Agronomic Crop Insects – August 28, 2015

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

Alfalfa and Grass Hay Crops
We are starting to hear reports from consultants about an increase in armyworm levels in grass hay crops. Significant damage can occur in grass hay fields and alfalfa 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 when larvae are small and before significant defoliation occurs.

Soybeans
Corn earworm larval populations still remain relatively low; however, we are hearing reports of increasing numbers in some double crop fields in Sussex County. In addition, since pheromone trap catches remain steady you will need to watch fields closely over the next few weeks. As full season fields approach the “worm-safe” growth stages (late R6-R7); they will be less susceptible to attack. However, double crop soybeans will still be susceptible to attack. Since population levels will vary from field to field, the only way to know if you have an economic level will be to scout all fields. 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 seen an increase this week in soybean looper populations (i.e. a new hatch of small larvae) in some areas of the state. There are also reports of increased populations of this migratory pest in soybean fields in states to our south. Green cloverworm populations have increased in some areas and decreased in other areas. So, the only way to know if you have an economic level will be to continue to scout all fields for defoliators. Please refer to past newsletters for decision making guidelines and chemical controls, especially as it relates to soybean loopers that are known to be resistant/tolerant to pyrethroids in states to our south.

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 continue to find soybean aphids in an occasional field. The economic threshold for soybean aphid established in the Midwest is 250 aphids per plant. Populations should be increasing and most of the plants should be infested (>80 percent) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5). Spraying at full seed set (R6) has not produced a consistent yield response in the Midwest.

Sorghum
If you are following newsletters from my colleagues to the south, you are aware that there is a new aphid, the sugar cane aphid, that is moving our way and it can quickly cause significant damage in sorghum. So far this season, it been detected in North Carolina and recently there is a report of an infestation in VA that has not yet been confirmed. Sugarcane aphids are yellow and can be distinguished from other aphids in sorghum by the presence of black tailpipes on the tail (cornicles) and black feet below their yellow legs. These aphids will often infest entire fields, which is rare for our native aphids. If you suspect a sugar cane aphid infestation, be sure to contact us for confirmation.

In some years, we can see issues with insects causing economic damage in sorghum heads. The following decision making and management information is from my colleague at Virginia Tech, Dr. Ames Herbert

“Sorghum is susceptible to several insect pests. Both stink bugs and corn earworm are highly attracted to the heads once seed begin to form and both feed directly on those seed. Later planted sorghum is especially attractive to these pests as late sorghum heads offer a nutritious food source when many other host crops are reaching a stage that is no longer preferred. We have seen sorghum heads in Virginia with large numbers of worms and severe head damage. We have also seen heads with stink bugs feeding. Growers should check all fields to determine if insecticide sprays are needed. The best and only efficient way to sample heads is to shake individual heads into a white 5 gallon bucket. Worms and stink bugs show up well in these buckets and can be easily counted. Sample several heads throughout the field and determine the average number of stink bugs and worms per head. Thresholds taken from several other states are pretty consistent:

Head worms (mostly corm earworm in Virginia)—an average of 2 worms per head

Stink bugs—2-4 per head at seed milk stage; 4-8 per head during soft dough stage”

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