Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. We continue to find the first nymphs as well as adults in fields. Although both life stages can damage alfalfa, the nymphs can cause damage very quickly. Once plants are yellow, yield loss has already occurred. 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.
Field Corn
Continue to watch fields that are next to maturing small grains for true armyworms, cereal leaf beetles and native brown stink bugs
(I) True Armyworms (TAW) – Fields planted next to barley and wheat fields, especially untreated fields, should be scouted for armyworms moving from small grains into adjacent corn fields. Control will be difficult once larvae move deep in whorls. Remember, worms must be less than 1 inch long – some labels indicate that larvae need to be even smaller – to achieve effective control. The treatment threshold for true armyworms in corn is 25% infested plants with larvae less than one inch long.
(II) Cereal Leaf Beetles (CLB) – We continue to find a few fields with CLB adult feeding. Beetle adults can be found moving out of untreated small grains and feeding on the edge of corn fields. Although we do not have any firm thresholds for this insect on corn, as a general guideline controls may be needed if you find an average of 10 beetles per plant and 50% of the plants exhibit feeding damage. In the Midwest, it has been reported that the adult beetle is a vector of maize chlorotic mottle virus (MCMV) that causes corn lethal necrosis disease. Thresholds for beetle feeding would be much lower if this disease is an issue. We have not seen this virus in Delaware corn fields; however, please let us know if you suspect a problem.
(III) Native Brown and Green Stink Bugs –As indicated in past articles, sampling in wheat can give an indication for the potential for problems in nearby corn fields. Information from North Carolina indicated that if you capture 5 or more stink bugs in 20 sweeps in nearby wheat you should be vigilant for movement into corn. If you do not have a nearby wheat field, you will want to check weeds and forested areas near your corn, especially if you have weedy ditches nearby. These areas can provide holding places for stink bugs that can move in and out of fields. For more information on management of stinkbugs in field corn, please visit the following link:
https://entomology.ces.ncsu.edu/2016/05/how-to-avoid-a-stink-bug-disaster-in-corn-2/?src=rss.
Soybeans
As soon as soybeans emerge, be sure to begin sampling fields for the following seedling stage insect pests:
(I) Grasshoppers: We continue to find grasshoppers present in emerged no-till full season fields. As barley is harvested and soybeans are planted, these fields will be especially susceptible to attack by grasshoppers which can cause stand loss. If stand reductions are occurring from plant emergence to the second trifoliate, a treatment should be applied. Although no precise thresholds are available, a treatment may be needed if you find one grasshopper per sweep and 30% defoliation from plant emergence through the pre-bloom stage.
(II) Bean Leaf Beetle: As a general guideline, a treatment may be needed for bean leaf beetle if you observe a 20 – 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages. These treatment thresholds should be reduced if bean pod mottle virus is present in your area and/or you suspected virus the previous season.
(III) Thrips: Thrips can feed and reproduce on the leaves and buds of soybean seedlings. Their feeding creates bleached-out lesions along the leaf veins and gives a silvery/bronzed appearance to the leaf surface when damage is severe. These insects are very small (less than one tenth of an inch) and are torpedo shaped. While thrips always occur on seedling stage soybeans, it is only during outbreak years that they cause concern. In particular, during dry weather and on earlier planted full-season soybeans, thrips populations can explode when plants are growing slowly. Under these circumstances thrips injury will occasionally kill seedlings. Other stressors, such as nutrient deficiencies and herbicide injury, can add to thrips damage and cause plant loss. Yellowing can occur from thrips but there are also a number of other factors that can cause yellowing so it is important to scout fields to identify what is causing the yellowing. Although no precise thresholds are available, as a general guideline, treatment may be needed if you find 4-8 thrips per leaflet and plant damage is observed.
As a reminder, OP insecticides (examples – dimethoate or Lorsban) cannot be combined with SU/ALS herbicides. Since other materials may also state restrictions regarding combinations of insecticide and herbicides, you should be sure to check all labels carefully before combining insecticides and herbicides.
Slugs can also still be found damaging soybeans, especially in fields with heavy cover crop residue. The slugs are able to survive under the cover crop and feed on the soybean seeds underground, especially if the seed slot is not closed, as well as on cotyledons as they are trying to emerge. If a field needs to be replanted, tillage before replanting is critical to help manage the slugs. If a treatment with a bait product is needed, labeled products include Deadline MPs (the most common metaldehyde formulation labeled and available in Delaware), Iron Fist and Sluggo. Be sure to read all labels before making an application. If you plan to use Deadline MPs be sure to look at the new label since the maximum rate allowed per acre per application is lower on soybeans compared to corn and it can only be broadcast up to the V4 stage. http://www.cdms.net/ldat/ld2GN006.pdf