Yellow-Striped Armyworm and Blister Beetle Problems in Vegetable Fields

Jerry Brust, IPM Vegetable Specialist, University of Maryland;

There have been several reports and sightings of blister beetles Epicauta spp. and yellow-striped armyworm Spodoptera ornithogalli feeding on various vegetable crops in our area. This is not too unusual as both of these pests tend to become more of a problem later on in the season – late summer/early fall.

Yellow-striped armyworm larvae vary from dark grey to black with two conspicuous yellow stripes along each side of its body (Fig. 1). Below this yellow stripe there often is one pink or orange stripe running along the length of the worm. There are two dark triangle-shaped marks on the top of the yellow stripe on each segment (Fig. 1), although this is difficult to see in the dark form. And there is a dark spot just behind the true legs. The female moth lays her eggs in clusters of 200 to 500 on the undersides of leaves. A single female may deposit more than 2000 eggs. Eggs hatch in 5 to 7 days and the larvae feed for about 3 weeks before moving to the soil to pupate. There are three to four generations during the year in the mid-Atlantic with the late-summer generation being the most common.

Yellow-striped armyworm is a general feeder and will consume large quantities of foliage. It will feed on asparagus, bean, beet, cabbage, carrot, cucumber, lettuce, onion, pea, rhubarb, sweet potato, tomato, turnip, and watermelon and will also feed on blackberry, clover, grape, peach, raspberry, soybean, sweet clover and sunflower. It will also feed on several of our more common weed species such as: dock, horse nettle, jimsonweed, lambsquarters, morning glory and redroot pigweed. Most of its damage is caused by defoliation of a plant, but later in the season larvae often will feed on the outer surface of vegetables scarring them badly making them unmarketable (Fig. 1).

It is easier to control small larvae vs large larvae as large larvae are more tolerant of several insecticides. Transplanted crops, young plants and fruit bearing vegetables, especially tomatoes in high tunnels, should be observed regularly for early detection of yellow-striped armyworm. In tomato, small yellow-striped armyworm larvae can be managed with several insecticides including Lannate, Pyrethroids, Radiant, Confirm, Avant, Coragen and Exirel. Larger larvae will be more difficult to control and often require the aforementioned pesticides be applied at high rates with high gallonages of spray material. For organic growers Bacillus thuringiensis (Bt) works well for small larvae control while Spinosad tends to work better on larger larvae. In the field yellow-striped armyworms have natural enemies, which include parasitic wasps and Tachinid flies, although these natural enemies are less commonly found in high tunnels.

Yellow-striped armyworm feeding on a green tomato fruit

Figure 1. Yellow-striped armyworm on tomato

Blister beetles have been reported feeding and defoliating swiss chard, eggplant and other vegetables. The presence of blister beetles now is not unusual as they are often found in large clusters in late summer-early fall. They can arrive in large groups, seemingly overnight and can do a great deal of damage in a short period of time.

Adults are large, oblong beetles with relatively large heads, long ‘necks’ and usually with some stripes (but not always) (Fig. 2). Striped blister beetles are shades of gray or brown with yellow stripes running lengthwise on their wing covers (Fig. 2). The ash-gray blister beetle is gray, the black blister is completely black, and the margined blister beetle is black with a grayish band around the edge of each wing cover (Fig. 2). Blister beetle abdomens usually extend past their leathery wings. Striped blister beetles hide beneath plants during the hotter periods of the day, becoming active when temperatures are more suitable for them. If disturbed when on plants beetles will immediately fall to the ground and run. Adults begin laying eggs in late spring or early summer and continue through most of the season. A female can lay one to two hundred eggs just beneath the soil surface and eggs hatch within a couple of weeks.

If you look up blister beetles most of the literature deals with the beetles as a threat to horses and livestock. The beetles secrete and contain within them a blistering agent called cantharidin. Cantharidin is toxic if ingested and it persists in beetles long after they are dead. Humans who ingest the beetle can suffer severe damage to the urinary tract and gastrointestinal lining.

Blister beetles begin feeding on the edges of leaves eventually leaving only stems. They will feed on just about any leaf that grows in a vegetable field such as tomato and other solanaceous vegetables as well leafy greens, crucifers, spinach and others.

Pyrethroids can be used to control blister beetles on most vegetable crops. Pyrethroids will reduce the damage, but there is often a 7-day pre-harvest interval (phi) with some of the chemicals depending on what the crop is. So be sure to check the label to find the correct phi for the particular product you are using on the particular crop you are using it on. It should be noted that once established, beetles are difficult to eliminate completely. Organic growers have an even more arduous task of managing them. Row covers will keep this pest as well as harlequin bugs off your plants. However, if row covers are not used then I often see diatomaceous earth (DE) recommended for beetle control. If it rains DE does not work very well and overall, I have not had much luck with it controlling the beetles. Spinosad alone or mixed with other products such as neem or kaolin clay have been found to reduce feeding damage in 24-48 hours.

Figure 2. Margined blister beetles (top) and striped blister beetles (bottom)