ISSUE 15 August 14, 2003
SUNFLOWER INSECTS TOPS ON THE FIELD SCOUTING LIST
Sunflower fields should be monitored to determine the need to treat for seed weevil, banded moth, and Lygus. The primary insect scouting will likely focus on is the Red Seed Weevil. Field populations have generally been greatest and prompting treatment in the southern counties of ND. Fields in the northern counties have had lower numbers. Confection fields should still be treated twice to minimize quality losses due to all three of these insects being present. Refer to the Around the State article for the North Central region for more specific details. The southwest part of the state is also reporting some areas with high seed weevil numbers, according to Roger Ashley, Area Extension Agronomist.
SOYBEAN APHID - READY TO TAKE FLIGHT ?
Soybean aphid populations have been causing some problems in the central valley and northwest Minnesota. Recent reports of populations reaching very high numbers, 1,000's per plant, have been received. In these cases, treatments are being applied. Fortunately, these are small areas of activity, and still associated most often with small fields, wooded areas, and field margins.
As we reach this point in the season, several things are expected in the aphid population. Last year, a slow down in population growth was observed in mid-August. In many cases field infestations declined. However, this decline was followed by a very rapid increase and the production of many winged aphids. The winged aphids subsequently abandoned fields quickly in search of other soybeans or perhaps buckthorn.
Fields are also reaching a growth stage where risk of economic damage from aphid infestations is much lower. Fields are in approaching or are in the R-5 growth stage. Descriptions of R-4 and R-5 stages are provided.
Identifying Nymphs of Winged Soybean Aphids
Adult aphids can be winged or wingless. When observing field infestations, outbreaks largely consist of wingless adults. However, there are conditions that stimulate production of winged adults. What are all the conditions? They may not be completely known but include crowding in large colonies, declining quality of the food (e.g., plants are maturing), shortening day length, and others.
An inspection of the nymphs provides a clue whether populations in fields are preparing to be winged and take flight. If a nymph has squared shoulders, a closer look reveals the developing wing buds. Nymphs with wing buds become winged adults in a only a few days and take flight. If the majority of the nymphs are alatoid (developing wings) then migration from fields is expected. If the populations is preparing to take flight, insecticide treatments have not proven to be economical.
Soybean Growth Stages:
Full Pod (R-4)
This stage shows rapid pod growth and the beginning of seed development at the beginning of the full pod stage. Dry weight of pods is greatly increased from R4_R5. There is a pod at this stage which is at least ¾ inch long on at least one of the four upper nodes of the main stem. This stage is the most crucial period for seed yield. Any stress from R4_R6 causes more yield reduction than at any other time. Late pod formation at R4.5 to early seed fill at R5.5 is most critical. Yield reduction at this time is mainly from fewer pods. The last flowering will occur at the main stem tip (through R5).
Beginning Seed (R-5)
Seed filling during this stage requires much water and nutrients from the plant. Redistribution of nutrients in the plant occurs with the soybean providing about half of needed N, P and K from the plant's vegetative parts and about a half from N fixation and nutrient uptake by the roots. Leaf loss of 100% at this stage will reduce yields by 80%; the plant is less able to compensate from stress and vegetative damage. Stress can actually lower yields by reducing pod number and the number of beans per pod, and to a lesser extent by reducing seed size. This stage has seed at least 1/8 inch long in one of the pods on one of the four upper nodes of the main stem. About halfway through this stage, the plant attains its maximum height, node number and leaf area. Nitrogen fixation peaks and begins to drop and the seeds continue a steady period of dry weight accumulation.
Toward the end of this stage, the nutrient accumulation in the leaves peaks and then begins the process of redistributing to the seed. Seed accumulation will continue until shortly after R6.5, with about 80% of total seed dry weight being accomplished.
Source: McWilliams, Berglund, and Endres. 1999. Soybean Growth and Management Quick Guide. NDSU Extension Circular A-1174.
SPECIAL LOCAL NEEDS (SLN) LABEL ISSUED FOR ASANA XL USE IN CRP FOR GRASSHOPPER MANAGEMENT
The North Dakota Department of Agriculture recently issued a Section 24(c) Special Local Needs (SLN) registration for the use of Asana in CRP to control grasshoppers. This registration does NOT allow haying or grazing following an application. However, since CRP is only hayed under special circumstances, this should not be as big an issue as the haying and grazing restriction for Asana in other non-crop sites. In the future, if haying of CRP is anticipated under emergency conditions, the user will need to be aware of these limitations for use of Asana.
Product: DuPont Asana XL (EPA Reg. No. 352-515)
Site: CRP lands
Registrant: DuPont Crop Protection
Target Pest: Grasshoppers
Issue Date: August 7, 2003
Expiration Date: August 4, 2008
STORCIDE FOR WHEAT, OAT, AND FEED BARLEY
In November 2002, a label was issued for Storcide, sold by Gustafson, and its use to control stored product insects in wheat, oat, and feed barley. This stored grain protectant was used by some people in the region last seaason. More people are likely to use it this year.
