Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to see an increase in populations – both the adult and nymph stage. As indicated before, the nymphs can cause damage very quickly so sample fields on a weekly basis for both stages. Once plants are yellow, yield loss has already occurred. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Field Corn
Last year we received reports of fields with stinkbug damage to developing ears. In years past, most of the damage we encountered in field corn occurred in the seedling to whorl stage, resulting in stunted and distorted plants. We have started surveying field corn for brown stink bugs and so far populations are sporadic. We did see higher numbers in some wheat fields this spring and there is some evidence from the South that brown stinkbug damage is most severe in corn fields next to wheat fields – that is stinkbugs are moving from wheat to corn. However, we have little experience and data from our area. Reports from Georgia indicate they are seeing higher than normal stinkbug populations this spring. David Buntin, Grain Crop Entomologist from the University of Georgia, provided the following information in his June 2008 newsletter on stinkbug damage in field corn, thresholds and timing of applications in Georgia:
“Stink bugs are back. After low levels the last two years, spring rains have increased bug populations especially brown stink bugs. Corn is most susceptible to stink bug injury during ear formation before tasseling. Bugs will feed through the sheath, causing a dead spot on the ear. As the ear expands it becomes distorted and curves, usually outward. Feeding during silking and pollen shed also will kill kernels on the ear. Once the ear has elongated, stink bug feeding during the blister and milk stages blasts individual kernels, usually causing them to abort.
Recent cage studies at Tifton have found no significant affect on corn grain production at infestation of 1 brown stink bug per plant. The old threshold (5% infested plants) that is in the Handbook is too low. I suggest using a 25% infested plants (1 bug per 4 plants) as a threshold during ear elongation to pollen shed and 50% infested plants (1 bug per 2 plants) during the later part of pollen shed and blister/milk stage. Bug is defined as adults and/or large nymphs. Initially stink bugs tend to be more prevalent on the field edge, so only a perimeter spray may be needed.” Here is the link to his newsletter http://www.caes.uga.edu/commodities/fieldcrops/gagrains/newsletter.html.
Be sure to note that a most of products listed in his newsletter are not labeled in our area on corn ( i.e. Declare, Methyl Parathion, Capture, Brigade, Karate are not labeled in Delaware) so be sure to check all labels for use rates and restrictions. Others can not be used during pollen shed if bees are foraging in the area (Penncap-M).
Soybeans
Be sure to sample fields in the seedling stage for bean leaf beetles, grasshoppers and thrips. We are also seeing an increase in green cloverworm activity, so be sure to scout soybeans for all of these defoliators.
We continue to receive reports of no-till fields that need to be replanted due to slug damage. Unfortunately, this is not unusual due to the fact that soybeans were at a very susceptible stage when slugs are able to do the most damage – i.e. plants were behind in growth and slugs are larger. In addition, the mild winter resulted in an increase in the number of slugs, especially adult stage. Although we had a few hot days last week, spring conditions have been very favorable for slugs (i.e. prolonged periods of favorable temperatures of 63 to 68°F combined with evenly distributed rainfall, maintaining the soil moisture at 75% saturation). As a reminder, tillage will be needed when replanting to allow plants to grow ahead of the damage. The slugs will be less active under hot , dry conditions; however, they will still be present in fields all season and if conditions become cooler and wet again soon after replanting they can still cause damage.
Continue to watch carefully for spider mites. Early detection and control is needed to achieve spider mite suppression. In addition to dimethoate and Lorsban, we now have Hero labeled on soybeans. The bifenthrin component in this mix is the material that will provide spider mite suppression. However, to be effective it should be applied before mites explode. Please refer to the label for use rates and restrictions – you will need the high rate for spider mite control. It should also be noted that the label states do not make applications less than 30 days apart (http://www.cdms.net/LDat/ld80Q005.pdf).