IPM
STUFF – SW MN
2002 - 5
5/31/02
Crop status
Since I only look at a limited number of fields there may be many disasters that I am unaware of. Slow crop development and thin stands have already lowered the 400-bushel yield goal in some fields. However, crop production and especially stand evaluation is a little like a poker game. Moderate stands still have yield potential and a lot of low pair hands win. On the other hand, there is plenty of time for things to happen and I’ve seen a lot of 2 pair, 3 of a kind and even smug-faced straights and 4 of a kind get beat late in the game.
Dryer areas occur in some parts of SW MN and corn has curled in recent hot weather. Soybeans in those areas that recently received downpours are at risk for root rots. Assess stands.
White grub and wireworm problems became more noticeable the past week.
Corn
rootworms
I’ve been receiving calls from some of you who plan corn rootworm control with post emerge insecticide (Furadan). Although the spring has been cold, the rootworms are not that far behind. Ann Journey, U of M, Entomology ran a two corn rootworm developmental models. As of May 30, these indicate that egg hatch at Lamberton should occur beginning to be about 10% complete on June 12 with normal weather. These models are approximate but provide an estimate of timing. According to Ken Ostlie, U of M, Entomology, poor control can result if no rainfall occurs between application and hatch, when excessive rainfall before hatch occurs or with late applications. I’ll try to post the development graphs on the SWROC web site.
Cutworms
Although several reports of cutworm injury have been received, they appear to have been primarily dingy cutworms. Cutworms usually feed on leaves of the crop until they are large enough to cut plants. They may also leaf feed and cut weeds before moving to corn or soybeans. As mentioned in previous issues, dingy cutworms tend to feed at or above the soil line and rarely cut corn below the growing point. Plants cut above the growing point will recover and therefore this species does not often cause economic damage to corn. Above ground cutting of crops like sugarbeets and soybeans below the cotyledons will kill the plant.
Although scouting is still required, black cutworms are not likely to cause the widespread destruction that occurred in SW MN as last year. This migrant species, like a few overwintering species, is apt to feed and cut plants below ground. This makes these subterranean feeders a threat in corn.
Unneeded insecticide applications needlessly cost money, hurt the bottom line and panic the neighbors. Don’t be misled. Other than sandhill and glassy cutworms, both subterranean feeders, most cutworms appear “black” especially as smaller larvae. Dingy cutworms have a broad light band on the back, which tends to have a sawtooth appearance at the edges. They also have tubercles (small dark bumps) of similar size on the edge of each segment as opposed to dissimilar tubercles on black cutworm. Under magnification, black cutworms also have a much more grainy appearance to the skin than other cutworms. On the other hand, cutworm feeding below ground is a cause for concern and some already thin stands may not tolerate 3-5% cut plants.
Refer to http://www.ipm.iastate.edu/ipm/icm/2000/5-8-2000/cutworm2000.html for more information on cutworm id and some good pics.
Most corn fields were worked before flights arrived and less attractive to migrant moths. Later planted fields, in ridge-till, strip-till and to a lesser extent no-till systems may be at risk, and especially those fields with heavy early infestations of common ragweed and lambsquarters.
Alfalfa insects
First cutting of alfalfa will start soon. As always, regrowth should be checked for insects. Plant bugs and potato leafhopper are two insects that cause the most problems in SW Minnesota. Some potato leafhoppers recently made the trip from the Gulf coast. 0.2 potato leafhopper adults/sweep were found at the Southwest research and Outreach Center (SWROC) on May 24. This is well below economic levels. Although the leafhoppers may have been present earlier in the week, even a highly trained professional can’t operate a 15” sweep net in 40 mph winds. Try to avoid sweep net sampling during windy weather, as the results will tend to be artificially low.
Bean leaf beetles are active
Bean
leaf beetles are tan but occasionally red individuals can be observed. Winter
mortality did not reduce bean leaf populations to a low level. The beetles evidently overwintered quite
well in soybean residue as beetles were commonly observed in corn (a non host)
this spring. Bean leaf beetle injury is
characterized by round holes between leaf veins.
Thresholds for bean leaf beetle control where virus transmission is a concern are conservative. We do not have high levels of bean pod mottle virus in Minnesota and I would be less anxious to treat soybean production fields.
People tend to overreact to soybean defoliation and agriculturalists are much more worried about early season defoliation than the soybeans are. The defoliation in the image, although impressive, is not enough to economically justify an insecticide treatment. I would, however, keep a close eye on a whole field of plants with this degree of defoliation. From hail loss research, early season soybean plants can tolerates quite a bit of defoliation. For example, 50% defoliation at VE-V4 results in only a 4% yield loss. Soybeans become more sensitive to defoliation approaching and during pod fill.
The need to treat early season MN soybeans for overwintering bean leaf beetle is very rare. In most early season production soybean fields, 50 % defoliation of the plant is the minimum level where I would treat bean leaf beetle. Beetles should be still present and abundant. Remember, the soybeans may be growing faster than the beetles can defoliate them. Insecticide treatments will also negatively impact beneficial insects that may help keep other pests under control.
Iowa State has developed early season thresholds based on crop stage, beetles/plant and control costs and these are listed in their 5/27/2002 ICM newsletter ttp://www.ipm.iastate.edu/ipm/icm/2002/5-27-2002/.
Counting bean leaf beetles can be difficult and beetles are less active and visible in cool, cloudy weather. The generation present during pod fill is the one we need to really watch for.
Soybean aphid watch
Soybean aphids have not yet been observed in SW MN during the spring of 2002. I have been checking soybeans, red clover and defoliated a lot of buckthorn looking for aphid colonies. The buckthorn at the SWROC has more crown rust than I remember in recent history. Our allies in the fight against soybean aphid are getting ready. I have been observing three species of ladybird beetles in emerged soybean fields. Hopefully, enough multi-colored Asiatic ladybird beetles avoided the brooms and vacuum cleaners this winter to help provide early season soybean aphid control.
Bruce Potter
IPM Specialist SW Minnesota
University of Minnesota Extension Service
Department of Entomology
University of Minnesota Southwest Research and Outreach Center
PO Box 428
Lamberton, MN 56152
Ph: 507.752.7372
Fax: 507.752.7374
E-mail: bpotter@tc.umn.edu
http://swroc.cfans.umn.edu/SWMNPEST/swmnpest.htm
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Created on 5/31/02 by B. Potter with assistance from M. Werner. Last revised 6/3/02.