Soybean Moth causing major damage in soybeans

Soybean moth Aproaerema simplexella is a very common pest of soybeans but is usually only present in very low numbers. This season high numbers of moths and caterpillars were found especially in the Wide Bay Burnett region with some fields sustaining extensive damage. Similar leaf miners attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax)

Identification
The soybean moth is a small narrow winged moth, up to 6 mm long. The forewings are dark brown/grey, each with a white bar across them, and pale brown hind wings. In soybean crops with large infestations, moths can be seen flying up from the foliage when disturbed.

Small elongated eggs are laid on both the top and underside of leaves, generally near leaf veins. Larvae of soybean moth are pale green/grey and grow up to 7 mm (in length).

Other caterpillars that mine and web soybean leaves include soybean leafminer Lithocolletis aglaozona, a much smaller and less abundant species, and legume webspinner Omiodes diemenalis, common in coastal regions, but which is much larger (up to 15 mm) and is bright green. Legume webspinners also web leaves together, rather than mining leaves (feeding inside them).

Similar leaf miners also attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax).

Damage
Larvae initially feed inside leaves (i.e. in leaf mines) and emerge after approximately four days to feed externally, folding and webbing leaves together to form a protective shelter. Infested leaves are often crinkled and pulled together in the middle and this together with webbing of leaf is the most obvious symptom of damage. In low numbers the larvae only cause cosmetic damage. While larvae normally feed on leaves only, extremely high populations will also graze on the surface of pods, after they have denuded the crop of leaves.

Soybean moth infestations are favoured by hot, dry weather, with crops under severe moisture stress most at risk. Large populations can cause extensive damage by stripping all leaves from crops and so reduce photosynthesis and grain production.

Monitoring and thresholds
Monitor crops regularly for the early warning signs of rare plague events. Look for numerous small, pale patches (leaf-mining) on the leaves and large numbers of soybean moths in the crop or around lights at night.

The indicative threshold is based on defoliation, with. 33% pre flowering and 15-20% defoliation during early to mid pod-fill.

Control
In most years control is not required but large infestations in the Bundaberg region will need chemical control to prevent total crop loss. Check thoroughly before spraying, as larvae may have already pupated (as black pupae within the webbing) or reached full size (7 mm) and stopped feeding.

There are no specific registrations for the control of soybean moth. However pesticides effective against Helicoverpa (except Helicoverpa virus and Bt), and targeting that pest in soybeans will most likely also give reasonable control of soybean moth.

DPI&F plans to investigate the effectiveness of a number of pesticides registered in soybeans (against other pests,) as part of GRDC-funded pulse IPM research project DAQ00086. The hope is to identify at least a moderately selective pesticide to preserve soybean moth parasites such as Temclucha sp., a small Ichneumonid wasp (8 mm). This species has been observed in very high numbers in some crops infested with soybean moth.

Article by Kate Charleston and Hugh Brier