Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Alfalfa
Continue to sample for potato leafhoppers on a weekly basis. Although adults are the main life stage present, we should start to see the first nymphs in the next week. 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
Be sure to watch carefully for armyworms moving out of small grains and into adjacent field corn. In some cases larvae are large and are on the move out of small grain fields. Remember, worms must be less than 1 inch long to achieve effective control. The treatment threshold for armyworms in corn is 25% infested plants with larvae less than 1 inch long. Large larvae feeding deep in the whorls will be difficult to control.
Small Grains
ARMYWORM ALERT – We have received numerous reports this week of high levels of armyworms in both wheat and barley fields that did not receive an earlier insecticide treatment. As reported in previous newsletters, trap catches were unusually high this spring. Just to review: Young larvae (less than 1/2 inch long) generally feed on the upper leaf surface. Larger larvae feed heavily on the leaf blades and weeds. The last instar (1.5 inches long and greater) will consume 80 percent of all the plant material eaten during their larval development. This stage lasts six to eight days before moving into the soil to pupate. Heavy defoliation of the flag level can result in significant economic loss. Armyworms generally begin head clipping in wheat when all vegetation is consumed and the last succulent part of the plant is the stem just below the grain head. Larvae can feed on the kernel tips of wheat, resulting in premature ripening and lower test weight. On barley, head clipping occurs sooner and significant clipping can occur in a short time.
In some cases worms, the worms are large and may pupate soon – however, this can be hard to judge. We are receiving reports of armyworms moving out of fields and into neighboring homeowner yards. However, in many cases there is a mixture of worm sizes so the potential for head clipping is high, especially in barley. Although beneficial insects and pathogens can help to reduce populations, it is unlikely that the recent rains will result in a crash in populations. Be sure to scout all fields carefully and watch for head clipping. The treatment threshold in barley is one per linear foot of row and in wheat it is one to two per linear foot of row.
Since barley is close to harvest, the only available control option for barley at this point is Lannate which has a 7-day pre-harvest interval (PHI) between application and harvest (http://www.cdms.net/LDat/ld183010.pdf).
As far as wheat, Lannate (7 day PHI) or Mustang MAX (14-day PHI – http://www.cdms.net/LDat/ld8G1002.pdf ) will be the only available control options at this time due to the PHI of the other products.
As a reminder, the preharvest interval for Warrior is 30 days between application and harvest. Baythroid, which is only labeled on wheat, also has a 30-day PHI and the label lists armyworm (first and second instars). Be sure to check all labels before spraying for rates, restrictions and days between last application and harvest.
Soybeans
Continue to sample for bean leaf beetles and grasshoppers. We continue to see an increase in activity for both insects. In general, the treatment threshold for grasshoppers is one per sweep and 30% defoliation. Sprays may be needed sooner if stand loss is occurring. Early detection and control of small grasshoppers is necessary to achieve control. Numerous products are labeled for grasshopper control including a number of pyrethroids, dimethoate, Furadan (currently under review by EPA for cancellation but FMC rep says it should be available this year), Lorsban, Orthene 97 and Sevin XLR. As far as bean leaf beetle, we are seeing high numbers in areas of the state where they were also present in previous years. In those same areas, bean pod mottle virus was also detected for the first time in 2007. Even though the feeding by first-generation beetles on soybean leaves has seldom resulted in economic yield losses (except if virus is vectored), fields should be scouted carefully to assess the damage. There are numerous treatment guidelines available. However, as a general guideline, a treatment may be needed if you observe a 20 to 25% stand reduction and/or 2 beetles per plant from cotyledon to the second trifoliate stages. The Iowa State economic threshold for cotyledon stage is 4 beetles per plant. Once plants reach the V1 and V2 stages, their thresholds increase to 6.2 (V1 stage) and 9.8 (V2 stage) beetles/plant. These treatment thresholds should be reduced if virus is present or you suspected virus the previous season.
Grass Hay Fields
If you have not done so already, be sure to also check grass hay fields for true armyworm larvae. During the last outbreak, high populations could be found in grass hay fields and economic losses occurred in a number of fields. Although no thresholds are available, Mustang Max and Warrior are now labeled for armyworm control in grass hay fields. Be sure to check the labels for use rates and restrictions (including days to harvest) – Mustang MAX – http://www.cdms.net/LDat/ld8G1002.pdf and Warrior – http://www.cdms.net/LDat/ld5JH041.pdf.