This product is a mixture of two different insecticide active ingredients, cyfluthrin, a pyrethroid formulated by Bayer, and chlorpyrifos-methyl, an organophosphate insecticide formulated by DowAgrosciences. The product has a broad spectrum of activity, controlling most of our common stored grain pests, including activity against the lesser grain borer.
These two active ingredients are found alone in Tempo, which has been used in empty bin treatments, and Reldan, the protectant that is being phased out of production by DowAgrosciences. It is our understanding that Storcide has been approved for use, but will be a transition product to help bridge the stored product protectant needs to a time when alternatives to Reldan are available.
LYGUS BUGS IN VALLEY SUGARBEET FIELDS
Low to moderate numbers of Lygus bugs have been observed in reservoir crops such as alfalfa and canola during the past few weeks, and small numbers of adults and nymphs (immatures) are beginning to appear in area sugarbeets. Localized infestations have been observed in the central and northern Valley; however, populations are not yet at economically significant levels. The coming week or two should determine whether Lygus densities will be likely to reach damaging levels in the area. Fields should be monitored during the next two weeks as other crops and small-seeded broadleaf weeds continue maturing to determine if treatment will be necessary.
Description. Adults are about 1/4 inch in length, 1/8 inch wide, and their color can range from dark greenish yellow to a dingy, mottled brown. Older adults will usually have the distinctive mottled coloration with lightened wing tips and a pale yellow V-shaped mark near the middle of their back (Fig. 1). Sometimes the "V" is less prominent than seen in this figure.
Figure 1. Lygus lineolaris adult
The Lygus bug passes through five nymphal stages (instars) before reaching adulthood, with each progressive instar increasing in size. First-instar nymphs are very small (1/25 inch long), wingless, and look like a robust, bright green aphid. Also, they have a faint black spot in on the center of their back. Later-instar nymphs will have a total of 5 spots (Fig. 2).
Figure 2. Lygus nymphs
(courtesy, Univ. CA.-Davis)
Damage. Lygus bugs use piercing and sucking mouthparts to feed on plant tissue. They pierce the plant and inject a salivary toxin as they feed that liquifies and kills plant tissue. Feeding injury in sugarbeets is usually concentrated in new leaves and petioles. Symptoms of Lygus feeding injury include curling and wilting leaves, seepage of a black oil-like exudate (Fig. 3.a.), swollen tumor-like feeding scars on petioles (Fig. 3.b.), and often a blackened sooty appearance on new growth near the plant crown.
Figure 3a. Black exudate seeping
from Lygus feeding site injury
Figure 3b. Healed scar
from Lygus feeding
Injury often causes plants to respond by using carbohydrate reserves to produce new leaves and stems. Unfortunately, this later in the season when these reserves should be building up in the root and can result in a reduction of sugar produced by the plant. Both adult and immature stages of the Lygus bug are capable of causing this injury.
Sampling. Lygus bugs are very elusive. Adults often fly away and nymphs usually hide or drop from the plant as soon as the canopy is disturbed. Also, young nymphs blend in well with the beet canopy due to their green color and are, therefore, difficult to detect. Thus, sampling should be done with care to ensure accurate population estimates. Currently, there is no published economic threshold for Lygus bug control in sugarbeet. However, treatment may be justified if at least 1/3 of plants are infested with one or more Lygus bug (adult or nymph).
Control. This insect usually infests beet fields around mid-August. Thus, consideration of pre-harvest interval may be a critical factor in choosing an insecticide (refer to Table 1 for treatment options). Also, border treatments may be effective if highest Lygus densities are along the edges of a given field. If beets are within two weeks of harvest when populations reach the level of "1/3 of plants infested", fields will not likely benefit from an insecticide treatment.
A number of insecticides approved for use on sugarbeets have tarnished plant bug (or the generic term Lygus) as a target pest in their labels for other crops; however, the tarnished plant bug that infests ND and MN sugarbeet is not listed as a target pest in the sugarbeet portion of those labels. These insecticides include Asana, carbaryl (Sevin), Lannate, Lorsban, and malathion. It is legal to apply an insecticide if it is labeled for use in the crop; however, if the target pest is not listed for that crop, efficacy is not implied by the manufacturer and growers that choose to use the product assume their own liability for any unsatisfactory performance. Mustang is also a legal option; however, the preharvest interval (50 days) makes it an unlikely choice because most infestations occur so late in the season.
Table 1. Insecticide options for Lygus bug management in sugarbeet for 2003.
Use Rate / Acre
Preharvest Interval (days)
RUP – Restricted Use Pesticide
Always read, understand, and follow all label instructions and precautions!
Research & Extension Entomologist