Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to scout fields on a weekly basis for leafhoppers and defoliators including corn earworm, webworms, fall armyworm and beet armyworm. Larvae must be small to achieve effective control. Defoliators can be destructive, especially during drought conditions. When defoliators are present, early harvest may eliminate the problem. Although there are no specific thresholds, as a general guideline if the crop is more than 2 weeks from cutting and 25 to 30 percent of the terminals are damaged, treatment is suggested.
Soybeans
Be sure to watch for defoliators in both full season and double crop soybeans. In full season soybeans, the defoliation drops to 10-15% defoliation at the bloom to pod fill stages. Double crop soybeans cannot handle as much defoliation as full season fields at the pre-bloom or pod-fill stages. In addition to our normal mix of defoliators (i.e. grasshoppers, green cloverworm, and Japanese beetles), be sure to watch for beet armyworm. There are reports of a few fields with beet armyworm in Virginia. Since beet armyworm larvae have been found for the last 3 weeks in pepper and watermelon fields in Sussex County we know it has migrated to our area. Corn earworm larvae can also be found at low levels in full season and a few double crop fields. With the drought stressed corn in our area, the potential for earworm populations may be higher this season.
During the winter season, we discussed the potential for a new insect pest of soybeans, the Kudzu Bug. This insect has been attacking soybeans in Georgia, South Carolina, North Carolina and now a number of counties in southern Virginia. The Delaware Soybean Board has provided us with funding for a survey for this insect pest, So far we have not detected any on soybeans or Kudzu. We will keep you posted on what we find. The following is a link to a webcast by Dr. Jeremy Greene from Clemson University. The webcast contains information on tentative treatment threshold recommendations, expected control with insecticides, the history of the distribution of the insect in the USA, identification of the life stages of the species, and data demonstrating potential yield losses due to Kudzu bug.
http://www.plantmanagementnetwork.org/edcenter/seminars/soybean/kudzu/
With the extended drought conditions in our area, we continue to find significant populations of spider mites in both full season and double crop fields as well as in irrigated fields. In most cases, infestations at this time are field wide, so edge treatments will not be effective. As we have 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, rotation among available control options and multiple applications are often needed under drought stress conditions. Under high population pressure, a single treatment is often not adequate to kill all the life stages. Mite eggs will not be affected by the initial knockdown/control of adults and nymphs and thus hatch after a few days. The only available materials for spider mite management in soybeans in Delaware are dimethoate, Lorsban (as well as generic chlorpyrifos products), bifenthrin (numerous generics available) and Hero. (Be sure to read all labels before spraying for restrictions and rates). Unfortunately, we do not have a selective miticide labeled for soybeans like we do in vegetable and fruit crops. The following is a summary of what we have seen so far this season as well as a summary of results from past seasons.
Hero and bifenthrin Products – A number of products containing the active ingredient bifenthrin are available for spider mite management in soybeans. Some examples include Brigade, Bifenture, Frenzy and Sniper. In addition, Hero, a combination product including both bifenthrin and zeta-cypermethrin (two pyrethroids) is also available. In many cases, these materials have provided good initial control but a second treatment has been needed in some fields, especially if populations were exploded at the time of treatment and numerous mite eggs were present. Early detection and control is needed as with all of the materials available for mite management in soybeans. In addition, most of the labels for products containing bifenthrin state “do not make applications less than 30 days apart or do not apply more than once every 30 days“. Therefore, you will need to rotate to a material with a different mode of action if a second application is needed.
Dimethoate – In past years, dimethoate has not provided effective spider mite management under drought stress conditions. However, this year we have received reports of fairly good control in some situations but it should be noted that rain was received in those areas. Although dimethoate is the only systemic material available for spider mite management in soybeans it 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.
Note – the dimethoate label states it has a “ 7-day reapplication interval. “
Lorsban (chlorpyrifos) – We have seen good initial control of mites with Lorsban and other generic chlorpyrifos products this season. A second application with another material has been needed, especially if populations were exploded at the time of treatment and numerous mite eggs were present. Lorsban (and other generic chlorpyrifos products) can provide good contact control of motile mites when applied in enough water to get good coverage. Since Lorsban is not a systemic product, a second spray of non-chlorpyrifos product may be needed in 5 to 7 days 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 of Lorsban 4E or other product containing chlorpyrifos within 14 days of the first application.
So before applying any material, be sure to read the label for rates as well as all restrictions including but not limited to the total number of applications allowed, rotation between materials, minimum number of days required between applications as well as the pre-harvest interval between last application and harvest. Spider mite management is never easy under drought stress conditions. Early detection and multiple applications of materials with different modes of action are often needed to reduce losses from this pest in soybeans. As a reminder, under heavy mite pressure and extended hot, dry weather, it often takes an extended period of free moisture on leaves, high humidity during the day and cool evening temperatures to get an increase in the fungal pathogens that can significantly reduce exploded mite populations.