Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Soybeans
We continue to see a number of defoliators (grasshoppers, blister beetles, Japanese beetles, green cloverworm, bean leaf beetles, yellow striped armyworms, etc.) present in full season and double crop beans. As fields enter the bloom to pod fill stages, remember that the defoliation threshold drops.
Before the recent rain, we continued to find fields with economic levels of spider mites, especially in drought stressed areas of the state. I am sure some will assume that the rain controlled the mites and others will wonder. The answer will depend on a number of factors. In areas of the state where you experienced heavy thunderstorms, those storms may have been forceful enough to cause direct mortality; however, scattered and infrequent showers will not reduce a heavy mite infestation. The rains of course will also help plants to grow ahead of the spider mite damage. However, if economic levels of mites are still present after fields have recovered and greened up, you should get better control, especially with dimethoate. It will also be important to spray before plants are stressed again. You will also want to check carefully to see if populations are starting to decline. Under heavy mite pressure, it often takes a period of favorable conditions to significantly reduce exploded mite populations. So if we see a quick return of hot, dry weather you may not see the decline you would have hoped for. A combination of high humidity, dew and cool night temperatures is often needed to favor the development of the naturally occurring fungus that can crash mites in a few days. So bottom line – you will still need to scout for spider mites.
So what if you still need to control mites. As we learned in 2007, drought will seriously stress plant growth, 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. As indicated in past newsletters, dimethoate, Lorsban (chlorpyrifos) and Hero are labeled on soybeans for spider mite control. The bifenthrin component in Hero is the material that will provide spider mite suppression. However, to be effective it should be applied before mites explode. Please refer to the label for use rates and restrictions – you will need the high rate for spider mite control. It should also be noted that the label states do not make applications less than 30 days apart (http://www.cdms.net/LDat/ld80Q005.pdf). We do not have replicated trial experience with Hero. We plan to evaluate it this year in research trials. In two grower experiences that I am aware of, it appeared to control the populations; however, it also rained within a few days of the applications. In years past, under drought stress conditions, the combination of a timely rainfall before or right after a miticide application has greatly improved control.
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 adjuvants and in 2007 the use of adjuvants did not always help improve control with dimethoate in extremely stressed fields. Like any foliar systemic, 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.
In 2007, Lorsban (chlorpyrifos) provided good contact control of motile mites in situations when applied in enough water to get good coverage. However, since Lorsban is not a systemic product, a second spray 3 to 5 days later 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. (http://www.cdms.net/LDat/ld02A000.pdf).
In speaking with the research and development rep for our area from Dow AgroSciences in 2007 (manufacturer of Lorsban), he suggested that the addition of a petroleum oil may help to improve control with Lorsban. He ran trials with cooperators in 2006 using petroleum oils plus Lorsban on soybeans (such as Damoil and stylett oils) and did not see phytotoxicity. However, I have not had any experience with the use of these oils in soybeans, especially under drought stress conditions. As we know, the Lorsban will only provide control of the motile mites and he suggested from work in other crops that the oil, when applied correctly, should help to smother eggs present and therefore provide a longer period of control.
As indicated last week, continue to watch full season beans that are starting to set pods for stink bugs. We continue to see an increase in both brown and green stink bug populations. Economic damage is most likely to occur during the pod development and pod fill stages. You will need to sample for both adults and nymphs when making a treatment decision. Available thresholds are based on beans that are in the pod development and fill stages. We are currently following the same guidelines that are being used in Virginia. Thresholds are also based on numbers of large nymphs and adults, as those are the stages most capable of damaging pods. As a general guideline, current thresholds are set at 1 large nymph/adult (either brown or green stink bug) per row foot if using a beat sheet, or, 2.5 per 15 sweeps in narrow-row beans, or 3.5 per 15 sweeps in wide-row beans.
We have also found our first corn earworms in a few fields in Sussex County. As soon as pods are present, you will want to start watching for this insect as well.