High soybean looper (Thysanoplusia orichalcea) populations (20–40/m2) have been reported in many regions in flowering and podding soybeans. While soybeans can tolerate 33% defoliation with no yield loss during the vegetative stages, their defoliation tolerance slips to 16% during podset/early podfill. Consequently, consider taking action if high looper populations are threatening your crop.
Large soybean looper (38 mm). Note the tapered body (towards the head) and 2 pairs of ventral prolegs.
The preferred IPM option, where loopers are the sole caterpillar in the crop, is a Bt-based biopesticide such as Dipel (1–1.5L/ha) or BioCrystal (0.5–0.75L/ha), both with an amino feed adjuvant at 1L/ha.
Biopesticides such as Bt are best applied in the early morning or evening to minimise their breakdown due to ULV. If you are experiencing very heavy early morning dew, then evening spraying is preferable. Note that for ingestion products such as Dipel, good spray coverage is critical. This also applies to helicoverpa virus products such as Vivus and Gemstar (which have zero impact on loopers), and indoxacarb (which is very effective against loopers).
Small looper larva (10 mm) and typical early damage leaf windowing symptomatic of early damage
Using a selective option such as Bt preserves predatory insects in the crop and reduces the risk of flaring helicoverpa. In flowering crops in particular, the use of Bt against loopers gives growers the option of keeping the moderately-selective indoxacarb (Steward®) option in reserve, should above-threshold populations of the more-difficult-to control helicoverpa make an appearance.
Article by Hugh Brier