By the OMAFRA Field Crops Team
Western Bean Cutworm
Western bean cutworm (WBC) traps have been set on corn and dry edible bean fields across southern Ontario, and low numbers of moths are being caught. Traps are used to monitor moth activity and indicate when scouting is required, and are not intended to determine when to spray insecticide. Up to date trap capture data and more information on WBC can be viewed at www.cornpest.ca/wbc-trap-
Spraying for WBC control in corn is warranted if 5% or more of plants have egg masses. WBC eggs are laid on the upper surface of the top 3 to 4 leaves of corn plants. WBC prefer fields in the whorl to pre-tassel stages of corn. Once the corn crop is in tassel or beyond, the moths prefer to lay their eggs on the dry bean crop or later planted corn fields still in pre-tassel stages. Peak moth flight is typically the last week of July, first week of August (starting first in the SW and then into E ON). Only corn hybrids containing Vip3A provide protection against WBC.
In dry beans, it is extremely difficult to find eggs or larvae. The larvae are active at night and hide in the soil during the day. It is more productive to scout for signs of feeding holes in the pods. Pod feeding is expected to start approximately 10 to 20 days after peak flight. If entry holes are observed in the pods, an insecticide application is necessary. Effective control can occur if pod feeding is spotted early. Select insecticides with some residual. Dry bean fields next to corn fields that have reached threshold are at risk, especially if the corn is beyond the pre-tassel stages.
Insecticide application is most effective when it is done close to or shortly after egg hatch. Eggs hatch occurs one or two days after turning purple. Pay attention to pre-harvest intervals, particularly in dry beans. Consult the Field Crop Protection Guide- Publication 812 for product information ontario.ca/ctaw.
There have been reports of soybean aphids, and for many it seems to be early in the season. Neonicotinoid seed treatments provide protection against soybean aphids for just two weeks or so after planting, according to several recently published reports in the US. This research also confirms that the IPM approach of a well-timed foliar insecticide at threshold in the R1 to R5 stages has greatest yield response where aphid infestations exist.
Early planted fields and fields close to buckthorn are more likely to have soybean aphid colonizing at this time, and may have very high aphid counts per plant in pockets near field edges. However, scouting a few more metres into the field will likely show few or no aphids. Very high aphid counts in these pockets attract natural enemies. Spraying too early when the colonies have just started will delay natural enemy populations from building up and responding to these aphids. Insecticide trials have not been able to show any yield response to sprays done on these vegetative stage infestations. In fact, a spray application during the V stages can actually lead to problems because they easily wipes out the natural enemies. This results in aphids rebounding quickly and potentially requiring a second spray again once the crop does reach the R stages.
Research has found that an insecticide application is required once 80% of the plants in the field have at least 250 aphids per plant and it is apparent that the population is on the increase during the R1 to R5 stage of the soybeans. This threshold gives an approximate 7–10-day lead time before the aphids would reach the economic injury level, where cost of control is equal to yield loss. Experience has shown that natural enemies can keep the aphid population fluctuating around the 250 aphid threshold. This fluctuation means they are working hard at controlling the aphid population. It is only when the aphid populations continue to rise instead of fluctuate, that the natural enemies are not plentiful enough to keep up.
The Aphid Advisor (www.aphidapp.com) is a helpful tool to use when scouting for soybean aphid in Ontario. Developed by University of Guelph, based on field research conducted in Ontario, it determines if there are enough natural enemies to keep aphid populations in check or if an insecticide application may be needed. Just enter the number of aphids and natural enemies present during R1 to R5 and the app will calculate the potential buildup based on weather and natural enemy presence. This tool helps to take the guess work out of your spray decision.
A new version of Agronomy Guide for Field Crops – Publication 811 is now available in print or for download. For a copy, contact firstname.lastname@example.org or 1-877-424-1300 or visit ontario.ca/c9uz.
Updated daily at www.weatherinnovations.com/
This Report includes data from WIN and Environment Canada.
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