My mom mentioned in the weekly email this week that I discovered that caterpillars had eaten about 500 broccoli plants on Saturday night and that I was going to post some more info about it. Here’s what she wrote:

On Saturday night we discovered that one of the caterpillar pests we mentioned last week has eaten about 500 broccoli plants from our latest planting. These worms have been a problem this spring in the greenhouse, but not outside under row cover. Our neighbors have had problems in their pac choi outside, so we’ve been keeping an eye on each planting. However, the last planting we set out was split between two fields, and those 500 plants happen to be in a field we don’t walk by very much and they’re under row cover. The one field has been looking great so we were shocked to discover the infestation in the other and confused at the difference. It turns out that we used different cover crops in the two fields, and the caterpillars preferred the one that had been planted with sweetclover rather than rye and vetch. For now, we’re investigating what strategies work to control the little buggers.

So the name of the pest is the Variegated Cutworm. You may have read about it in the Duluth News Tribune near the end of May–while we’ve had other varieties of cutworm up here for a long time, this variety cannot typically overwinter here and hasn’t made it this far north before. We first noticed them in the greenhouse, eating spinach and carrot seedlings just as they emerged. They are generally nocturnal so we had a difficult time observing them at first, but now they have thoroughly infested three of our 6 greenhouses. We’ve never experienced a pest with as wide a range in appetites as this. Their favorites thus far seem to be spinach, broccoli, greens mix, beet greens, turnip greens, and any type of small seedling. They have also eaten off the stems of 6″ tall tomato plants, small cucumber plants, rhubarb leaves, and they are really attacking the hostas and mint that we have planted around the house. Their latest victim is green onions–they like to eat the tops off and crawl inside the hollow leaves and eat their way back out again. In my research, it also sounds like they will eat potato plants, which are mildly poisonous and tomatoes; we haven’t had any damage yet, but we should be seeing some begin to ripen in the next two weeks and that’s when we’ll really get nervous. That is an amazing variety of plants that play host to these caterpillars, and it has us worried that they will continue to be a significant problem this year, especially as plants become more stressed due to excess moisture.

As far as controlling them, our hope is that on outdoor crops the birds will help with control. The broccoli that were eaten were under row cover in order to protect them from the root maggot fly, but that protected caterpillars from bird predation. Root maggots aren’t as much of an issue after late June, so the last three plantings of broccoli won’t have to be under row cover. There are a few naturally-derived substances that are organically approved for insect control. DiPel, a variant of bT that’s targeted to caterpillars, hasn’t been very effective on cutworms when we’ve tried it in the greenhouse. Two other products are both expensive and affect a broader range of bugs, so we’re not very excited about using them. We are in the process of experimenting with Neem Oil, which is extracted from the seed of the Neem tree, an evergreen tree in India. It has been used traditionally for medicinal uses, as well as a pesticide and repellant for insects. While it doesn’t kill them outright, it appears to repel caterpillars and disrupt their reproductive cycle. None of these are very good solutions, but hopefully they will be enough to reduce the caterpillars’ destructiveness and get us through the year. I, for one, am hoping for a normal Minnesota winter so that they can’t overwinter here and are driven back to warmer climates where they have more natural enemies. Another long-term solution is tilling under cover crops a week or two earlier so the caterpillars don’t have a food source and die before the regular crop is planted.

As organic farmers, the goal is not to simply replace a chemical-based pesticide with one that’s less harmful, it’s really a change in mindset. What we try to do when a new pest, disease, or weed shows up is try to understand the organism and use that knowledge to disrupt its habits and lifecycle in order to minimize the damage. At times we still use products such as DiPel to reduce populations of Imported Cabbage Worm, for example, but when we do it’s in the context of a system that is organized around the goal of strengthening crops and beneficial organisms, and looks at the long-term health of these as the key rather than sacrificing these to the short-term goal of pest eradication.

This photo was taken on June 10th, 3 days after planting the broccoli (under the rowcover).

A closer view of the most beautiful sweetclover cover crop we’ve ever grown. This should have provided much of the fertility needed to grow the broccoli, breaking up the subsoil with its deep taproot, and feeding billions of beneficial soil bacteria. Unfortunately, it also appears to have attracted hundreds of cutworm moths whose larvae soon hatched and destroyed the broccoli.

Those little sticks used to be vibrant transplants!

If you look closely, there are 5 caterpillars on this poor little plant. The average was about 4 per plant.

The caterpillars seem to have left a little bit of leaf matter on this plant, so the slugs have joined the party to finish it off!

These are the broccoli plants in the other field that has not been affected by the cutworms yet. They look great! (Please note, these pics were all taken before the 9.1″ of rain we got Tuesday night and Wednesday morning. I’ll have another depressing post about that later.)