Figure 1. A late instar beet armyworm larvae with inset (upper right) showing detail of the characteristic black dot behind its head.  (Photos by Jeff Bradshaw)

August 1, 2012

August 9, 2012 Update: A Special Local Needs label has been issued under Section 24(c) of FIFRA. The SLN label is for the use of Intrepid 2F insecticide/insect growth regulator applied by aircraft at the spray volume of 5 gallons per acre to control armyworm in sugarbeets and dry edible beans.

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In May of this year Bob Wright, extension entomologist, reported the occurrence of beet armyworm, Spodoptera exigua, at Clay Center. He noted that this insect occurs uncommonly in corn in Nebraska, but that it has a wide host range. This was the first noted occurrence of this insect in Nebraska this year. I noticed a limited number of larvae about one month later in sugarbeet at the Mitchell Research Station in early July; however, I have received numerous phone calls regarding this insect just in the past couple weeks. Some have reported the complete defoliation of large pivot sections of sugarbeet in Colorado and Nebraska. This is the first record that I know of that has indicated any economic number of beet armyworms in sugarbeet in Nebraska.

Identification

The adult moth appears similar to many of the small noctuid or “miller” moths found in this region; mottled forewings with light-colored to white hind wings. The small white eggs are laid in a cluster and covered by a cottony material on the undersides of leaves. The larvae can be light to dark green and are characterized by a black dot behind the head (Figure 1). Multiple sizes of beet armyworm larvae are typically found on the same plant (Figure 2).

Biology

The beet armyworm is native to Asia; however, it has been introduced worldwide and can be found anywhere its hosts are grown. Its host range is huge — it is destructive to more than 90 plant species — and includes cereals, sugarbeets, asparagus, corn, potatoes, and beans. It is not known to overwinter in cold climates, but is thought to migrate northward each year (much like its cousin the fall armyworm, Spodoptera frugiperda). It is possible that our unusually warm winter this past year has contributed to a more northerly survival of this insect. In regions where this insect overwinters, it will survive in the soil as a later instar larva or a pupa. As the soil warms in the spring, moths will fly for a longer period, very similar to army cutworm or “miller” moth activity.

Beet armyworm moths can develop from egg to moth in 21-24 days and a cold period is not required to complete development. As long as it’s warm enough, this moth can continue to develop further generations. Small larvae will feed primarily on leaf parenchyma and will leave behind the epidermis, cellophane-like leaf material (Figure 3). Larger larvae will consume larger areas of leaf tissue and also may burrow into thicker stems or, possibly, into the beat root or bean pod. Note that if large larvae burrow into beet leaf petioles, this damage may result in flagging (or die-off) of large portions of the leaf.

Scouting

There are no thresholds for beet armyworms in beets or beans. A nominal threshold of 25% is used in cabbage and may be an adequate threshold for defoliation in sugarbeet. Beet armyworms are known to be voracious feeders and can consume large amounts of plant tissue over just a couple days of warm weather. As the larvae grow, their feeding rate increases dramatically. Larvae are more active feeders at night and may not be visible on leaves. Instead, larvae may be more likely found hidden in the crown of the beet (Figure 4) or in shaded, cooler parts of the plant during the heat of the day. Larvae also may slightly web leaves together to protect themselves from the heat or predators (Figure 5).

Management

Few insecticide labels include efficacy information specifically for beet armyworm in sugar beet. Control may be unsatisfactory with many widely used insecticides as this insect has developed resistance to many commercial insecticides throughout the world. Newer chemistries may provide better control (Table 1). To avoid further resistance development, rotate product chemistries. ( See www.irac-online.org/content/uploads/LAPHEG_Poster_v5.0.pdf).

Beet armyworms have also been reported in some bean fields, especially in association with pigweed. It will be important to watch for beet armyworm in most western Nebraska crops.

Currently, the best chemical control options (i.e., products for which beet armyworms are less likely to have resistance) cost $22-30 per acre (Table 1). This cost does not include the applicator or surfactant costs. When reading the product label pay careful attention to surfactant requirements and water carrier recommendations as coverage can be particularly important for good control. Also note that if you use a pyrethroid pesticide, which is not recommended, early-harvest contracted beets may be limited by pre-havest intervals if treatment can not be made during the next couple weeks.

Jeff Bradshaw
Extension Entomologist, Panhandle REC, Scottsbluff