Using Insect Thresholds For Soybean Management Decisions
- Thresholds are not correct anymore because it should be a formula of cost of application and chemical and prices for the commodity, and
- Damage present or expected (numbers of certain insects in sweep net will determine that as well as already occurring damage to plants/pods)”.
This prompted the following article about using insect thresholds as one of the soybean farming tools available to Mississippi soybean producers.
Controlling damaging populations of insects is always a production issue in Midsouth soybeans. The decision to apply control measures is usually based on “thresholds”, which is the number of insects of a particular species that have reached the level thought to reduce yield.
Generally, two methods for counting insects in the crop are used: sweep nets and drop cloths. Both of these methods give objective measures of the insects that are present, so counting them is no problem.
The contents of this article are not meant to agree or disagree with the farmer’s statement in the opening paragraph. In fact, the thresholds for triggering insect control measures that appear in the below tables are the best tool available without committing to years of expensive research involving cage trials in various production environments. However, since the subject was raised, I thought I would at least “open this can of worms” (no pun intended, of course) that is one of the many soybean farming tools available to growers.
| Suggested threshold levels for insect pests in soybeans–Tennessee (TN). | |
| Three-cornered alfalfa hopper | Treat if 10% of young plants (up to 10-12 in.) are infested with adults and/or nymphs. Threshold of 1 hopper per sweep for plants taller than 12 in. |
| Defoliating pests (bean leaf beetles, green cloverworm, blister beetles, loopers, grasshoppers, etc.) | Treat at 30% defoliation until bloom (R1), 20% from R1 to full seed (R6), and 30% after R6 to beginning maturity (R7). Alternatives to defoliating thresholds during R1-R6:Bean leaf beetle: 50 beetles per 25 sweeps;Green cloverworm: 38 larvae per 25 sweeps;Loopers: 19 larvae per 25 sweeps. |
| Corn earworm or fall armyworm | After R1, treat when an average of 9 or more larvae is found per 25 sweeps (or 3 to 4 larvae are found per foot of row). |
| Stink bugs | From R1 to R7, treat when an average of 9 or more stink bugs is found per 25 sweeps (or 1 stink bug is found per foot or row). |
| In soybeans planted on 38-in. or wider rows, sweep only one row. In narrow-row soybeans, allow the normal arch of a sweep net to continue through adjacent rows. | |
| Treatment levels for various larval insect pests of soybeans for different row spacings (RS) using a drop cloth–Arkansas (AR). | |||
| Larvae per foot of row | |||
| RS (in.) | CEW | SL, CL, BAW, VBC | Comments |
| 38 | 4 | 6 - 8 | Treat when worms are ½ in. or longer. |
| 30 | 3 | 4.5 - 6 | For loopers and other defoliators, the number of larvae is in addition to 25% defoliation after bloom. |
| 19 | 2 | 3 - 4 | For drought-stressed fields, reduce CEW threshold levels by one for 30-38 in. rows and by ½ for 9-19 in. rows. |
| CEW = corn earworm; SL = soybean looper; CL = cabbage looper; BAW = beet armyworm; VBC = velvetbean caterpillar. | |||
| Equivalent economic threshold conversion between drop cloth and sweepnet–Arkansas (AR). | ||||
| Drop cloth | Sweepnet | |||
| Insect | No./ft. of row | No./25 sweeps | No./50 sweeps | No./100 sweeps |
| Stinkbugs | 1 | 9 | 18 | 36 |
| SL, CL, VBC, GCW | 6 | 29 | 58 | 116 |
| CEW | 4 | 15 | 30 | 60 |
| SL = soybean looper; CL = cabbage looper; VBC = velvetbean caterpillar; GCW = green cloverworm. Threshold numbers in association with 40% defoliation before bloom and 25 percent after bloom. Number represents medium and large larvae.CEW = corn earworm. Since CEW are difficult to sample with a sweepnet, sweep deeper into the canopy using extra force. Supplement with visual check for bloom and pod feeding. | ||||
| Recommendations for control of insects in soybeans–Louisiana (LA). | |
| Insect | When to treat (economic threshold) |
| Southern green, green, and brown stinkbugs | After pods appear or beginning bloom (R3), treat for 1 stinkbug (1/4 in. long or longer) per ft. of row or 36 in 100 sweeps. |
| Redbanded stinkbug | Treat for 24 bugs in 100 sweeps. |
| Bean leaf beetle* | After R3, treat for 2 beetles per sweep, or when 10% of pods are damaged. |
| Three-cornered alfalfa hopper | Starting at R3, treat for 3 nymphs per ft. of row or 1 adult per sweep. |
