Friday, August 6, 2010

Corn Earworm Outbreaks In Soybeans: Early and Heavy in Some Fields, Not in Others

From Ames Herbert, Virginia Extension Entomologist

Corn earworm has moved into soybean fields across much of Virginia. This is earlier than normal and most are faced with the decision to treat (or not) flowering stage beans. Infestation levels vary a lot from field to field from almost no worms in lush canopy fields, to drought stressed open canopy fields with as many as 25+ per 15 sweeps.

Although we have decent thresholds for foliage feeding and pod feeding, there is little guidance as to the impact of flower feeding by corn earworms. Several years ago researchers in North Carolina (Eckel et al. 1992) concluded that flower feeding contributed to yield reductions by delaying pod set, but no exact thresholds were developed. Without good guidelines as to how many can be tolerated, we are suggesting that low to moderate numbers (6 or less medium plus large worms/15 sweeps) could be safe, especially if beans are growing well with good soil moisture. More than that may represent a threat.  Again, just a grab.

We are also getting a lot of calls from folks finding live worms after pyrethroid sprays, in cotton, peanut and soybean fields. Most are using either Karate Z or Baythroid XL and the misses are about equal among the products. The most obvious cases are where there were very high numbers of worms so the survivors are still quite noticeable.

We visited one field that had been sprayed 24 hours earlier and found an average of about 4 to 8 worms/15 sweeps. Since it was still not forming pods, I suggested that it not be retreated, yet. Of course, any field that has been treated will be very susceptible to any subsequent infestations, as beneficial insects would have been mostly eliminated so be sure to begin re-scouting 7 days after the last application. If another treatment is needed, strongly consider a non-pyrethroid.

We are also seeing a mix of worm species including yellow striped armyworms, green cloverworms, and maybe some tobacco budworms. Corn earworm is still the primary species, so I would direct the spray program to those.

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