Agronomic Crop Insects – July 24, 2009

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Soybeans
We are starting to see an increase in the levels of bean leaf beetles in full season soybeans, especially on the western half of the state from Greenwood through Middletown. Remember, at this time you will need to consider a treatment both for defoliation as well as consider their ability to feed on the pods. At the pod fill stage, the defoliation threshold drops to 10-15% defoliation. This insect can also feed on pods. Bean leaf beetles can clip pods or plant diseases may enter the pod through their feeding sites. This can result in seeds that appear shrunken, discolored, and moldy, resulting in a reduction in seed quality. Although we have not established thresholds for pod feeding in our area, the following link provides information that is used in the Midwest (http://www.ipm.iastate.edu/ipm/icm/2000/8-21-2000/lblroof.html). When possible, a material with residual control should be used for bean leaf beetle control. However, the presence of other pests, especially mites, may impact your selection of a control material.

Although isolated at this point, we are seeing a few fields where soybean aphid population levels continue to increase in New Castle County. Population increases are favored by cooler temperatures. The treatment threshold established in the Midwest is 250 aphids per plant from R1 through R5 stage of growth. Updates on Soybean Aphid can be found at the USDA Public PIPE website: http://sba.ipmpipe.org/cgi-bin/sbr/public.cgi?host=All%20Legumes/Kudzu&pest=soybean_aphid. The following links from the University of Wisconsin provide good information on sampling, stages of soybean growth and development, thresholds and treatment guidelines: http://www.plantpath.wisc.edu/soyhealth/aglycine.htm and http://www.plantpath.wisc.edu/soyhealth/pdf/aphid_thresholds.pdf.

Spider mites continue to be found in fields throughout the state, especially along field edges. Remember, early detection is needed to achieve control. In addition to dimethoate and Lorsban, we now have Hero (zeta-cypermethrin + bifenthrin) as well as a number of stand alone bifenthrin products (not all are labeled so be sure to check the label) available for spider mite control in soybeans. All of these products need to be applied before mites explode. Be sure to read the labels for use rates and restrictions — there is a limit on the number of applications as well as the time between applications on all of the materials labeled for spider mite control.

When it comes to spider mite control, the following are a few things to consider:

(1) Drought conditions favor mite development and create plant growth conditions that make it difficult to achieve effective control. Early detection and multiple applications are often needed under drought stress conditions. Under high population pressure, a single treatment may not be adequate to kill all the life stages. Mite eggs may not be affected by the initial knockdown and thus hatch after a few days.

(2) When dimethoate was used in past years, growers reported that the addition of a penetrating surfactant helped to improve control, especially in drought stressed fields. Although we have also observed this in grower fields, we do not have any research data on the use of adjuvants. Dimethoate must be absorbed and translocated by the leaf tissues to provide residual action; otherwise, it undergoes rapid photodecomposition from sunlight. This leaf absorption process is greatly reduced in drought-stressed plants that have “shut-down” physiologically. Another important factor that plays a role in the performance of dimethoate is the pH of the water used as the carrier. Many pesticides, especially dimethoate, are subject to breakdown by alkaline hydrolysis (http://www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/ICG_08/01_Intro_08.pdf). “In alkaline water (high pH), there is a break in certain bonds in the dimethoate molecule, causing two or more new molecules to form. This increases the decomposition rate of the insecticide and can result in poorer than expected field performance. Dimethoate degradation is also accelerated by the mineral content of the water, especially the presence of iron. If a high pH situation exists, you can lower the alkalinity of the water in the spray tank by adding an acid-based buffer. An important consideration is to select a buffering product that lowers the pH to the acid range without causing phytotoxicity. Also, the buffer must be added to the spray tank first, before the addition of dimethoate.” Also, be sure to read the new dimethoate labels for use restrictions including maximum number of applications and reapplication intervals.

(3) Lorsban (chlorpyrifos) has provided good contact control of motile mites in situations where it is applied in enough water to get good coverage. However, since Lorsban is not a systemic product, a second spray of another material may be needed to kill newly hatched mites. The Lorsban label states that: “(1) When large numbers of eggs are present, scout the treated area in 3-5 days and if newly hatched nymphs are present, make a follow up application with a non-chlorpyrifos product and (2) do not make a second application within 10 days of the first application.” So before applying be sure to read the label (like all products) for restrictions, maximum number of applications, etc.

(4) Hero and bifenthrin products – In fields where these products have been applied this season, we are seeing control at 7-10 days after application. However, treated fields have not been evaluated this week to see if the control is holding. Last season, we did see resurgence of populations after application if populations were exploded at the time of application. Like all spider mite control products, these materials must be applied before mites explode and you will need the highest labeled rate for spider mite control. It should also be noted that the labels state: “Do not make applications less than 30 days apart”.

You should also scout for stinkbugs and podworms as we enter the pod set and pod fill stages. As corn earworm trap catches increase, open canopy blooming fields will be attractive to egg laying earworm moths. A treatment should be considered for earworms if you find 3 podworms per 25 sweeps in narrow fields and 5 podworms per 25 sweeps in wide row fields (20 inches or greater).

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