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ISSUE 14    August 4, 2005

SOYBEAN APHID

We are entering the critical period for most fields for soybean aphid and some growers are experiencing treatable populations. The soybean aphid is small 1/16" or less in length, pale green in color, and has two tubular structures (cornicles) protruding from the top of the back end of the aphid. The tips of the cornicles of the soybean aphid are colored black.

You should check from 20 to 30 plants per field and tally the numbers of aphids per plant. An average of 250 aphids per plant is an indicator that the field may be approaching economic levels. Check the field again in two or three days to see if the aphid population is increasing. Continue monitoring your fields for aphids until the populations begin to decline, usually mid to late August.

An insecticide application is advised if you have an average of 250 aphids per plant and the population is actively increasing.

To know if the population is increasing requires a second sample 2 to 3 days later. Aphid populations may be in decline because there are a large number of predators present, especially lady bugs. If you see a number of winged aphids in the field, this is another indication that the population is in decline. Disease can also rapidly reduce aphid numbers.

The best return on an insecticide application will be when the beans are in the late vegetative stage (3/4 to one inch pods). Insecticides will give you 7 to 10 days of protection. Early applications made before the onset of flowering may allow the aphid population to recover and cause damage. Late applications made after full bloom have not given a consistent return.

 

SUNFLOWER SEED INSECTS

As sunflowers begin flowering, yield and quality can be impacted by insects. Larvae of the red sunflower seed weevil, the banded sunflower moth, and Lygus bugs feed on developing seeds and reduce yield. Honey bees and wild bees are also active in sunflower at this time and, in contrast with the pest insects, they increase yield by enhancing pollination.

Red Sunflower Seed Weevil. Reports are showing increasing numbers of seed weevils from around the state, especially in the SC region. Red sunflower seed weevils lay eggs into developing seeds of plants that are at about the R5.3 stage (30% of the disc flowers open) of development. In addition to a yield loss, in confection seeds larval feeding reduces quality.

Scouting for adult weevils should begin when plants are showing yellow ray petals (R5.0) to 30% of the head shedding pollen (R5.3). Continue scouting until most of the plants have reached 70% pollen shed (R5.7). To sample, spray the sunflower heads with mosquito repellent containing DEET and wait 15 or more seconds for the adult weevils to move to the front of the head. Then knock the weevils into a white container for easy counting. Take counts from 5 heads and from 5 locations in the field and calculate the overall field average. The typical economic threshold is one adult weevil per head for confections and about 5 to 6 adult weevils per head for oils.

Red sunflower seed weevil
Red Sunflower Seed Weevil
(Photo courtesy Kirk Mundal, NDSU Entomology)

Banded Sunflower Moth. The banded sunflower moth has been active for some time now but the larvae only impact yield from the start of flowering (anthesis) through seed development. Larval feeding on florets reduces seed set. Older larvae will feed directly on the seed kernels and reduce yield.

If your field is still at the middle stage of bud development (R3), you can sample for banded sunflower moth eggs using the method described in the July 21 issue of the Crop and Pest Report. Otherwise, you will need to scout for adult moths. Count the moths on 20 plants at five sites in the field in the early morning or evening, avoiding the field margins. This is best done from late morning to early afternoon when the moths are resting on the plants. For most growers, an average of one adult moth per 2 plants will be the economic threshold. Because most of the moths stage outside of the field during the day, counts done during the day will be much lower. The economic threshold for moths counted mid-day is one moth per 50 to 70 plants. Avoid sampling the field margins.

Banded sunflower moth
Banded sunflower moth
(Photo courtesy Kirk Mundal, NDSU Entomology)

Lygus bug. Lygus bugs can be a problem for confection seed growers. See the July 28 issue of the Crop and Pest Report for a description of Lygus bugs. Scout for adults or nymphs on the sunflower head or foliage. In confection sunflower, one Lygus bug per plant is the economic threshold. Lygus bugs usually do not reduce yield in oilseed sunflower sufficiently to warrant control.

Adult Lygus bug
Adult Lygus bug

Pollinators. Bees can provide a yield boost to sunflower and should be protected. If an insecticide is to be used, make the application late in the day when most bees are no longer working the field and use an insecticide with low bee toxicity.

Insecticide Application Timing. For all three pest insects, the best time to treat is when three out of 10 plants in the field are beginning to shed pollen. If spraying is done too early, weevils and Lygus can re-infest the field requiring a second treatment. Confection fields, because of their lower threshold, usually require a second treatment 5 to 7 days later.

Gary Brewer
Research Entomologist
North Dakota State University


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