Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields on a weekly basis for leafhoppers. In past years, we have also seen an increase in thrips when weather conditions turn hot and dry. Reports from other areas of the country indicate that thrips feeding on the developing leaf tissue can cause the leaves to distort as they emerge. Leaves may also be curled, with a cupped or puckered appearance. Although there are no thresholds for thrips in alfalfa, the following information from other areas of the country may be helpful when considering the need for thrips management: “(a) high populations of bean or onion thrips may cause damage, especially in dryland conditions and (b) if a thrips treatment is contemplated, it is best to cut as soon as possible and treat the regrowth if the infestation persists. Thrips are very difficult to control in alfalfa, so excellent coverage is important and two applications may be required for satisfactory results.”
Field Corn
As expected, Japanese beetle populations have increased in fields that are in the silk stage. As discussed in previous newsletters, damage from silk clipping generally occurs before 50% pollination. The following link to a fact sheet from Purdue provides good information on scouting and decision making. There is also an IPM tip at the end from Bob Nielsen about how to determine what percent of the pollen has been released.
http://extension.entm.purdue.edu/fieldcropsipm/insects/corn-japanese-beetles.php
As a general rule, treatment for Japanese beetle may be needed if silks are clipped back to less than half an inch when less than 50% of the plants have been pollinated and Japanese beetles are still present and actively feeding. Pollen shed for an individual tassel generally takes 2-7 days to complete and 1-2 weeks for an entire field (information from Bob Nielson, Purdue University).
Additional information and research from states to our south can be found at the following link: http://news.utcrops.com/2013/06/japanese-beetles-on-corn-silks/ .
Soybeans
We continue to see a mix of defoliators (grasshoppers, Japanese beetles, yellow striped armyworm and green cloverworm) in full season and double crop soybeans. As a general guideline, treatment decisions for defoliators should be based on the following thresholds:
(a) Full Season Plantings – 30% defoliation pre-bloom; 15% defoliation from bloom through the end of pod fill; 35% defoliation- once fully developed seeds are present
(b) Double Crop Plantings (especially if growth is poor) – 20% defoliation pre-bloom, 10% defoliation from bloom through pod fill; 15% defoliation – once fully developed seeds are present.
We are starting to see soybean fields in Kent and Sussex counties with economic damage from spider mites. Early detection and control before populations are exploded is necessary to achieve effective control. Although edge treatments can be effective in some cases, in other cases mites have already been transported into the field interiors on wind current so you still need to sample the entire field for mites. We can often find economic levels starting in field interiors.
We are also starting to see an increase in native stinkbug populations (native green and brown) in soybeans. Economic damage from stinkbug occurs during the pod development and pod fill stages. You will need to sample for both adults and nymphs when making a treatment decision. Available thresholds are based on beans that are in the pod development and fill stages.