Agronomic Crop Insects – September 11, 2015

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

Alfalfa and Grass Hay
Continue to watch for defoliators in grass hay crops and alfalfa including earworm, webworms and all armyworm species. If a field was cut recently and has started to regrow be sure to also watch for damage to the re-growth. In addition to checking labels for rates and pre-harvest interval (time needed between last application and harvest), be sure to check for all restrictions, including, but not limited to, comments on control under high populations and size of larvae; days to harvest and forage/silage restrictions. No thresholds are available; however, controls should be applied before significant defoliation occurs.

Soybeans
Corn Earworm – Corn earworm larval levels still remain generally low in most fields throughout the state. Most trap catches have significantly decreased over the last week but there are still a few locations with high trap catches and in some cases moths can be found laying eggs in late double crop soybeans. So, late planted soybean fields that still have susceptible pods will still be at risk from pod damage.

Defoliators – This past week the predominant defoliators found were still soybean looper (although populations have dropped off), grasshoppers and newly emerged bean leaf beetle adults. If economic levels are present, you will also need to consider the maturity of the crop as well as the health of the leaf canopy to make a treatment decision. In an article printed in 2010 regarding defoliation from soybean loopers, entomologists and agronomists in the South suggested that “if economic levels of defoliation are present, fields will need to be protected as long as the pods are still green and until the lower leaves are just beginning to yellow. This should correspond, more or less, with the R6.5 stage (10 days after R6.0 = full green seed). If leaves are beginning to yellow up the stem from the maturity process, and there are any pods on the plant that are beginning to yellow, the field should be safe, that is, no need to treat. Next you have to determine the health of the leaf canopy: is it robust, average, or thin. Each can tolerate different amounts of leaf loss before reducing yield potential. Robust fields (mid chest or higher) can tolerate a lot of feeding. Average fields (upper thigh to mid chest) can tolerate normal amounts of feeding. Thin canopy fields (mid thigh or below) cannot tolerate additional leaf loss. Also you need to estimate defoliation. Be sure to look at the entire canopy from top to bottom not just the more affected top leaves to come up with an overall average.”

Stinkbugs – When it comes to stinkbugs, you should continue scouting until the latest planted fields reach the mid R-6 stage, when beans should no longer be susceptible to direct loss from stink bug feeding. Once soybeans reach mid R-6 to R-7 (beginning seed maturity) , studies from the South say that scouting may still be needed to avoid quality damage from stinkbugs which can include underdeveloped or aborted seeds, green stem syndrome, reductions in seed vigor and viability, and a reduction in the storage stability of harvested seeds

Pod Scarring – You also need to consider the potential for grasshoppers and bean leaf beetles to feed on pods. With the recent increase in bean leaf beetle populations in some areas of the state, you will need to examine pods for feeding damage. During the last wet fall, we did see significant pod scarring from bean leaf beetles late in the season that resulted in moldy beans. The following link from Ohio State University gives guidance on treatments related to percent pod injury from actively feeding beetles on beans that are still filling:
http://corn.osu.edu/newsletters/2013/2013-27/late-season-soybean-insects).

The following link from Purdue also gives additional information on decision making regarding the number of bean leaf beetles per sweep, percent pod injury and when you need to consider a treatment.
http://extension.entm.purdue.edu/fieldcropsipm/insects/bean-leaf-beetle.php

Soybean Aphid – We continue to here reports of economic levels. To review treatment decision making (information developed in the Midwest): (a) the current economic threshold for aphids is still set at 250 aphids per plant through the R5 growth stage (3 mm long seed in the pod at one of the four uppermost nodes on the main stem) with an increasing population. (b) You should check fields at least twice before making a treatment decision. If you find 250 per plant you need to re-check in 3-4 days to see if the population is increasing. This insect can be controlled by beneficial insects so be sure to watch for natural enemies including lady beetles, parasitized aphids and fungal pathogens that can help to crash populations.

Reminder — If you do need to treat for any of the above insects, be sure to check the label for the pre-harvest interval (time needed between last application and harvest) as well as other restrictions, including rotational restrictions.

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