“Nature will bear the closest inspection. She invites us to lay our eye level with her smallest leaf, and take an insect view of its plain.”
-Henry David Thoreau
I don’t know about you, but this weather has me a bit stumped – at least in regard to when I should be expecting certain bugs and other damaging insects. I typically watch for the chicory to bloom along the roadsides to give a heads up that it will soon be Japanese beetle and squash vine borer season. As of yet, I have still seen very little blooming chicory. I also always keep an eye out for the imported cabbageworm moths as they love to lay their eggs on my broccoli and once they’ve hatched, the larvae love to eat it. That’s when it comes in handy to understand degree days. Insects and various plant diseases need certain temperatures for optimal life cycle conditions.
Degree days (DD) are a measurement of heat units over time, calculated from daily maximum and minimum temperatures. Degree days are used to predict insect life cycles, and in turn, time insecticide treatments to those life cycles. For example, the imported cabbageworm moth, the first-generation adult flight starts around 150 DD, peaks around 240 DD, with larval feeding expected through 630 DD. Second generation adult flight starts around 830 DD, peaks around 1030 DD, with larval feeding expected through to around 2000 DD. There are a number of models out there to help you calculate degree days. I find some of them to be very confusing but I found the UW Extension AgWeather models very user friendly. I particularly like the Vegetable Disease & Insect Forecasting Network – https://agweather.cals.wisc.edu/vdifn. This model allows you to select a disease or insect, an area (ie. Wisconsin or the Upper Midwest), crop/host and insect, and date range. It then displays a map of severity levels. It allows you to zoom in almost to your very yard. Using the imported cabbageworm as my example, I selected cole crops as my crop/host, allowed the model to select the recommended date range, and found that the cumulative degree days was 1043.8. This puts my location at very high risk for imported cabbageworm damage. I’ve also started noticing a number of the white moths, so I know it’s time to take action.
My course of action will be to first put row covers over my broccoli, cauliflower, kale, and cabbage. I also will do a much closer inspection of the leaves to see if I see any velvety green worms. If I see any worms or significant leaf damage, I will break out the Bt (Bacillus thuringiensis var. kurstaki). Bt is a naturally occurring bacterium, common in some soils, that causes disease in certain insects, most notably leaf and needle feeding caterpillars. The infected insect stops feeding and starves to death. I also have plenty of the cabbageworm’s natural predators hanging around.
All these steps I just mentioned are Integrated Pest Management (IPM) techniques. IPM’s key components are (1) Monitoring and Inspection, (2) Identification, (3) Prevention, and (4) Control Methods. Monitoring degree days is just one of those tools in your IPM toolbox. Using IPM techniques, you can solve a lot of insect and disease problems while minimizing risk to people and the environment. Combine your knowledge of what insects and diseases you’ve seen in your garden with a knowledge of IPM techniques, and you can combat and/or minimize these issues without the use of costly insecticides, fungicides, bactericides, and more. To learn more about Integrated Pest Management, visit https://vegento.russell.wisc.edu/ipm/.
Time to start planning your visit to the Sauk County Fair and the Sauk County Master Gardeners Association booth. Ask all your gardening questions, purchase a ticket to their upcoming home garden tour on July 26, or attend one of our demonstrations. We’ll have demonstrations on proper tomato plant care and pruning, adaptive gardening techniques to ensure you can garden your entire life, and planting small take-home gardens for kids.
This week’s article is written by Jeannie Manis, a Wisconsin Certified Sauk County Master Gardener Volunteer. If you have any gardening questions, please contact the Extension Sauk County by emailing to trripp@wisc.edu or calling the University of Wisconsin Madison Division of Extension Sauk County office at 608-355-3250.
