Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu
Reminder – Warrior II is now present in the market place so be sure to read the label to be sure you are applying the correct rate. This new formulation is twice as active (2.08 lbs of active ingredient per gallon) as the old Warrior (1.04 lb of active ingredient per gallon) so be sure to read the label for the correct rates as well as restrictions. (http://www.cdms.net/LDat/ld8JD000.pdf).
Alfalfa
If you have not been scouting fields, be sure to keep a close watch for leafhoppers. Although the recent rains can help to get alfalfa growing ahead of damage, we continue to see an increase in adult and nymph activity. Remember, the nymphs can quickly cause damage and once yellowing is present significant damage has already occurred both in season as well as to the long term health of the stand. During extended periods of hot, dry weather you may need to reduce treatment thresholds by one-third to one half.
Field Corn
We are starting to see an increase in Japanese beetles and corn rootworm beetles feeding on corn silks. Both of these insects can potentially interfere with pollination. However, silk feeding by these beetles will not reduce pollination if they cut the corn silks after pollination has already taken place. As a general guideline, an insecticide treatment may be needed if two or more Japanese beetles or corn rootworm beetles are present per ear and silks are clipped to less than ½ inch prior to pollen shed. As far as stinkbugs, we have no thresholds for our area; however, the following is information from Georgia:
“Corn is most sensitive to stink bug injury during ear elongation before pollen shed. The treatment threshold at this stage is 1 bug per 4 plants (25% infested plants). Once pollination occurs, feeding though the husk causes damage to individual kernels. Kernels are susceptible to damage up until the milk stage (R3) and possibly early dough stage (R4). The threshold at this time is 1 bug per 2 plants (50% infested plants).”
Soybeans
In areas receiving rain it appears to be helping soybeans to grow ahead of some of the defoliators. However, in areas that have not received adequate rain or if conditions become hot and dry again, you will need to keep a close watch on defoliation levels. We have a wide variety of defoliators present in fields including Japanese beetles, green cloverworm, grasshoppers, blister beetles and silver spotted skipper. The best way to make a treatment decision in full season soybeans is to estimate defoliation. With the increased value of soybeans, you should consider using a lower defoliation threshold to make a treatment decision. As a general guideline, you may want to reduce thresholds by at least 1/3. As full season beans enter the reproductive stages, the defoliation threshold also drops. Remember that double crop soybeans can not tolerate as much defoliation as full season beans so be sure to watch newly emerged fields carefully.
As full season beans start to set pods in the next week to 10 days, you will also need to consider stink bugs. We have started 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.
Spider mites can still be found in fields throughout the state. In many cases, if economic levels are still present after a rain it would be a good time to treat, especially with dimethoate, since the plants are actively growing and will be more able to benefit from the systemic action of dimethoate. As a reminder, dimethoate is very susceptible to alkaline hydrolysis (chemical breakdown from high pH water) so pH of water is an important factor to consider with dimethoate applications.