Eric Sideman's Pest Report
August 7, 2000

Crop Condition

The warm weather has started the summer surge in plant growth, especially weeds. Some fields that looked clean 2 weeks ago now look unkept. Remember to prioritize your efforts. Many crops are well on their way to maturity and harvest and will not be set back by weed pressure. On the other hand, one of the best ways to get control of weeds is to never let any go to seed and restock the weed seed bank in your field. If you are unable to keep the weed seeds from falling it may be a sign that your are trying to manage too much land. Think about cutting back and keeping better care of a smaller acreage. It may result in equal or maybe even higher yields.

In the same light as tolerating weed presser, crops may be able to tolerate insect pressure this late in their growth cycle with no reduction in yield. But you have to be aware of the particular situation. For example, potatoes can tolerate a fair amount of Colorado potato beetle feeding this late. Ten or even twenty percent defoliation late in the season will not reduce yield enough to justify the cost of another spray. On the other hand, complete loss of foliage will. Like usual, scout your fields and learn about your pests.

POTATOES

Growers should be aware that potato late blight has been found in the Grand Falls and Drummond area of New Brunswick. No late blight has been reported anywhere in the state of Maine as of this writing, however, we strongly encourage all growers and industry personnel to be actively scouting for this disease. The earlier that it is found the more control options that are available.

Please remember that late blight is a community disease. What happens on one farm, has the potential to effect others. Neighbors working together will be a key management strategy if late blight is found.

CORN and other GRASSES

This critter is widespread but spotty. It has been found in corn in the sourthern part of the state, but perhaps more devastating is that the larvae are feeding heavily on some grains and hay fields. I have heard that in some timothy fields that the larvae are so thick it is like walking on cushions. I do not know whether spraying hay or grains with Bt is cost effective. Also, even in corn, the large larvae are very difficult to control. I think the only crop worth the spray is silking corn.

Here is a fact sheet on armyworms from Vern in Vermont :

By G.B. MacCollom and G.R. Nielsen, Former Extension Entomologists, Plant and Soil Science Department

There are two species of armyworms that can occur in Vermont: Armyworm and Fall Armyworm.

1. Armyworm

Description: The armyworm* (true, common) adult is a buff or sand colored 1 ½ inch moth with a small white dot in the center of each forewing and dark margins on the hind wings. The full grown larva is a 2 inch, nearly hairless, smooth, striped caterpillar, varying in color from green to brown. The stripes, one along each side and a broad one down the back, are dark, often nearly black. The stripe along the back usually has a fine light-colored broken line running down its center. The head is pale brown with a green tinge and mottled with dark brown.

Damage: The armyworm eats the leaves of host plants. Small 6 to 8 inch corn plants are consumed entirely. Larger corn will have the midrib left. Damage starts first at the field edges. The larvae feed at night and hide in the soil during the day.

Hosts: Armyworms attack all grass-type crops, especially corn and the small grain. Timothy, millet bluegrass, and some legumes are also attacked. When armyworms are marching, many other plants are devoured.

Life History: Armyworms overwinter as partly grown caterpillars and complete development in May. Two to three weeks later, adults emerge and lay eggs. There are three generations, and it is usually, but not always, the late generation that does the damage.

2. Fall Armyworm

Fall Armyworm** is usually a southern insect, but in some years migration northward causes problems in northern states and Canada in late summer and fall. It also develops the marching habit, crawling in great "armies."

Description: Fall armyworms larvae have a prominent white, inverted Y-shaped suture on front of head, and hairs arise from black tubercles on the body that are more conspicuous that those on the true armyworm.

Damage: Initially fall armyworm damage starts by feeding holes in leaves and grass collection in whorls. As the population increases and/or larvae pass into succeeding instars, feeding accelerates and entire leaves may be stripped leaving only the veins. If worms are found on 10 to 20% of plants and are in the early instars (small), control should be considered. Time is crucial because a good-sized population can destroy a field in 72 hours. When marching in "armies," almost all crops in their path can be invaded and consumed in a few nights.

Hosts: Fall armyworm food plants are more varied than the true armyworm. It feeds on alfalfa, beans, clover, grasses, all grain crops, and many vegetable crops, as well as corn and sorghum. Larvae will on occasion bore into the ears of corn.

Life History: A permanent resident in the Gulf states, the fall armyworm is unable to survive where the ground freezes hard. The moths fly northward each spring. In its northern range, there are never more then two fall armyworm generations. Eggs hatch in 2 to 10 days and are full grown in 20 days. Larvae enter soil and pupate for 10 days. Moths emerge and may migrate many miles before females lay eggs. None of these stages survive the winter in Vermont. The small larvae feed gregariously at first, especially down in the base of the unfurling leaves. They usually go unnoticed until half grown or larger when damage becomes alarming. The larvae do not leave the plant during the day and hide in the soil like the armyworm and climbing cutworm do. There is normally only one generation per year in Vermont. Control: Cold, wet spring weather favors the fall armyworm over its natural parasites. Watch for young worms in grassy fields, etc. Keeping crop fields as free of grass as possible will help to reduce crop damage by this pest, because the infestations normally start in grass.

*Armyworm, Pseudaletia unipuncta (Haworth); Lepidoptera: Noctuidae.
**Fall Armyworm, Spodoptera frugiperda (J.E. Smith); Lepidoptera: Noctuidae.

Vern Grubinger
Vegetable and Berry Specialist
Director, Center for Sustainable Agriculture
University of Vermont Extension
157 Old Guilford Rd.
Brattleboro, VT 05301-3669
(802)257-7967 257-0112 FAX

Catches are spotty around the state. Call the pest hotline at the U Maine Extension office (933 4647) for updates in your local area. Check past Pest Reports for control practices.

NITROGEN

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)