Here is the latest insect degree day information based on daily temperature data from the Minnesota Climatology Working Group.
As of May 22, and based on the temperature data collected from the Ft. Snelling station, our area has accumulated approximately 526 insect degree day units using the simple degree day calculation method (insect base temp 50 degrees F).
One way to use degree day information is to look at insect life cycles and take the steps needed to protect plants. For example, cabbage butterfly adults should have started to emerge at 150-240 DD, and they should continue to be present in gardens until at least the end of May. Mated females will start laying yellow eggs around the edges of cabbage leaves and other crucifers. Yellow sticky card traps should be set out for monitoring. Plant leaves should be checked from underneath for yellow eggs laid singly or along a line; these can be crushed on sight. Note: not every yellow egg should be crushed – clusters of yellow eggs on cabbages could be ladybeetle eggs. Don’t crush them. If in doubt, take the suspect leaf and let the eggs hatch out in a petri dish; if black alligator-like creatures emerge, they are beneficial ladybeetle larvae and you can return them gently to your plants with a fine paintbrush.
Upcoming insect attractions
Cabbage flea beetles (120-200 degree days initial emergence…and will continue to emerge from 200-500 degree days). Set out yellow sticky cards to attract them and monitor for emergence levels. Some hairy-leafed cabbage varieties respond well to kaolin clay (product: Surround) applications that are applied wet and then dry as a thin protective barrier of mineral dust. Adults are very difficult to spray since they jump, but plants that are treated – or pre-treated – with insecticidal soap that contains neem oil may get growers better results than plants left unprotected. At least one of the active ingredient in neem oil acts as an anti-feedant and in some insects it can also function as a deterrent for egg laying.
Information about neem oil from Cornell University: http://web.pppmb.cals.cornell.edu/resourceguide/mfs/08neem.php