Angie Peltier, UMN Extension crops educator, Bob Koch, UMN Extension soybean entomologist, and Bruce Potter, Extension IPM specialist
While some may confuse soybean or alfalfa looper larvae for green cloverworm larvae and vice versa, there is a quick way to tell them apart. Both loopers and green cloverworms have three pairs of true legs on their thorax and one pair of prolegs on their rear end, but they differ in the number of pairs of prolegs on their abdomens. Alfalfa and soybean loopers have two pairs of abdominal prolegs (Figure 3) and green cloverworms have three pairs of abdominal prolegs (Figure 1).
Another way to tell the loopers apart from green cloverworm is that when disturbed (for example when knocking one from a plant into your hand), green cloverworm wiggles around in a surprisingly 'spirited' fashion.
Green cloverworms, thistle caterpillars, loopers, grasshoppers, and other insects feed on soybean foliage, resulting in lost photosynthetic material. As multiple defoliators can cause defoliation injury simultaneously, treatment thresholds are based on the presence of abundant defoliating insects and the combined defoliation injury severity from insects and other sources. Treatment thresholds change with time as the soybean crop progresses from vegetative (average of 30% injury) to reproductive (average of 20% injury) growth stages.
Keep in mind: when a treatment threshold is 20% defoliation severity, this is an average of 20% severity throughout the entire plant, not just the lower leaves, or the leaves in the mid-canopy or leaves toward the top of a plant. Many insect defoliators prefer to feed on new upper leaves. It is easy to overestimate the effect of this highly visible injury on soybean yield. However, lower, previously shaded soybean leaves can compensate for lost leaf area and new leaves will be produced into the R5 stage.
Then, calculate a whole-canopy average defoliation severity to determine whether thresholds have been reached for plants at your crop's stage of development. Lastly, to avoid making an unnecessary insecticide application, before loading up your sprayer double check that the caterpillars have not pupated or moved. To verify caterpillar presence, use a sweep net or a white sheet laid between soybean rows. Soybean rows are then shaken so that caterpillars fall onto the sheet.
Table 1. Insecticides registered for use in soybean with activity against defoliating caterpillars. Modified from: 2023 North Dakota Field Crop Insect Management Guide, NDSU Extension.
Caterpillar ID 101
The moths that lay eggs that eventually hatch into green cloverworms (Figure 1) and thistle caterpillars (Figure 2) migrated into some Minnesota soybean fields this year and have been causing injury symptoms on leaves as they feed and progress through their caterpillar stages towards pupation. |
| Fig. 1. Green cloverworm larva on a soybean leaf. Notice that the larva has three pairs of legs on its thorax (left) three pairs of prolegs on its abdomen and one pair of prolegs on its rear end (right). Photo: Bruce Potter, UMN Extension IPM specialist. |
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| Fig 2. A thistle caterpillar rolled up in a soybean leaf it is using for protection from predators while it feeds. Photo: Angie Peltier, UMN Extension |
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| Fig. 3. Alfalfa and soybean looper larvae are difficult to tell apart; both have three pairs of true legs on their thorax (right, note that one pair is tilted forward towards its head), one set of prolegs on their rear end (left) and two sets of prolegs on their abdomens. Photo: Bruce Potter, UMN Extension IPM specialist. |
Another way to tell the loopers apart from green cloverworm is that when disturbed (for example when knocking one from a plant into your hand), green cloverworm wiggles around in a surprisingly 'spirited' fashion.
Green cloverworms, thistle caterpillars, loopers, grasshoppers, and other insects feed on soybean foliage, resulting in lost photosynthetic material. As multiple defoliators can cause defoliation injury simultaneously, treatment thresholds are based on the presence of abundant defoliating insects and the combined defoliation injury severity from insects and other sources. Treatment thresholds change with time as the soybean crop progresses from vegetative (average of 30% injury) to reproductive (average of 20% injury) growth stages.
Keep in mind: when a treatment threshold is 20% defoliation severity, this is an average of 20% severity throughout the entire plant, not just the lower leaves, or the leaves in the mid-canopy or leaves toward the top of a plant. Many insect defoliators prefer to feed on new upper leaves. It is easy to overestimate the effect of this highly visible injury on soybean yield. However, lower, previously shaded soybean leaves can compensate for lost leaf area and new leaves will be produced into the R5 stage.
How best to estimate defoliation injury
In order to avoid unnecessary insecticide applications that will have negligible effect on yield potential, it is recommended that one learn what 20% and 30% defoliation severity looks like. Keep in mind that these defoliation thresholds for treatments are an average of the severity of defoliation throughout the entire canopy, not just the lower leaves, or the leaves in the mid-canopy or leaves toward the top of a plant and not just one spot in the field. To estimate percent defoliation, select multiple plants located in multiple areas of each soybean field, collect a leaf from the lower, middle and upper canopy of each of those plants and estimate percent defoliation for each leaf. One can 'train their eye' to better estimate soybean defoliation severity by taking the "Soybean Insect Defoliation Training" quiz until comfortable.Then, calculate a whole-canopy average defoliation severity to determine whether thresholds have been reached for plants at your crop's stage of development. Lastly, to avoid making an unnecessary insecticide application, before loading up your sprayer double check that the caterpillars have not pupated or moved. To verify caterpillar presence, use a sweep net or a white sheet laid between soybean rows. Soybean rows are then shaken so that caterpillars fall onto the sheet.
