Insect issues appear for Ontario growers

Cereal leaf beetle (CLB) populations have reached threshold in some spring cereal fields this week. CLB larvae and adults feed on the tissue between leaf veins, leaving long scratch marks that can make plants appear silver or frosted from a distance. Before boot stage, the threshold is an average of three larvae per tiller. One CLB adult or larvae per stem warrants control after boot stage but prior to heading. Once headed, protecting from injury to the flag leaf is key in early heading stages. Control in early stages of heading is only warranted if CLB are still actively feeding directly on the flag leaf before the grain fill period (Zadok 75) is complete.

Thrip infestations have been spotted in a few corn fields again this year. Typically, not a pest of concern, thrips have become an annual problem for some corn, soybean and spring cereal fields in Ontario. These tiny cigar shaped insects feed on the underside of the leaves, leaving scars along the veins of the leaf. When young plants are stressed from lack of rain or a nutrient deficiency, thrips may thrive, moving beyond the lower leaves, causing feeding injury up to the newest leaves. No thresholds are available as typically the plants can grow ahead of the infestations. Control may only be warranted if infested fields continue to be significantly moisture stressed with no rain forecasted, and there are several thrips per leaf throughout the plant, but especially if causing injury on the newest leaves.

Potential upcoming pest issues

Two-spotted spider mite (TSSM) populations in Ontario soybeans and dry beans are becoming tolerant to dimethoate (Lagon® or Cygon®). Despite last year’s field season not being ideal for TSSM, samples were collected in ten fields in southwestern Ontario that had reached threshold. Populations from these sites were found to be resistant to dimethoate. In a hot dry year, we expect to see significant problems in soybean and dry beans. Through a collaborative research project with Western University, AAFC – London and OMAFRA, we hope to understand the distribution and extent of this resistance. Currently, dimethoate is the only active ingredient registered for spider mites in soybeans and dry beans, so this finding is very concerning. It also increases our need to test these populations to find what possible alternative products are still effective and can be registered to replace dimethoate. If you find any spider mite populations in soybeans or dry beans before an application is made (preferable), or after a dimethoate application that was not successful at controlling the population, please contact Tracey Baute ([email protected]). Samples will be taken back to Western University for testing. Fields with a history of spider mite problems or fields close to greenhouse operations are of special interest, since they could influence which products they have been previously exposed to.

Corn rootworm (CRW) populations could be heavy again this year, as Bt resistant populations continue to spread and new counties are identified with problem fields. Hot, dry conditions are ideal for adult activity which is expected to start over these next few weeks. In continuous corn fields with repeated use of Bt rootworm hybrids, monitor for beetles in July and August. Those fields that had heavy adult pressure last year will have experienced abundant egg laying in late summer. The larvae that hatched from those overwintered eggs are happy to find corn roots planted in those same fields again this year. Look for signs of lodging and goosenecking to dig the roots to determine the extent of the root feeding. Contact your seed provider or Tracey Baute, OMAFRA if unexpected damage in Bt rootworm hybrids is found.  

The Regional Corn Rootworm Trap Network will continue in 2022 and we are looking for more trap participants in Ontario. Trap supplies are free to growers and crop consultants, thanks to funding from the Grain Farmers of Ontario (GFO) and the Agricultural Biotechnology Stewardship Technical Committee (ABSTC). Monitoring sticky traps weekly during adult activity helps us gain insight as to whether it is a good rootworm year and alerts us to regions and fields where rootworm activity may need further scouting and monitoring for potential resistance issues. It is also improving our understanding of where both northern and western CRW populations are in Ontario and across North America. Trap sites in continuous corn with a history of repeated use of Bt rootworm hybrids are preferred trap sites. If you are interested in getting free trap supplies in Ontario, please contact Tracey Baute ([email protected]) so we can get traps to you before July when trapping will start.

– Click here to read the June 22 field crop report at the Field Crop News website.