Agronomic Crop Insects – August 31, 2012

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Alfalfa
Continue to sample fields on a weekly basis for defoliators including earworm, webworms and all armyworm species. Economic levels of defoliators, continue to be found causing damage.

Soybeans
We can still find fields with economic levels of corn earworm (mainly double crop), stink bugs and defoliators. There have also been a few fields that need a second corn earworm spray for recently hatched larvae.

As soybeans begin to mature and insects are still active in fields, there is always the question regarding the susceptibility of fields to insect damage, especially full season soybeans. So let’s discuss each group and what we know and/or what experience we have from past seasons.

Stink Bugs
As a general rule, soybeans are still susceptible to damage from stink bugs through growth stage R6.5 (“mid R6”). This has been described by some agronomists as the stage when pods are still green and the lower leaves are just beginning to yellow from natural senescence and not drought stress – approximately 10 days after R6.0 (full green seed). It has also been described by others as mid-way from full seed development until maturity. However, there are a few studies from the south indicating that scouting is needed until beans are in the R-7 growth stage (beginning seed maturity) to avoid damage from stinkbugs which can include underdeveloped or aborted seeds, green stem syndrome, reductions in pod fill, seed vigor and viability, yield loss and a reduction in the storage stability of harvested seeds. In New Castle and Kent Counties, we are starting to see fields with high levels of Brown Marmorated Stink Bugs on field edges of full season soybeans (most fields are growth stage R6). If a treatment is needed, an edge treatment (one spray boom width – at least 50 ft wide) was effective last year, and we are working on a regional project to document the outcome of this strategy in 2012. There are also areas of the state with high native brown and green stink bug populations so be sure to scout for them as well.

Corn Earworms
As far as corn earworms, the experience of most entomologists in the region is that soybeans are most susceptible to corn earworm damage when beans are in the R-5 and early R6 stages although there are cases where we see damage through the R6.5 stage. As of Aug 27, a number of our corn earworm pheromone traps where still catching high numbers of moths so it is too early to say for sure if populations are on the decline (http://ag.udel.edu/extension/IPM/traps/latestblt.html). In addition, trap catches in North Carolina are still high and weather patterns could bring these moths north. We sometimes see a late summer/early fall corn earworm flight which could result in new worm infestations. Most full season fields should be in the “bug-safe” late R6-R7 stage if this occurs; however, double crop soybeans will still be susceptible to attack.

Defoliators
When it comes to defoliators, especially soybean loopers, it is important to keep scouting for them as well through the R6.5 stage. We continue to find pockets of soybean loopers and beet armyworms. If leaves are beginning to yellow up the stem, not from drought but from the maturity process, and there are any pods on the plant that are beginning to yellow, the field should be safe from most defoliators. The exceptions would be grasshoppers and bean leaf beetles, which can scar pods later in the growth stage of soybeans.

NOTE: As we get closer to harvest, be sure to check all labels for the days from last application to harvest as well as other restrictions.

Small Grains
With the increase in no-till wheat acreage as well as our typical rotation of wheat following corn, it will be important to consider a number of insect pests that can present problems. The following article provides a good review of insect pests that pose a threat to wheat in the fall including aphids, the wheat curl mite, Hessian fly and fall armyworm. (http://www.uky.edu/Ag/kpn/kpn_08/pn080825.htm#wheins). In addition to the insect pests listed in this article, true armyworms have been a pest in the past as well as slugs if we have a wet fall.

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