Insecticides registered for soybean with activity against defoliating caterpillars
There are multiple insecticides labeled and registered for use in Minnesota soybean with activity against foliage feeding caterpillars (Table 1). Always read the pesticide label. Pay special attention as many of the labeled insecticides are “restricted use”, meaning that one needs to have a valid Minnesota Department of Agriculture issued pesticide applicator license. Caution is also warranted when considering whether to use insecticides with lamba-cyhalothrin as an active ingredient in those areas of the state that are abnormally dry or suffering from drought, as this active ingredient can flare two-spotted spider mite population densities.Table 1. Insecticides registered for use in soybean with activity against defoliating caterpillars. Modified from: 2023 North Dakota Field Crop Insect Management Guide, NDSU Extension.
| Soybean insecticide a.i. Ex. trade name |
Insecticide group~ |
Label rate per acre |
Pre-harvest interval |
|---|---|---|---|
| acephate Acephate 90 PRILL Acephate 90WDG Acephate 97 Acephate 97UP |
1B | 0.28 - 1.1 lbs 0.28 - 1.1 lbs 0.25 - 1 lbs 0.25 - 1 lb |
14 days Do not graze or harvest for hay or forage |
| afidopyropen + alpha-cypermethrin** Renestra |
9D + 3A | 6.8 fl oz |
21 days Do not feed or graze hay or forage |
| alpha-cypermethrin** Fastac CS Fastac EC |
3A | 1.3 - 3.8 fl oz |
21 days Do not feed or graze hay or forage |
| beta-cyfluthrin** Baythroid XL |
3A | 0.8 - 2.8 fl oz |
21 days for seed 15 days for hay and green forage |
| beta-cyfluthrin + imidacloprid** Leverage 360 |
3A + 4A | 2.4 - 2.8 fl oz |
21 days for seed 15 days for hay and green forage |
| bifenthrin** Bifender FC Bifenture EC Brigade 2EC Capture LFR Discipline 2EC Fanfare EC Sniper Sniper Helios Sniper LFR |
3A | 2.4 - 7.4 fl oz 2.1 - 6.4 fl oz 2.1 - 6.4 fl oz 2.8 - 8.5 fl oz 2.1 - 6.4 fl oz 2.1 - 6.4 fl oz 2.1 - 6.4 fl oz 2.1 - 6.4 fl oz 2.8 - 8.5 fl oz |
18 days |
| bifenthrin + chlorantraniliprole** Elevest |
3A + 28 | 4.8 - 9.6 fl oz |
18 days |
| bifenthrin + imidacloprid** Brigadier |
3A + 4A | 3.8 - 6.1 fl oz |
45 days for feeding dry vines 18 days for feeding green vines |
| bifenthrin + imidacloprid Skyraider Swagger |
3A + 4A | 2.1 - 6 fl oz 7.6 - 12.2 fl oz |
18 days |
| bifenthrin + sulfoxaflor Ridgeback |
3A + 4C | 4.5 - 13.8 fl oz |
18 days |
| bifenthrin + zeta-cypermethrin** Hero |
3A + 3A | 2.6 - 10.3 fl oz |
21 days Do not graze or harvest for hay, straw, forage or feed |
| carbaryl Sevin XLR Plus |
1A | 0.5 - 1.5 qts |
21 days for seed 14 days for grazing or harvest for forage |
| chlorantraniliprole Coragen Prevathon Vantacor |
28 | 3.5 - 5 fl oz 14 - 20 fl oz 0.7 - 2.5 fl oz |
1 day |
| chlorantraniliprole + lambda-cyhalothrin**^ Besiege |
28 + 3A | 5-10 fl oz |
30 days |
| cyfluthrin** Tombstone Tombstone Helios |
3A | 0.8 - 2.8 fl oz |
45 days for seed and feeding dry vines 15 days for green forage |
| deltamethrin** Delta Gold |
3A | 1.4 - 2.4 fl oz |
21 days for seed Do not graze or harvest for hay, straw, forage or feed |
| esfenvalerate** Asana XL |
3A | 2.9 - 9.6 fl oz |
21 days for seed Do not graze or harvest for hay or forage |
| lamba-cyhalothrin**^ Grizzly Too Kendo Lambda-Cy EC LambdaStar Paradigm VC Silencer Silencer VXN Warrior II |
3A | 0.96 - 1.92 fl oz 1.92 - 3.84 fl oz 1.92 - 3.84 fl oz 1.92 - 3.84 fl oz 1.92 - 3.84 fl oz 1.92 - 3.84 fl oz 1.92 - 3.84 fl oz 0.96 - 1.92 fl oz |
30 days for seed Do not graze or harvest for hay, straw, forage or feed |
| lamba-cyhalothrin + thiamethoxam**^ Endigo ZC |
3A + 4A | 3.5 - 4.5 fl oz |
30 days for seed Do not graze or harvest for hay, straw, forage or feed |
| methomyl** Lannate LV |
1A | 0.4 - 1.5 pts |
14 days for seed, 12 days for hay 3 days for forage |
| methoxyfenozide Intrepid 2F |
18 | 4 - 8 fl oz |
14 days for seed 7 days for hay and forage |
| permethrin** Arctic 3.2 EC Permethrin Perm-UP 3.2 EC |
3A | 2 - 4 fl oz 2 - 4 fl oz 2 - 4 fl oz |
60 days for seed Do not graze or harvest for hay or forage |
| zeta-cypermethrin** Mustang Maxx |
3A | 1.28 - 4 fl oz |
21 days for seed Do not graze or harvest for hay, straw, forage or feed |
**Restricted use pesticide; requires a valid, commercial or non-commercial Minnesota Department of Agriculture-issued pesticide applicator license.
^Caution: insecticides or mixtures containing lambda-cyhalothrin can flare two-spotted spider mite infestations when present.
~Rotating among insecticide groups is a strategy to reduce the speed at which pest populations shift to resist an insecticide.
References
Knodel, J. et. al. 2023 North Dakota Field Crop Insect Management Guide. NDSU Extension.
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