Spider Mites in Soybean

Spider mites often show up when it’s hot and dry outside. Given our current weather (and the promise of more heat to come), it’s a good time to review our scouting and management recommendations. Spider mites feed on a wide variety of plants, and usually enter soybean fields from grassy edges – especially right after the edges have been mowed, which causes the mites to seek out a new food source. (If you have the option, consider holding off on mowing those roadsides and grass margins when it’s hot and dry). When scouting for spider mites, look for areas of the field with a yellow-brown or bronze coloration, especially near the edges. Spider mites feed on cellular fluid, which leads to a characteristic stipled or “sandblasted” appearance on the leaves. If you look closely at an infested leaf, you will likely see the tiny mites moving around, and if you examine the leaf with a hand lens you will see their small, globe-shaped eggs.

Spider mite damage within a soybean field

Isolated spider mite damage within a soybean field. Worth keeping an eye on, but not a reason to spray.

Close-up of spider mite damage to soybean

Close-up of spider mite damage to a soybean leaf. Note the stipled or “sandblasted” appearance.

Application decisions for spider mites can be tricky. Use an economic threshold of approximately 20-25% of foliage discolored before pod set, and 10-15% of foliage discolored from pod set until pods are filled (mid-R6). Consider the extent of the damage and whether it is expanding, the growth stage of the crop (R4-R5 are especially critical), and the likelihood of continued hot, dry conditions. While spider mites are not insects, several insecticides will provide control, including the organophosphates chlorpyrifos and dimethoate, and the pyrethroid bifenthrin. Revisit the field 5 days after an application to determine if eggs or small mites are still present; these materials do not kill the eggs and have short periods of residual activity, so a follow-up application might be necessary. (If a follow-up application is necessary, switch the mode of action).  Newer miticides, including products containing abamectin, are more effective and have a longer period of residual activity, but are usually more expensive.

Using the wrong insecticide when spider mites are in the field (whether the mites were your target or not) can be costly in more ways than one. Most pyrethroid insecticides will not provide control, and can actually make the problem worse by removing predators that normally keep mite populations in check. Remember, if an application is not going to make you any money, it’s best to keep it in the jug.

Contact: Dr. Nick Seiter, Field Crop Entomologist | nseiter@illinois.edu


Japanese Beetle Management Guidelines

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:

Japanese beetle adult

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.
Soybean defoliation levels

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 | nseiter@illinois.edu | 217.300.7199