Delaware Soybeans, Sorghum: Scout Fields Before Making Insecticide Decisions

Soybeans

We continue to find a variety of defoliating insects in soybeans. Although green cloverworm populations remain high in some fields, we are also seeing a large number of moths in fields indicating larvae have pupated and a new population of egg laying moths can be found in fields. A new hatch of small grasshoppers can also be found in many fields, both full season and double crop fields.

As a reminder, in addition to defoliation, grasshoppers and bean leaf beetles being found now can feed on and/or scar pods. In general, pyrethroids will provide control of the general complex of defoliators present (green cloverworm, cabbage loopers, grasshoppers, and bean leaf beetles).

However, soybean loopers and beet armyworm (also present in fields) are not effectively controlled by the pyrethroids. So knowing what species is present will be needed to make an insecticide selection.

Materials labeled for soybean loopers and beet armyworm including Besiege, Blackhawk, Coragen (NOTE – Prevathon is not labeled in DE) Radiant or Steward will be needed. The highest labeled rate is generally needed for soybean looper and beet armyworm control.

Continue to watch for stink bugs in all fields during the pod development and pod fill stages. We continue to see an increase in populations, especially green stink bugs. You will need to sample for both adults and nymphs when making a treatment decision.

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 corn earworm (CEW) larvae in both full season and double crop fields but populations remain low in most cases. With the recent increase in our CEW moth catches in pheromone traps and economic levels of larvae now being found in soybean fields on the Eastern Shore of VA, you will need to sample fields for this insect pest on a weekly basis.

Although CEW populations have been lower the last 2 seasons, this is a more typical year where we will see hot spots of economic levels. 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 which estimates a threshold based on the actual treatment cost and bushel value you enter.

You should also follow what is happening in areas to our south related to CEW pyrethroid resistance. Most of the population we will see in soybeans will come from moths migrating from the south.

NOTE – not all materials mentioned in southern newsletters are labeled in Delaware so be sure to check both the federal and state labels before applying an insecticide.

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. It has been found in Virginia this season so be sure to check fields for this aphid.

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 corn earworm in Virginia): an average of 1- 2 worms per head during seed formation
  • Stink bugs: 2-4 per head at seed milk stage; 4-8 per head during soft dough stage”

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