ISSUE 9 July 10, 2008
WHEAT MIDGE SCOUTING AND THRESHOLDS
The updated wheat midge degree day (DD) maps for North Dakota indicate that wheat midge should be starting to emerge (1,100 for male and 1,300 for female wheat midge DD) in the northern tier this week (see wheat midge DD map). Wheat in the heading to early flowering stages will be susceptible to infestation by wheat midge. After 50% flowering is reached, wheat is no longer susceptible to egg laying by female wheat midge and insecticide applications should be avoided to prevent killing parasitoid wasps that attack and kill the wheat midge.
Scout for adult wheat midge at night (after 9:00 PM) during warm temperatures( 60 degrees F) and light winds ( 6 mph). The adult wheat midge (see photograph) is an orange colored, fragile, very small insect approximately half the size of a mosquito. It is about 0.08-0.12 inch (2-3 mm) long with three pairs of long legs. There is one pair of wings, which are oval in shape, transparent and fringed with fine hairs. The eyes are conspicuous and black in color. Typically, the most significant flight period for the entire wheat midge population extends over a 14 to 18 day window within a region. Individual adult midge may survive from 3 to 7 days, depending on favorable conditions (warm, calm, humid weather).
Adult wheat midge
Economic thresholds are:
- Hard Red Spring Wheat (HRSW) = one wheat midge per 4-5 heads
- Durum = one wheat midge per 8-10 heads
The economic threshold for adult wheat midge is a ‘nominal’ threshold, which means it is based on entomologists’ experiences and not on research data describing the relationship between adult wheat midge infestations and wheat yield / grade. As a result, it is difficult to adjust for the high wheat prices. However, entomologists recommend that producers use the lower wheat midge threshold for both HRSW and durum this year - one wheat midge per 8-10 heads.
TIME TO SCOUT FOR SOYBEAN APHIDS
Soybean aphid populations have been low and below economic threshold levels so far (see NDSU Extension IPM Survey maps). However, aphid populations will continue to increase and soybean fields should be scouted. The current recommended threshold for late vegetative through R5 stages is 250 aphids per plant (field average) on 80% of the plants and increasing aphid populations. Scout 20-30 plants per field and cover at least 80% of the field. To determine if populations are actively increasing, check field over several visits. This threshold provides a 7-day lead time between scouting and insecticide treatment. Research from numerous trials in the upper Midwest, including North Dakota, indicates there is no yield loss at 250 aphids per plant. With higher commodity values, the threshold remains the same. However, response interval time will decrease to only 5 days. It is important to remember that there still is no yield loss at this threshold. Spraying early (below 250 aphids per plants or tank-mixing insecticide with glyphosate applications) is not recommended in North Dakota. Early insecticide or below threshold treatments simply remove any natural enemies in the field, resulting in a field that is open to re-infestation by immigrating aphids. This could lead to the necessity of re-spraying later in the season (source: Ian MacRae, UMN).
Two sources of weekly soybean aphid reports from multiple locations in North Dakota are available:
- the NDSU IPM website
- the Pest Information Platform for Extension and Education (PIPE).
In PIPE, sentinel plots are monitored throughout the U.S. for soybean rust and soybean aphid. State commentaries are also provided.
POTATO LEAFHOPPERS HAVE ARRIVED
Low populations of potato leafhopper have been observed in alfalfa, dry beans, soybeans, and potatoes in eastern North Dakota. This year, migrating potato leafhopper invaded North Dakota in late June from southern states where they overwinter. The adults, which are quite mobile and move from field to field, may migrate from freshly cut alfalfa fields.
The small ( inch), pale green, wedge-shaped adults move rapidly by jumping (see photograph). Nymphs are paler green, lack wings (see photograph) and exhibit a characteristic sideways walk when disturbed. Nymphs can be found on the undersides of leaves. Nymphs are generally more damaging than adults, since they feed for several weeks on the leaves where they hatched.
Adult potato leafhopper (photo courtesy S. Brown,
Univ. of Georgia, Bugwood.org)
Potato leafhopper nymph (photo courtesy F. Peairs,
Colorado State Univ., Bugwood.org)
Sweep nets are useful for confirming potato leafhopper presence in a field. Fields should be scouted to determine whether economic populations are present. Follow these economic thresholds to help make insecticide spray decisions:
Dry bean = 1 leafhopper per trifoliate leaf
Alfalfa = 1-2 leafhoppers per sweep when alfalfa is 8-14 inches high
Soybean = 5 leafhoppers per plant in vegetative stage and 9 leafhoppers per plant in early bloom stages
Potato = 10-20 adults per 20 sweeps, or 1 nymph per 10 leaves
BARLEY THRIPS HIGH IN CENTRAL NORTH DAKOTA
High numbers of barley thrips (>10 thrips per stem) have been reported in the central regions of North Dakota near Harvey and McClusky. Sampling for thrips should begin when the flag leaf is first visible and continue until the head is completely emerged from the boot. Most thrips can be found under the top two leaf sheaths. The dark brown to black thrips can be found by unrolling the leaf sheaths away from the stem. Direct feeding on reproductive tissues also may result in reduced seed weight. With the high market value of barley, the economic threshold is lower in 2008 than in previous years. Using a cost of control of $8.00 per acre and value per bushel of $8.00, the current threshold is only 2.5 thrips per stem. Insecticide treatments are recommended before heading is complete. Methyl parathion (8 - 12 fl oz per acre) is the only insecticide registered for control of barley thrips in North Dakota. The pre-harvest interval is 15 days. Do not enter treated field within 48 hours after methyl parathion application.
Janet J. Knodel