larva of yellow-striped armyworm, showing strong yellow stripe and pale pink stripe (left, photo B. Lingbeek, Penn State), and black triangles positioned above the yellow stripes (photo by Steve Bogash, Penn State, retired). Colors will vary among specimens.

In Kentucky, adults are active in April or May. Eggs are laid in clusters of 200-500 eggs per cluster, on the underside of leaves. Females are capable of depositing up to 3000 eggs. Young instars initially are gregarious, and later disperse, sometimes using silk strands carried by the wind, as they mature. There are typically six instars. Typical life cycles in Kentucky are 5-7 days in the egg stage, about three weeks as larvae, followed by pupation, and 3 to 4 generations per year.

Yellow-striped armyworm damaging tomato in 2012 (left). Photo: Steve Bogash, Penn State, retired. Adult yellow-striped armyworm (right). Photo: Marlin Rice, Iowa State

Resistance is very common in its relative, the fall armyworm. Insecticidal control can be expected to be more effective when targeting early instars. Scout for early signs of infestation. In southern states, problems tend to start in the vegetative stage of plants. However, the larvae may move rapidly to fruit. In Florida tomatoes, the recommendation is to treat with an insecticide if there is one larva or more per six plants before bloom; after bloom, treat if one egg or larva is found per field. Pheromone lures might help. Great Lakes IPM carries a pheromone lure for yellow-striped armyworm. Also, the lure currently used to trap the Western bean cutworm is capturing yellow-striped armyworm adults. Field research should be conducted to determine if the lure is sufficiently selective – does it only, or mostly, attract yellow-striped armyworms, or do we have a significant non-target capture in our area? Another option is to use blacklight traps, but you would need to sort through the captures of many species.

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