Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa and Grass Hay Crops
Continue to watch for defoliators in grass hay crops and alfalfa. We continue to see economic levels in an occasional field. Significant damage can occur in grass hay fields from true armyworm and fall armyworm. It is important to catch populations before significant damage has occurred and when larvae are small. In addition to checking labels for rates, 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 populations still remain relatively low; however, we can still find spotty infestation of economic levels of small larvae in double crop fields. Trap catches as of early in the week have started to show a slight decrease. Although it is extremely dry in all areas of the state, a combination of early morning dew with hot, humid temperatures may help increase the incidence of fungal pathogens that can help to regulate populations. However, only scouting will tell if this is occurring. It is also important to continue scouting to determine if a treatment is needed since population levels will vary from field to field. Once pods are present, the best approach to making a decision on what threshold to use for corn earworm is to access the Corn Earworm Calculator developed at Virginia Tech (http://www.ipm.vt.edu/cew/) which estimates a threshold based on the actual treatment cost and bushel value you enter.
As far as defoliators, we have started to see an increase in hatch of small grasshoppers and newly emerging bean leaf adults (generally grey in color) in double crop soybeans. Although both insects can defoliate soybeans it is also important to consider their ability to feed on pods resulting in the potential for moldy beans this fall. For more information on decision making for bean leaf beetles and pod feeding please check the following link http://extension.entm.purdue.edu/fieldcropsipm/insects/bean-leaf-beetle.php.
Soybean loopers continue to be present in many fields in Sussex and Kent counties and they are also increasing in states to our south as of last week. This is a migratory pest and although we have been finding low levels for the last month we have seen an increase in populations over the last 10 day period. In addition, it is difficult to control and pyrethroid resistance has been documented in states to our south. Be sure to select a material that lists soybean looper control on the label. In most cases, higher labeled rates will be needed so be sure to read all labels for rates and restrictions, including but not limited to days to harvest and rotational restrictions.
We continue to find fields with economic levels of native stink bugs (mainly green stink bugs), especially in fields that have reached the R-5 stage (beginning seed – seed is 1/8 inch long (3 mm) long in the pod at one of the four uppermost nodes on the main stem). 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. Studies from the South say 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. As a general guideline, we are using a new threshold in the Mid-Atlantic Region — 5 stink bugs per 15 sweeps. This is the threshold for soybeans produced for grain. If you are producing soybeans for seed, the threshold is still 2.5 per 15 sweeps. We are also starting to see an increase in brown marmorated stink bugs (BMSB), only on field edges bordered by woods. A new BMSB threshold of 3 to 5 total adults + medium and large nymphs in a 2-minute visual count is being used this season. For BMSB, a single, well-timed field-edge-only treatment can provide season-long control.
Small Grains
As you make plans to plant small grains, you need to remember that Hessian fly can still be a problem. Since the fly survives as puparia (“flax seeds”) in wheat stubble through the summer, you should still consider this pest as you make plans to plant small grains. Although damage in our area has generally been the result of spring infestations, we can see damage in the fall. Plants attacked in the spring have shortened and weakened stems that may eventually break just above the first or second node, causing plants to lodge near harvest. Plants attacked in the fall at the one-leaf stage may be killed outright. Wheat attacked later in the fall will be severely stunted, with the first tillers killed and plant growth delayed. Plants infested in the fall can be recognized by their darker than normal bluish coloration and leaves with unusually broad blades. The following combinations of strategies are needed to reduce problems from Hessian fly:
(a) Completely plowing under infested wheat stubble to prevent flies from emerging.
(b) Avoid planting wheat into last season’s wheat stubble, especially if it was infested with Hessian fly.
(c) Avoid planting wheat next to last season’s wheat fields – the most serious infestations can occur when wheat is early planted into wheat stubble or into fields next to wheat stubble.
(d) Eliminate volunteer wheat before planting to prevent early egg-laying.
(e) Do not use wheat as a fall cover crop near fields with infestations.
(f) Plant after the fly-free date. (Oct 3 for New Castle County; Oct 8 for Kent County; and Oct 10 for Sussex County).
(g) Plant resistant varieties. You should look for varieties that have resistance to Biotype L. You will need to check with your seed dealers to identify varieties that our adapted our area.
The following link from Alabama provides additional information on Hessian Fly Management (http://www.aces.edu/dept/grain/HessianFly.php).