Crop insect concerns low, some soybean crops report brown spot

Manitoba Insect and Disease summary for August 17

Larvae on sclerotinia infected plant tissue.


Insects: Insect pest concerns are currently low. Most canola crops are now past the stage where Lygus bugs would be of concern. Only trace levels of soybean aphids have been found so far. Grasshopper counts have so far generally been low.

Plant pathogens: Various diseases continue to be reported in field crops..

Disease Update

Corn: Goss’s wilt is being reported in fields with multiple years of corn. For more information, on Goss’s Wilt on the Manitoba Agriculture website, click here.

Soybeans: Brown spot and Phytophthora root rot is being reported in some fields.

Canola: Fusarium wilt was reported in a field. A typical symptom of Fusarium wilt is brown discoloration on only one side of the stem. As a result, the other side of the stem remains green.

Insect larvae on diseased plant tissue

Numerous enquiries have come in regarding small insect larvae found on white mold (sclerotinia) infected parts of canola, soybeans and dry beans. My best guess is that these are larvae of a species of gall midge (Cecidomyiidae). Gall midges are a fairly big group of insects that includes some crop pests such as wheat midge, swede midge (which this is not) and Hessian fly. This family of flies also includes some predaceous species, and some species feed on decaying organic matter and fungi, which is what I suspect is being found in these diseased portions of the crop. They would likely not be specific to white mold, but would likely be attracted to areas where decay may be starting (for various reasons). I am currently trying to rear some of these larvae to adults to determine the species involved.

Late-season control of plant bugs in seed alfalfa  – Is it economical?

After pollinators are removed from alfalfa seed fields, it is good to assess the health of the crop. But are plant bugs (Lygus and alfalfa plant bug) still a potential threat to a good yield? Research in Manitoba attempted to develop an economic injury level for August populations of plant bugs in seed alfalfa, and concluded that there was no yield benefit to suppressing these late-season populations of plant bugs (Canadian Journal of Plant Science. 2008. 763-770). This may be related to both crop staging and plant bug biology. At the end of the growing season in the Prairie Provinces, adult Lygus bugs are in reproductive diapause, and may feed less as they prepare to overwinter. If alfalfa pods are already yellow or brown, Lygus feeding will not likely affect yield. If pods are still green, plant bug populations similar to the spring threshold of 50 plant bugs per 10 sweep are unlikely to result in economic yield loss, although it is possibly that much high numbers could cause significant losses. A nominal late-season threshold of 12-16 plant bugs per sweep (120 to 160 plant bugs in 10 sweeps) has previously been suggested, but could not be validated by the study in Manitoba because in 3 years of research mean late-season levels of plant bugs never got to this level.

Northern corn rootworm

When corn is grown in the same field for several years in a row, it becomes more susceptible to various potential pests. One such pest is northern corn rootworm (Diabrotica barberi). Until last year only the occasional specimen of northern corn rootworm had been found in Manitoba, and not at levels that appeared to be an established population in a corn field. Last year we did find a well established population in a field in the Souris area. This year we are looking more intensively for them, and have found established populations in corn fields near Morden and Winkler. All fields where they have been found so far have had a long history of consecutive corn being grown in the same field.

This time of year you will see the adult beetles, often on the silks of the corn plants. These adult beetles are generally not of concern, and will lay eggs in the soil of the corn field they are in. When larvae hatch from these eggs the next spring, if there is corn in the field again they will feed on the corn roots. If corn is not in the field they will starve to death. Thus crop rotation is the easiest and cheapest way of dealing with them.

If anyone finds corn rootworm on their corn, or insects they think may be corn rootworm, we are trying to verify the range of this insect in Manitoba. So samples would be welcome and can be sent to John Gavloski, Manitoba Agriculture, Box 1149, 65-3rd Ave. NE, Carman, MB, R0G 0J0.

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