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Soybean aphid attacking late-maturing fields in N. Indiana
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By John Obermeyer & Larry Bledsoe
Purdue University
8/29/2008 10 a.m. CDT WEST LAFAYETTE -- Recent phone calls from pest managers in northern counties of Indiana, especially where very dry, indicate that some soybean fields still in R4/R5 growth stages are “dripping” with aphids. To boot, in some of these fields, spider mites populations are building.
Because of the tremendous variability in aphid and mite numbers from field to field, diligent scouting NOW can pay big dividends. The following is a quick scouting review. Remember, if your beans are at the R6 growth stage, there is little benefit to treating aphid or mite populations unless the field is under drought stress.
Sampling: Count aphids, primarily on the undersides of leaves, on at least 20 plants in various areas of the field. When aphids are just beginning to colonize soybean plants, they will be concentrated on the most active growing points -– the newest unrolled leaves and the developing pods. Shortcuts: if aphids are observed on the petioles and stems, that plant is over 250 aphids. If honeydew and sooty-mold are obvious as you walk the field, the threshold has been exceeded.
Aphid Number: Should you find an average of 250 or more aphids/plant during the pod and seed development stages (R1-R4), a treatment is justified. Threshold of ≥250 aphids/plant includes a week to get field sprayed. During the seed-fill stages (R5-R6), treatment is not as clear-cut. If aphid numbers are increasing and plants are under stress, however, a treatment is justified, see this online threshold graphic. Do NOT treat soybean beyond the R6 stage of growth, the plant is already beginning to senesce and any aphids that are there will not impact yield.
Spider Mites: Exact thresholds are not developed because of the complex-interacting factors between crop, environment, and the mites. Basically, if extensive leaf discoloration is apparent, spider mites are positively identified in the field, and hot, dry conditions are expected to persist, then controls are recommended.
Weather: High temperatures (90s) are less than optimal for soybean aphid growth and reproduction, but don’t stop them. Rainfall, including some hard downpours, has had some negative effect on aphid populations, but does not wipe them out. Spider mites only thrive under dry conditions, because moisture stressed plants actually provide a more nutritious feast than do healthy ones and dry conditions reduce naturally occurring pathogenic fungi that kill mites. Complicated, isn’t it!
Predators: Most pest managers calling to report aphid numbers are reporting fewer “good guys” than normal. Seems as though this sudden surge of aphid and mite activity has overwhelmed the slower-developing beneficial insects.
Treatment: Should control for aphids and/or mites be necessary, complete coverage on the foliage seems to be the key. Ground driven rigs applying at least 20 gallons per acre with 40 PSI at fine to medium droplet size will help penetrate the canopy. Aerial application success is dependent on finished spray volume (we recommend 5 gallon/acre) and air movement into the canopy. If spider mites are significant within fields with soybean aphid, then organophosphate products, i.e., chlopyrifos should be considered alone or in a tank mix.
Products labeled for soybean aphid and spider mite control can be viewed online.
Source: Purdue University.
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