3/13/2024 | Volume 27 number 1a
This newsletter and the advice herein are free. You usually get what you pay for.
A SW MN insect forecast for 2024.
The warm, dry winter in Minnesota has doubtless affected some insects, crop pests, and otherwise. Barring extreme cold, it should be late enough in the year to make predictions on 2024 pest problems. While the warm temperatures should improve insect survival, exposure from a lack of snow cover and desiccation from low moisture may hurt some species. Remember, many of our insect crop pests cannot tolerate our cold winters and migrate into Minnesota from the south each year.
My predictions on crop pests are mostly for the southwest quarter of the state but may apply to crops elsewhere. Over the next week or so, I’ll make pest predictions for several crops. Heck I might even guess 50% right. I might as well start with soybeans. As 2024 pest problems develop, I’ll try to provide ideas on the reason for infestations, thresholds, and other management tips.
While you’re waiting for a crop pest outbreak to happen, and who isn’t, here are some points to ponder and consider.
2024 IPM Stuff Soybean Pest Forecast
Soybean Aphid
- Soybean aphid (SBA) issues reappeared in some areas of Minnesota during 2023 after several years of low populations.
- Egg-laying on buckthorn likely went well but the long, warm fall should have allowed predators a bit more time act on buckthorn-colonizing aphids and their eggs.
- The SBA eggs on buckthorn should have wintered well with relatively warm temperatures. SBA could still be vulnerable to a severe spring freeze after the eggs hatch on buckthorn, or delayed soybean planting and colonization.
- Growing season weather and mid-season migration have a large influence on SBA populations.
- Early season aphids can be concentrated in early planted fields near buckthorn. Lower soil moisture, low potassium, or high nitrogen increase the soybean plant sap’s suitability for aphids.
- Moderate temperatures and dry weather favor aphid populations as they increase. Populations can increase rapidly, particularly on early reproductive stage soybeans. As soybean vegetative growth ceases, aphids often leave soybeans in large numbers and migrate over larger distances.
- Scouting should increasingly focus on less mature soybean fields in late August.
- Resistance to pyrethroid insecticides should be presumed, but despite that, SBA can easily be controlled with other available insecticides. Chlorpyrifos will once again be available for use on soybeans. However, only those chlorpyrifos containing insecticides that have been re-registered with your state will be legal to apply.
- Scout and use economic thresholds. You do not need to other control pest insects or spider mites if they are below economic levels. It is usually safest to leave them, and any beneficial insects, alone.
- As always, by July we will start to know whether SBA hotspots are developing and statewide risk.
Bean leaf beetle
- Overwintering bean leaf beetles (BLB) can be killed during cold winter but likely wintered well this year.
- Expect populations to continue as high or higher in 2024. For the past few years, areas in the west central part of Minnesota have seen the highest BLB populations. I expect that will continue and populations may increase in other areas.
- Watch the earliest planted soybean seedlings for overwintering BLB populations. In most cases, early season defoliation is not yield-threatening. Fields with early-season bean leaf beetle infestations need to be watched for leaf feeding and for potential pod feeding in July and August.
- If needed, there are several labeled foliar insecticide options available to protect against early-season defoliation and late-season pod feeding. In seed production fields, seed-applied insecticides will minimize the early-season threat from this insect and the bean pod mottle virus it can transmit.
- I am not aware of insecticide resistance in any Minnesota BLB populations.
- When soybean planting is delayed, bean leaf beetles can sometimes be found in alfalfa before soybeans emerge and where they can provide the first clues to the season’s risk.
Twospotted spider mite
- Populations reached yield threatening levels in some fields in some areas of Minnesota again in 2023.
- Twospotted spider mite (TSSM) females overwinter on perennial vegetation where they should have had an easy time this winter.
- Spider mites are controlled by fungal disease most seasons.
- Hot weather and moisture stress, particularly on reproductive stage soybeans will favor TSSM infestations.
- Mites spin silk threads to disperse on the breeze. Mowing hay can help force mites into row crop fields.
- There are several insecticides and miticides that control two-spotted spider mites in soybeans. Be aware - except for bifenthrin, pyrethroids will not control spider mites and may flare populations.
- Undesirable outcomes often occur from unneeded and early insecticide applications to low TSSM populations.
- Long-term forecasts are iffy, but it will be weather that determines our 2024 TSSM threat.
Soybean gall midge
- I’m curious to see how this “Nebraska-like” winter affects this insect. The soybean gall midge (SGM) winter as cocooned larvae in the soil. Initial research from the UMN Koch lab indicates that overwintering soybean gall midge larvae are tolerant to short periods of cold exposure and their work continues to determine how temperature influences populations.
