Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
We have seen an increase in potato leafhopper populations this past week so be sure to sample fields for adults and nymphs. The treatment thresholds are 20 per 100 sweeps on alfalfa 3 inches or less in height, 50 per 100 sweeps in 4-6 inch tall alfalfa and 100 per 100 sweeps in 7-11 inch tall alfalfa.
Soybeans
We can find a number of defoliators in full season soybean fields including grasshoppers, green cloverworm, Japanese beetles, oriental beetles and bean leaf beetles. In some fields, the main defoliator is the green cloverworm. Larvae are light green with three pairs of white stripes running the length of the body. In addition to the three pairs of legs near the head, they have three pairs of fleshy legs near the middle of the body, and one additional pair at the end of the body. Larvae wiggle vigorously when disturbed. Smaller larvae may drop from the leaf when disturbed. Young larvae skeletonize the underside of the leaf. Older larvae chew irregular shaped holes in the leaves and can eat all of the leaf except large veins. Although populations of green cloverworm generally increase in number from July through September, if the weather turns dry, we often see an earlier increase in numbers. Fungal pathogens often crash populations; however, under dry weather conditions this will not occur. As a reminder, double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully, especially for grasshoppers.
Some consultants are also seeing an increase in leafhopper populations in seedling stage soybeans. As a general guideline, a control may be needed for leafhoppers if you see plant damage and you find 4 leafhoppers per sweep in stressed fields and 8 per sweep in non-stressed fields.
Economic levels of spider mites can be found in all three counties in both full season and double crop fields and in many cases infestations are field wide. As we learned in past years, 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), bifenthrin and Hero are labeled on soybeans for spider mite control in Delaware. (Be sure to read all labels before spraying for restrictions and rates).
Hero and Generic Bifenthrin (e.g. Sniper) – The bifenthrin component in Hero is the material that will provide spider mite suppression. Please refer to the label for use rates and restrictions – you will need the high rate for spider mite management. It should also be noted that both labels state: “do not make applications less than 30 days apart”.
Hero – http://www.cdms.net/LDat/ld80Q009.pdf
Sniper (generic bifenthrin) — http://www.cdms.net/LDat/ld7LQ018.pdf
Dimethoate – This material 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. (http://www.cdms.net/LDat/ld3LG000.pdf); (http://www.cdms.net/LDat/ld4PC002.pdf )
Lorsban (chlorpyrifos) – In past years, Lorsban has provided good contact control of motile mites 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 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 of Lorsban 4E or other product containing chlorpyrifos within 14 days of the first application. So before applying be sure to read the label for restrictions, maximum number of applications, etc. (http://www.cdms.net/LDat/ld02A011.pdf )