NDSU Crop and Pest Report


ISSUE 3   May 20, 2004


Reproductive Soybean Development Stages and Soybean Aphid Thresholds

A greater understanding of soybean aphid treatment decisions comes along with the completion of another season of research and field observations. Treatment decisions was the focus of many winter meetings throughout the areas where soybean aphid is now established.

The greatest development is the better understanding of the yield loss relationship between soybean growth stages and soybean aphid populations. The results of numerous replicated trials around the north central states have resulted in modeling yield loss in relation to aphid populations expressed as Aphid Days. Currently, it is recognized that a population of 4000 aphid days is resulting in approximately 4% economic losses (yield or quality losses equivalent to the cost of implementing a management strategy) when these populations occur from the R1 to R4 growth stages. Though these same populations will cause yield loss in R5 stage soybeans, the impact on yield loss has not been as great or the yield response as predictable.

An aphid day is defined as a single aphid feeding for 24 hours. Aphid days is a cumulative number used to express the total impact of an ever changing population over time. Aphid days also allows for better comparisons between treatments, locations and other variations observed when conducting these trails. However, aphid days is a cumulative number that does not necessarily provide a good "snapshot in time" treatment threshold approach.

To illustrate the different visualization of Aphid Days versus Aphids per plant, here are two graphs summarizing the soybean aphid populations recorded last season from the economic threshold study conducted at Prosper, ND.


For 2004, the recommended treatment threshold will be 250 aphids/plant on 80% or more of the plants when populations are actively increasing and the plant is in the R1 to R4 growth stage. Actively increasing aphid populations in the R5 stage are going to be a little more difficult to make decisions. However, if prices remain strong for soybean, even a 1 to 2 bushel advantage could provide a positive economic benefit in the R5 growth stage that in different economic times may not be realized.

The treatment threshold is a level of aphids where economic loss has not yet occurred, but if left alone could reach the level that economic injury does result. It is based on economic injury occurring when 4000 aphid days have accumulated. Since aphids per plant will be used for making treatment decisions, it is important to be able to predict when field populations are growing at a rate that will bring the population up to the economic injury level. If we model the aphids population growth, and accumulate aphid days at the same time, by selecting the 250 aphids per plant treatment threshold, the crop manager will have a 7 day window before the population is expected to reach the 4000+ aphid days level. This model assumes aphid populations are doubling every two days, an assumption based on laboratory observations of soybean aphid population growth. By taking this approach, it allows scheduling of applications or additional field scouting in time to prevent losses or continue monitoring for further changes in the population.

There are numerous revisions of publications that summarize this treatment threshold information. One very nice extension circular is from Wisconsin. It can be viewed and downloaded at:




Every spring people ask the question, "What are these bright red insects crawling on the ground, and will they damage the seedlings?"

First, these small creatures (1/8 inch) are not insects but mites (Acari: Trombiculidae) and are known as Red Velvet Mites. They are quite active following spring showers. Their bright color, in stark contrast to the browns and blacks of our soils, makes them very noticeable, even to a casual observer.

These mites are parasitic and predatory. The immature stages are parasitic on insects. The adults prey on insect eggs and other small arthropods. Since they are predatory, they pose no threat to young plants emerging in fields.

Red Velvet Mite
(Photo by John Kalisch, Univ. of Neb.- Entomology)

Phillip Glogoza
Extension Entomologist



First instar grasshopper nymphs (or young grasshoppers) were collected in sweep net samples in grassy ditches at Minot and Mohall this week. These were identified as our two-stripped grasshopper, Melanoplus bivittatus, which feeds as a generalist on our agricultural crops. Start scouting for these tiny grasshoppers in field edges and ditches. Young grasshopper feed in the general area that they emerge from and move only short distances making them easier to control now. If economic thresholds exceed 50-75 nymphs per square yard in field margins or 30-45 nymphs per square yard in fields, an insecticide spray is recommended. Insecticides registered in ditches and non-crop areas for grazing and haying are: carbaryl (Sevin), dimilin, and malathion. Insecticides registered for non-crop areas that do NOT include grazing and haying are asana and warrior.

Dimilin is an insect growth regulator and has several benefits compared to synthetic insecticides. For example, lower cost, longer residual about 30 days, and non-toxic to humans, honeybees and other beneficial insects. With the 30 days of residue, applications of dimilin could be applied in mid to late May to kill early emerging grasshoppers and subsequent emergence of young grasshoppers.

Dimilin (diflubenzuron):  Crompton Uniroyal Chemical

Grasshopper susceptible stage: Best results will occur when grasshoppers are in the 2nd to 4th instar stages. The growth stage between each molt is called an "instar." On average, grasshoppers will molt every six to nine days.

Mode of Action: Dimilin disrupts the formation and deposition of chitin in the grasshopper’s exoskeleton, interrupts the grasshopper nymph’s ability to molt, and affects its coordination and feeding habits.

How long before seeing control effects? Grasshoppers must feed on dimilin and then molt before populations are reduced. Effects of treatment typically being to appear within five to seven days after treatment, as the grasshopper nymphs fail to molt and die. In addition, dimilin causes grasshoppers to become malformed, and to exhibit impaired coordination and feeding habits. This makes them easy prey for natural predators like ground beetles.