- Most MN infestations have continued to be non-economic. Most MN SGM populations remained low during 2023 and infestations were not reported from new counties.
- One possible reason for the continued lower 2023 populations may have been dry conditions. This insect is still new to science so there may be other, not yet understood, reasons.
- SGM are vulnerable to several predators and a newly described parasitic wasp.
- Early planting tends to favor this insect.
- Insecticide performance has been inconsistent on this insect.
- In July and August, watch for wilting soybean and dry beans on field edges adjacent to 2023 soybean fields.
- You can find information on current soybean gall midge distribution, the timing of adult activity, webinars, and scouting/management at https://soybeangallmidge.org/.
- Register for an April 5th webinar and updates on the latest soybean gall midge research https://blog-crop-news.extension.umn.edu/2024/03/register-now-for-midwest-soybean-gall.html.
Grasshoppers
- Localized pockets of high grasshopper populations were observed in 2023. The extended warm, dry 2023 fall in SW Minnesota was again ideal for egg laying. High fall egg-laying grasshopper populations pose a potential risk for 2023 crops. Three consecutive dry summers likely increased the risk.
- Winter conditions have little impact on the egg pods the females placed in the soil.
- Depending on the species, nymphs begin to hatch in late May and early June. A grasshopper hatch is often triggered by a warm rain. Two-striped grasshopper nymphs hatch first, red-legged (currently the most abundant in SW MN) and differential grasshoppers hatch later.
- The nymphs need to feed soon after hatching. They starve or succumb to disease during prolonged cold, wet weather.
- Dry weather favors grasshopper populations by reducing entomopathogen levels and may increase movement due to reduced vegetation.
- Focus scouting where grasshoppers were abundant late last summer. Grasshoppers prefer to lay eggs in firm, bare soil such as field edges, CRP, and alfalfa. Red-legged grasshoppers often deposit their eggs in alfalfa fields and several grasshopper species, notably red-legged and differentials, will eggs within soybean fields, particularly in no-tilled fields.
- Unless crop yield is threatened, try to avoid insecticide applications before most nymphs have hatched. However, it is important to scout and control economic infestations when grasshoppers are small and easier to kill. Pyrethroid and diamide insecticides should be effective but don’t cut rates.
- Although the egg hatch of multiple species takes place over a long period of time, 2024 grasshopper infestations will show themselves in June.
Other soybean defoliators
There are other leaf feeding insect pests that can cause concern for soybean growers and their advisors.
Many of these are migratory moths. One advantage of our cold winters, even ones as mild as 2023-24, is that many pest species cannot survive them. As a result, last year’s population of pest locally means little for this this year. Weather conditions for pest populations in the overwintering areas and the right weather systems at the right time to bring insects north are determining factors.
Green cloverworm moths, painted lady butterflies (a.k.a. thistle caterpillar), and black cutworm moths are examples of soybean pests that migrate into the state each year overwintering areas to the south. We track the arrival of the black cutworm and provide updates each spring (see black cutworm reporting network.)
Look for these migrant pests to be more prevalent in growing seasons with periods of prolonged strong southerly winds – just like we saw in the spring of 2023. I neglected to include migrant insect pests in my 2023 insect forecast. Sure enough, it was a big year for green cloverworm in some areas.
And there’s more!
Some of the other defoliators that may create holes in your soybean leaves include red-headed flea beetles, soybean loopers, woolly bear caterpillars, Japanese beetles, and soybean tentiform leaf miner (recently added to the potential soybean pest list). These are rarely economic by themselves. Although specific thresholds for some pests (e.g. green cloverworm, grasshoppers) exist, it is faster and simplest to assess the defoliation by multiple species together. To calibrate your defoliation eye see: Crop protection network soybean insect defoliation training . Economically damaging levels of soybean defoliation from insects are not common in Minnesota.
Be aware of multiple pests in a soybean field but…
Biological control and competition for resources can limit the success of one or both species.
The crop stress from defoliation injury (loss of photosynthetic area) is different than the stress created from sap feeding (loss of water and nutrients). These types of injury are not necessarily additive. Any economic benefit from using a lower aphid treatment threshold, because a few grasshoppers, green cloverworms, or spider mites are present, is far from certain. Base your decisions on the species at an economic threshold. A “kill ‘em all strategy” may lead to less stress short-term but, treating a field early with broad-spectrum insecticides will remove beneficials and can make things worse.
Happy trails,
Bruce