| Velvetbean caterpillar^Green cloverworm^ | Treat for 8 worms (½ in. long or longer) per ft. of row or 300 worms in 100 sweeps. |
| Soybean looper^ | Treat for 8 worms (½ in. long or longer) per ft. of row or 150 worms in 100 sweeps. |
| Fall armyworm | Treat when seedling beans are reduced to 6 or fewer plants per ft. of row. In older beans, treat when defoliation becomes excessive*. |
| Beet armyworm^ | Treat for 12 worms (½ in. long or longer) per ft. of row, or 150 worms in 100 sweeps. If pod feeding occurs, treat for 10% damaged pods. |
| Corn earworm | After bloom, treat for 3 worms per ft. of row or 38 in 100 sweeps. |
| *Prior to R1, soybeans can tolerate 30-35% defoliation. From R1 to beginning podfill (R5), defoliation should not exceed 20-25% defoliation.^These defoliating caterpillars should be counted together and an insecticide to control them should be applied when any combination of the four reaches 300 worms in 100 sweeps. But treatment should be made anytime soybean loopers and/or beet armyworms exceed 150 worms in 100 sweeps. | |
| Insect Control Guide for Agronomic Crops–Mississippi (MS). | |
| Insect | Thresholds |
| BLB | Apply insecticide if beetles are present and defoliation reaches 35% before bloom or 20% after bloom; after podset, treat if have two beetles per sweep. |
| TCAH | Plants <10 in. tall, check for stand reduction; plants >10 in. tall, treat when have 25 hoppers per 25 sweeps. |
| GCW, VBC | Before R1, apply insecticide when have 8 or more worms ½ in. long or longer per ft. of row (drop cloth) or 75 worms per 25 sweeps, or 35% defoliation; After R1, apply insecticide when have 4 or more worms ½ in. long or longer per ft. of row (drop cloth) or 38 worms per 25 sweeps, or 20% defoliation. |
| SL | Before R1, apply insecticide when have 8 or more worms ½ in. long or longer per ft. of row (drop cloth) or 38 worms per 25 sweeps, or 35% defoliation; After R1, apply insecticide when have 4 or more worms ½ in. long or longer per ft. of row (drop cloth) or 19 worms per 25 sweeps, or 20% defoliation. |
| CEW | Before R1, treat at 35% defoliation; After R1, treat when have 3 worms per ft. of row (drop cloth) or nine worms per 25 sweeps. |
| BAW, FAW | Before R1, treat at 35% defoliation; After R1, treat at 20% defoliation. |
| SB | One bug (nymphs larger than 1/4 in.) per ft. of row (drop cloth) or 9 bugs per 25 sweeps. |
| RBSB | Four bugs per 6 ft. or row (drop cloth) or 6 bugs per 25 sweeps. |
| BLB = bean leaf beetle; TCAH = three-cornered alfalfa hopper; GCW = green cloverworm; VBC = velvetbean caterpillar; SL = soybean looper; CEW = corn earworm; BAW = beet armyworm; FAW = fall armyworm; SB = southern green, green, and brown stink bugs; RBSB = redbanded stinkbug. | |
Below is a summary of the four states’ recommendations for indicated species.
- TCAH–All states at 25 hoppers per 25 sweeps after plants are 10-12 in. tall.
- SL–19 (TN & MS), 29 (AR), and 38 (LA) worms per 25 sweeps, respectively.
- GCW & VBC–29 (AR), 38 (TN & MS), and 75 (LA) worms per 25 sweeps, respectively.
- CEW–All except AR (15) at 9 per 25 sweeps after R1.
- SB–All at 9 per 25 sweeps during reproductive development (R1-R7).
I recognize that the thresholds published by the various states represent the best information available, and the differences among states is related to latitude and/or the different production environments in each state. However, the question that begs to be asked is “Is it possible that the above thresholds do not represent all soybean production environments?”
It appears to me (but I am not an entomologist) that we should encourage and support the conduct of research that will produce specific insect control recommendations that are geared to specific major cropping systems (e.g., dryland vs. irrigated, early- vs. late-planted).
A good start might be to do this for dryland vs. irrigated soybeans because these two production systems have vastly different yield potentials and subsequently vastly different economic potentials. Further support for starting with these two production systems is past research that shows that:
- soybean looper larval weight is decreased and developmental period is increased by plants growing on drier soils, and
- yield reduction resulting from looper infestation is greater for irrigated than for nonirrigated soybeans [Crop Science, Vol. 31, 1625-1628 (1991), and Vol. 35:1657-1660 (1995)].
In the past few months I have been asked to identify “holes” or areas in our soybean farming tools that may need reevaluation (different from re-inventing the wheel). This may be one of those holes.
larryheatherly@bellsouth.net