Any control with adults? Dimilin does not control the adults. However, the consumption of dimilin by adults reduces their feeding rates, escape behavior as well as mating and egg production.

What is the application rate? Dimilin is registered in rangeland, pastures and non-crop areas at the rate of 0.5_1 fl oz per acre. If vegetation is dense in non-crops areas or pastures, a 2 fl oz per acre is also recommended.

Volume and adding a crop oil to solution? By air, use 1_5 gallons of water per acre. Add 1 to 8 pints of crop oil per acre to the solution when conditions favor water evaporation (high temperature, low relative humidity). Dimilin may be aerially applied in an Ultra Low Volume (ULV) solution containing at least 4 fl oz of crop oil for a total spray volume of 12 to 32 fl oz of solution per acre. Do not exceed 1 part crop oil to 2 part water. By ground, use 5 to 30 gallons of water per acre for non-crop areas and 5 to 20 gallons per acres for rangeland/pastureland. In ground applications, do not exceed 2 quarts of crop oil per acre.

What is the control effect? Studies conducted by the South Dakota Department of Agriculture and USDA-APHIS have shown that just 1.0 fl oz of dimilin per acre resulted in 61_86 percent grasshopper mortality seven days after treatment. Control increased to 98-99 percent mortality after 14 days of treatment and reached as high as 100 percent control 21 days after treatment.

How long is the residual? Dimilin remains active on the plant tissue for at least one full month. That means it’s still present to control delayed hatching nymphs, later hatching grasshopper species, and secondary infestations that may occur during this 30 day period.

Rainfast? Dimilin is rainfast and it will continue to control grasshoppers well after rains have occurred.

Safety? Dimilin is safe to livestock, birds, fish, honey bees, and beneficial insects. It also had low mammalian toxicity and is low risk for the pesticide applicator. Dimilin can be applied while bees are actively foraging. Dimilin is toxic to aquatic invertebrate animals. It should NOT be applied by ground within 25 feet, or by air within 150 feet of bodies of water including lakes, reservoirs, natural ponds, marshes, rivers, or streams.

Preharvest interval? NO haying or grazing restriction except on alfalfa forage. Observe a one-day preharvest interval when cutting alfalfa forage. There is no need to relocate cattle or other livestock when dimilin is applied.

High pH problems? Dimilin is NOT affected by water pH.

What is the cost? Dimilin costs about $235 per gallon. So, a 1 fl oz per acre rate would cost about $1.84 and 2 fl oz per acre about $3.67 per acre.



Will the seed treatments provide enough protection again flea beetles in the early planted canola? Crucifer flea beetles have been detected on yellow sticky traps and sweep net samples at Minot and Mohall. Spring emergence is now underway. There is concern with the early planted canola and the effectiveness of commercially applied insecticide-fungicide seed treatments, like Helix xtra and Prosper, against flea beetle control. The residue of these seed treatments is about 21_26 days after planting. After that, a foliar spray may be needed on top of the canola if heavy flea beetle feeding activity continues into late May and June. The economic threshold for flea beetles is 25% foliar damage in seedling canola.

Remember, flea beetles can move very fast when weather conditions are warm (>60°F) and quickly defoliate a small canola seedling in large numbers. Capture and Warrior (both pyrethroids) are labeled as foliar sprays on canola and can be safely tank-mixed with the herbicides labeled in canola. A residual of 7-10 days can be expected from these pyrethroids. Flea beetle feeding activity has continued into late June for the past several years and usually peaked during late May to early June. However, when the canola plant is 6_8 leaf stage, it can usually out-grow any flea beetle damage and compensate for feeding injury. However, flea beetles can also cause injury to the center growing point and kill the canola seedling under extremely high populations.

Research conducted at the NCREC, Carrington REC, and Langdon REC over the past several years has shown that a seed treatment plus a Capture (1.3 fl oz/acre) foliar spray applied at 21 days after planting has provided the highest and most consistent canola yields compared to seed treatments alone or a single or double application of Capture. This research was conducted under moderate to heavy flea beetle pressures.

Janet J. Knodel
Area Extension Specialist Crop Protection
North Central Research and Extension Center



Cutworms were found in sugarbeet fields in Southern Minnesota and the Red River Valley. Cutworms usually live in burrows or tunnels in the soil. Cutworms feed on plants above or below soil level, depending on species. Cutworms are most damaging to seedlings in sugarbeet, since they will feed and cut off the stem bases.

When scouting for cutworms, look for dead or wilting plants along the rows. Look carefully for disturbed soil near the base of cut plants. Cutworms can be traced by their tunnels of disturbed soil to their hiding place during the day.

Chemical control for cutworms is recommended when there is about 4-5% of cut seedlings. Insecticides labeled for control include Asana, Lorsban 4E, MustangMax, Lannate, and Methyl parathion. Since cutworms feed mainly at night, the best time to apply insecticides will be during late afternoon.

Mohamed Khan
Extension Sugarbeet Specialist

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