Southeast Missouri Pest Bulletin No. 10, July 22, 2004

INSECTS & OTHER PESTS
Insect activity has remained steady this past week in southeast Missouri. I've received reports about the following crop pests (ex. corn borers in corn; bollworms, plant bugs, spider mites in cotton; garden webworms in soybean) being active in the Bootheel. The latest moth trap counts for the crop pests that we're monitoring along with area consultants are in Table 1. (MLB)

CORN
Corn Borers: The southwestern corn borer moth counts at the Portageville trap site remained high again this past week. We checked the trap again yesterday, and another 204 moths were caught over a two-day period. Any fields with non-Bt corn should be closely monitored for infestations because this generation's feeding damage can lead to the greatest yield losses. The MU action threshold for SW corn borers is 25% of the plants infested with eggs and live larvae.

COTTON
Irrigation: One of the most difficult decisions in cotton production is when to stop irrigating. Terminate too early and bolls will prematurely open plus reduce lint yields should September be dry. On the other hand, stopping too late plus a wet September and the crop will try to produce more bolls that may not mature before harvest. One must make this decision based on the average year. For example, Lubbock, Texas is near the northern edge of the cotton belt just as we are. Their rule is to leave it wet on August 15th. Our last boll that has a 50% chance of making it will bloom on August 10th. By leaving it wet on the 15th the boll will be full size in about 16 days. At that point water use by the plant will start decreasing. (BP)

It is very tempting to irrigate late-planted cotton late in the season in order to make up for time lost earlier in the season. In October so few DD60's are received that the late bolls have very little chance of fully maturing; therefore, it is best to terminate irrigation at a normal date of August 15th. This will signal the plant to make what is already on it and not stimulate it to make a later crop. Don't gamble and risk the crop that you've already made!

Bollworm/Budworm: We continued to catch to a sizable number of bollworm moths in our pheromone trap at Portageville this past week. Counts at this and other locations are beginning to decline; therefore, I believe we've seen the peak moth flight for this generation. I still recommend closely scouting any fields that are non-Bt or Bollgard I because lush cotton will be a more attractive host than rapidly maturing corn plants. The recommended MU action threshold for bollworms and budworms in non-Bt cotton is to treat before 10% or more of the plants are infested with small larvae (less than ¼-inch in length). In Bt cotton fields a recommended threshold for bollworm control is to apply a foliar treatment when 4 or more larvae (greater than ¼-inch in length) per 100 plants are present. (MLB)

Plant Bugs / Cotton Fleahopper: Based on comments that I received from several different people, there still doesn't appear to be much of a letup in plant bug infestations. A couple of experienced consultants are still reporting they are spraying some fields each week. Reports that I've received plus observations made here at Delta Center indicate we're mainly fighting tarnished plant bugs. A few cloudy plant bugs plus cotton fleahoppers also are starting to appear in the area.

Spider Mites: Yesterday Jeff House, MU Extension Specialist in New Madrid County, and myself looked at field south of Gideon with a severe spider mite infestation. The plants were exhibiting signs of stress and damage (leaves red on top or dying). I'd caution you to continue monitoring for mite infestations as hot, dry conditions are ideal for this cotton pest. Again, to maximize control of mite infestations, I suggest these steps: 1) choose an appropriate miticide, 2) applications should be made at high volume and pressure, and 3) clean field equipment after driving through a field infested with mites to prevent spreading them to other locations.

RICE
Rice Stink Bug: Nothing new to report on rice stink bug infestations. Just remember to closely monitor any fields where the panicles are emerging for rice stink bugs. The MU action threshold is to treat when stink bug populations average 5 or more per 10 sweeps during the first two weeks of heading and 10 or more per 10 sweeps during the second two weeks of heading.

SOYBEAN
Garden Webworm: I received a call the other day from Steve Hawkins about a soybean field near Clarkton being severely defoliated in several areas. I got a chance to look at the field yesterday, and in several big areas (3-4 acres) the plants (about V6) were almost entirely defoliated (Figures 1, 2). The level of damage I saw by this insect is not very common in soybean fields, but localized "hotspots" may occur in association with pigweed infestations.

The garden webworm moth is small with dull yellow to brown coloration plus both pairs of wings have light and dark markings. Webworm larvae are usually green with conspicuous black spots on each segment. Typically, the larvae feed on leaves bound together with their silk webbing.

Insecticide treatments are rarely necessary unless large webworm infestations cause severe defoliation in a field. The MU action threshold for this pest is to treat when 10-12% of the plants show heavy webbing in the top trifoliates OR when 30% defoliation occurs prebloom (drops to 20% defoliation bloom to pod fill). Any labeled pyrethroid insecticide is recommended to control this insect.

Figure 1. Soybean field damaged by garden webworms near Clarkton, MO. Photo credit - Michael L. Boyd

Figure 2. Soybean plants defoliated by garden webworms. Photo credit - Michael L. Boyd