Japanese beetles (Fig. 1) have been arriving throughout Illinois over the last couple of weeks, and are becoming pretty conspicuous in some areas. Our crops are well behind their usual progress when Japanese beetle emergence occurs, which could impact scouting and management decision making. Several of my colleagues recently wrote an in-depth article on the history, distribution and management of this pest1; you can read the full open-access article here. Some notes on management follow by crop:
Fig. 1. A Japanese beetle adult hanging out on a corn leaf
Corn: Silk clipping is the primary concern with Japanese beetle infestations in corn. While the beetles will nibble on the leaves also, this does not amount to much. Many fields this season are likely to begin silking when Japanese beetles are at their peak, so scouting will be especially important. Silk clipping by Japanese beetles (as well as corn rootworms) can interfere with pollination. The effect of this feeding can be compounded by heat and drought stress2, which could be an issue in many fields this year given the late timing of pollination. Feeding tends to be concentrated on field edges, so thorough scouting within the field is necessary to determine if a treatment is justified. A rescue treatment with an insecticide is recommended if the following additions are observed:
Silks are being clipped to within ½ inch throughout the field
There are 3 or more beetles per ear (consider reducing this number if silk clipping is occuring under drought and heat-stress conditions)
Pollination is ongoing/less than 50% complete (especially during the first 5 days of silking).
Soybean: Control of Japanese beetles in soybean is rarely justified in Illinois, even though the damage is often conspicuous. Soybeans are fairly tolerant of defoliation in general. The only “wild card” this year is that, like corn, our soybeans are well behind their normal level of development when Japanese beetles (and other defoliators) become active. Making a rescue treatment decision for defoliators is a three-step process:
Determine the overall level of defoliation in the field. The recommended economic threshold is 30% defoliation prior to bloom, and 20% defoliation after bloom. Train your eye to accurately measure defoliation, and be careful not to over-estimate the extent of the damage (Fig. 2)
If a field is above the economic threshold, sample using a sweep net, shake sheet, or other sampling method to identify the insect responsible and verify that it is still present in the field. (Avoid “revenge” applications, which will not provide an economic return).
Choose an insecticide and rate that will provide effective control of the target insect. (Efficacy results from 2018 can be found in the 2018 Applied Research Results on Field Crop Pest and Disease Control report here. Results from trials conducted previously at the University of Illinois can be found in the “On Target” summaries of field crop insect management trials here.
Fig. 2. Soybean leaves with differing levels of defoliation. Most observers tend to over-estimate the actual level of defoliation in the field
Most insecticides that control Japanese beetles have a relatively short period of residual control. This is no big deal in corn, as the critical period to protect silks is short anyway. In soybean, the short period of residual activity is another great reason to abide by the economic thresholds for defoliating insects; yield-reducing numbers of Japanese beetles in soybean are rare, and multiple applications for this insect are usually a wasted expense.
1 Shanovich et al. 2019. Journal of Integrated Pest Management 10: 9
2 Steckel et al. 2013. Journal of Economic Entomology 106: 2048-2054
Author contact: Nick Seiter | firstname.lastname@example.org | 217.300.7199
Persistent wet field conditions have increased the likelihood that many farmers will opt to take the prevented planting option through their crop insurance policy. Even though no crop will be planted, weed control practices still should be implemented to reduce seed production from summer annual weed species. Any weed seed produced in 2019 will add to future weed control costs. The old weed science adage “One year’s seedling equals seven years weeding” reinforces the need to adequately manage weeds on prevented planting acres.
Many species of winter annual weeds already have flowered and soon will produce seed. Additionally, many summer annual weed species have emerged and are growing rapidly. We suggest the focus of weed management on prevented planting acres should be on summer annual weed species. Several options exist that could be used singly or in combination to keep weeds under control.
Tillage. Tillage implements that significantly disturb the soil (tandem disk, field cultivator, etc, but not vertical tillage implements) can effectively eliminate summer annual weeds. Generally, tillage is more effective when weeds are small and soils are not overly wet. Large weeds that escape a tillage pass can be very difficult to control later in the growing season. While usually effective at controlling established weeds, keep in mind that tillage can stimulate germination and emergence of additional weeds. Multiple tillage operations likely will be needed before a killing frost to prevent summer annual weeds from producing seed. Fuel consumption/cost and potential for soil erosion are additional factors to consider when using tillage to control weeds on prevented planting acres.
Mowing. Repeated mowing can help suppress weed growth, but might not prevent seed production of all summer annual species since some seed could be produced from plants that regrow or from tillers present on grasses below the height of cutting. Adjust the mower to cut as close to the soil surface as possible. Utilizing mowing followed by tillage likely would be more effective in reducing seed production than mowing alone. Alternatively, if vegetation is quite large, mowing that precedes tillage by several days might improve the effectiveness of the tillage operation in reducing seed production.
Herbicides. Non-residual herbicides can control many summer annual species, but will miss any plants/species that are resistant to it. Combining glyphosate with 2,4-D or dicamba would provide more consistent control of emerged waterhemp, marestail and giant ragweed than glyphosate alone. Waterhemp’s extended emergence duration will require at least two to three herbicide applications before the first killing frost. We do not recommend applying soil-residual herbicides as they are unlikely to maintain sufficient weed control in the absence of a planted crop.
Cover crops. A well-established grass cover crop (such as rye, wheat, sudangrass, etc.) can be quite effective in limiting emergence and growth of summer annual weed species. It is advisable to control any emerged weeds before seeding the cover crop. Tillage or non-residual herbicides can be used prior to seeding, but you should allow several days between herbicide application and seeding for the herbicide to control existing vegetation. Drilling cover crop seed likely will result in a good stand that can be very competitive with weeds and also help scavenge soil nitrates. A growth regulator herbicide (2,4-D, dicamba, etc.) could be applied after the cover crop has emerged to control broadleaf weeds if needed. Without vernalization, rye or wheat plants are unlikely to produce viable seeds by the end of the growing season but might still provide suppression of fall-emerging winter annual weed species. Be sure to plant weed-free cover crop seed, which might require cleaning bin-run wheat seed. Many references on cover crop establishment are available, including one published by the Illinois Nutrient Research and Education Council.
This summer we had a lot of questions around planting VT2 hybrids. Seed suppliers continue to come with new genetics that are only available in VT2 and these genetics are some of the highest yielding genetics out there. In an effort to help us make decisions for this coming year we participated in a joint effort with Wyffels and put yellow sticky traps in our bean plot fields. These sticky traps were part of over 500 traps put out by Wyffels this summer. We have attached the results from the Wyffels study. We also put out an additional 9 traps and checked those for a 4 week period from July 20th– August 17th. Results were the following.
Milmine- .32/day Atwood- .14/day
Camargo- .11/day Ivesdale- .46/day
Seymour- .18/day Mansfield- .18/day
Royal- .46/day Catlin- .07/day
We would appreciate the opportunity to help you make your seed choices next year and hopefully this kind of information helps. If you have any questions please contact one of our Seed Specialist.