I am going to use bacillus thuringensis for western grape leaf skeletonizers. When is the best time to apply the BT? My vineyard is in Santa Fe.
Santa Fe County New Mexico viticulture
Hello Mr. Kerr,
The best time to apply BT for the skeltonizers is when you see the young larvae.
They have to get the BT in their gut where it will take effect, and the best time to do this is in the spring time. You can catch them if you monitor your grape leaves typically around bloom time.
With the later generations, there doesn't seem to be anything that is very effective from an organic perspective. Even Entrust (spinosad) needs to get into the insects through their guts, and so are only effective when the eggs first hatch in the springtime. The only other options would be reduced-risk pesticides at this point for the "knock-down" effect - http://cekern.ucanr.edu/Entomology/Research/Completed_Research/Skeletonizer_Control_in_Grapes_with_R....
Next year, you may also want to see if you can use a granulosis virus (see: http://ipm.ucdavis.edu/PMG/r302301011.html), since it appears that it is fairly effective in California.
If you want/need more local help, please contact Bernd Maier at NMSU - firstname.lastname@example.org.