Author: Will Gay, Colquitt County Vegetable Agent
As pressure has been high this year, I would like to share some questions that I have been asked regarding management of DBM larvae.
- What does the pH of my tank water need to be to maximize efficacy of insecticides?
Answer: Generally, if pH is going to affect efficacy of a product, this will be specified on the label. I have searched most of the labels for insecticides used in cabbage. Lannate was the only insecticide with pH mentioned on the label, out of all of the ones I looked up. Lannate specified optimal pH of 7-7.5. Dibrom label does not specify but a separate publication of product information does note where hydrolysis is sped up in higher pHs causing dibrom half life to decrease much more drastically. (I have this along with a list of optimal pHs I have collected from companies for several insecticides we are using in cabbage, I can provide this to you). I feel we are going to be fine for most water sources; however, we may be in a gray area when we get to 8.5 or above for alot of chemistries
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Considerations
- Your pH of your tank water is subject to change each time you add a chemistry.
- Some chemistries may be dropping pH without you being aware. (observed this last week with grower and commonly used insecticide in cabbage)
- Kocide label specifies that potential for phytotoxicity (injury) may increase at or below a pH of 6.5.
- If you use an adjuvant to lower pH, you may not know how another chemistry would be affected by this adjuvant.
BOTTOM LINE
Test your water, read label or contact a rep and/or county agent if you have a concern, realize that pH may drop when you add a tank mix partner, if you are going to attempt to correct pH, do so cautiously.
2. Are BT Insecticides still working?
Answer: BT insecticides are an important tool to your DBM management program. To help maximize efficacy and protect these products, Dr. Stormy Sparks recommends that we alternate Xentari with Dipel every worm cycle. Out of 6 bioassays we have collected in the month of March, we recieved 10% control, 60% control, 60% control, 80% control, 80% control, and 100% control. This is when collecting 2nd and 3rd instar worms as well. Keep in mind that according to Dr. Sparks, two 1/2 pound applications twice a week are better than one 1 pound application once a week.
3. Has pressure decreased?
Answer: Pride cometh before the fall. I honestly do not know. It looked as if pressure was dropping every where I had been consistently looking, until I found a moderate number of freshly hatched out larvae on older cabbage yesterday with 50% of plants being infested by extremely small worms. I could not find a worm there last week, except for an older one here or there. On one hand, this is a testament to scouting and I am glad that we caught this population so early, but on the other hand……It gave me a sick sick sick feeling. Continue to scout. I would be glad to come look with you, these worms can be extremely small when they hatch out and the more eyes the better.
Will Gay, Colquitt County Vegetable Agent, 229-921-1992