Insect update for early August, 2011

August is the month for potential spider mite outbreaks. Also, high numbers of Asiatic garden beetles are being reported in southwest Michigan.

Spider mites

On July 28, Howard Russell in MSU Diagnostic Services reported his first two-spotted spider mite sample from soybeans in Shiawassee County. Despite heavy rainfall in some locations, the potential for a spider mite outbreak continues, unless humidity remains high under the crop canopy for an extended period. Under humid conditions, beneficial fungi can infect and kills mites, rapidly wiping them out. Just getting rain isn’t enough if the ground and canopy rapidly dry out. So continue to watch for mites – August is typically when outbreaks occur.

Asiatic garden beetles

For those of you in southwest Michigan, Bruce Mackellar, MSU Extension educator in Van Buren County, reports high numbers of Asiatic garden beetles in his western bean cutworm milk jug traps near Centreville, Bangor and Gobles, Mich. The beetles are attracted to the sweet tasting antifreeze used in the trap and they dive-bomb in at night and drown. Mackellar uses his western bean cutworm jugs to estimate the Asiatic garden beetle activity in his area, and he reports the highest numbers he’s ever seen.

The beetles lay eggs in the ground and small larvae (white grubs) are likely already present, feeding on roots. There is no threshold for Asiatic garden beetles in milk jug traps, but the high trap catch does alert potato and corn growers to a potential problem. In potatoes, larvae will feed on tubers in August. Potato growers in southwest Michigan should be on the lookout for tuber damage at harvest. In corn and soybean fields, the grubs will overwinter and potentially reduce corn stands next spring.

Growers in southwest Michigan should consider corn seed treatments or soil insecticides in 2012, because there is nothing that can be done after planting to eliminate Asiatic garden beetle grub damage.

Redheaded flea beetles

What’s that black, hoppy thing on your beets, soybeans, corn or other crop? Perhaps it’s the redheaded flea beetle, a shiny and black beetle with a red head and enlarged back legs for jumping. Redheaded flea beetles resemble corn flea beetles, but they’re larger and have an orangish-red head. See a picture of the redheaded flea beetle on Bugguide.net.

This year, redheaded flea beetles are reported in the Thumb and in Ontario, Canada, feeding on several crops and weeds. Like other flea beetles, it chews round holes in leaves. For management in soybean, simply include the chewing by redheaded flea beetles in the overall defoliation estimate from all insects (15 to 25 percent defoliation during pod fill).

Soybean aphids

The North Central Aphid Suction Trap Network is up and running and you can see aphid catches from five Michigan trapping sites. There is very little activity in soybeans so far this season. A low population on plants translates to low flight during July and August. However, low numbers on beans often means healthy aphids in late August (low infection by aphid-killing fungus), and we sometimes get a big fall flight of healthy winged aphids back to buckthorn. The network is most useful in the fall to track late flights and show the potential for aphids next season.

Dr. DiFonzo’s work is funded in part by MSU‘s AgBioResearch.

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