Agronomic Crop Insects – July 15, 2016

Joanne Whalen, Extension IPM Specialist;

Field Corn
As we begin to see more field corn entering the silk stage, there have been questions about silk clipping insects and management strategies. Although thresholds have not been developed in Delaware for silk clipping insects, the following information from other areas of the country can be used to help make a management decision.

Japanese Beetles and Other Related Species: As discussed in previous newsletters, silk clipping by these insects is related to damage 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:

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 indicates that they recommend treating for Japanese beetles in corn when an average of three or more beetles is found per ear during pollination. For more information on the research in Tennessee, especially as it relates to management of Japanese beetles in irrigated and drought stress corn, please see the following link:

Corn Rootworm: Although not a common issue in most areas of the state, we have seen problems in years past in an occasional irrigated continuous corn field in Sussex County as well as in areas of northern Kent and New Castle counties where continuous corn is planted. The following links provide information on management:

Stinkbugs: As indicated in past newsletters, most of the damage we see from stink bugs occurs before full tassel emergence. Information from Georgia indicates that corn is most sensitive to stinkbug damage during ear formation before silking. Stink bug feeding damage to small developing ears starting at V12 (vegetative tassel stage) can deform ears into a C or boomerang shape. These ears fail to develop properly and may be more susceptible to infection by diseases. Additional information from North Carolina on sampling and treatment timing can be found at sampling and treatment timing in field corn form North Carolina

During the pollination to blister stages, stink bugs can feed through the husk and damage individual kernels. Although we do not have thresholds for our area, information developed in states to our south can be used to make a treatment decision. From the end of pollen shed to blister/milk stage, the threshold used in the south is one stink bug for every two plants (50% infested plants). Please refer to the following link for more information on stink bug management in field corn

We continue to see a mix of defoliators (grasshoppers, Japanese beetles, 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.

Spider mites populations continue to be spotty but can be found in fields throughout the state. 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 currents so you still need to sample the entire field for mites. We can often find economic levels starting in field interiors. As a reminder, labeled materials include bifenthrin (numerous generics labeled), Hero (a combination of zeta-cypermethrin and bifenthrin), chlorpyrifos (Lorsban and a number of generics) and recently labeled Agri-Mek SC (reminder — this is the only labeled formulation). Be sure to read all labels carefully for all restrictions including but not limited minimum gallonage needed by air, days between applications, pre-harvest intervals, and adjuvants requirements that must be followed to avoid illegal residues.