OMG! What’s Eating the Broccoli?
There is nothing more demoralizing to a gardener than to lose your ready-to-be-picked produce to a sneaky pest. Cabbage family (also known as Brassicae or Cole) crops, including many popular vegetables like cabbage, kale, broccoli, collards, brussels sprouts, kohlrabi, cauliflower and others, are subject to damage, mainly from a variety of caterpillars. Luckily, there are both cultural and insecticidal practices that can help control the damage.
In Virginia, there are three major and several less-common culprits, all larval stages of various moth species that target cabbage family crops. The most common are the imported cabbage worm, the cabbage looper, and the diamondback moths. I would like to include a fourth, the cross-striped cabbage worm, because it is particularly problematic for me. These pests are often called “cabbage worms” even though they are actually caterpillars. While each has its unique characteristics, they share a common cycle and development process. The moths lay their eggs on the plants, typically on the leaves, singly or in clusters. Small caterpillars hatch in a few days and grow larger and change color as they feed. First, they create translucent windows in leaves, progressing to eating large holes as they grow. This stage lasts 2-3 weeks, then the worms enter a cocoon. During the warm seasons pupation lasts for about 10 days and a new cycle begins. The entire cycle lasts 4-5 weeks and there can be several overlapping generations each year. The pupae overwinter in plant debris and initiate the next season’s cycle in the spring. Rumors that fall crops suffer less from these pests are not accurate. They remain active and destructive up to the first killing frost.
The imported cabbage worm is probably the most conspicuous in the adult stage because it is active during the day. Young larvae are, on the other hand, hard to spot because they are tiny and green like the host plant.
The earliest arrival in central Virginia, or at least in my garden, is the cross-striped cabbage worm. They lay larger numbers of eggs and small “worms” seem to appear everywhere at once in mid- to late May. Their mixed colors may make them easier to spot than some others, but once they arrive, they are prolific and signal the start of the long fight ahead. They are typically found on the underside of leaves. Here is what they look like on my kale a few days after hatching.
Just hatched larvae are tiny miners that create windowpanes in leaves. As they grow, they eat progressively bigger holes in leaves, leaving only veins if left unchecked. They can also bore into heads, doing more than surface damage to head crops.
For the backyard gardener, there are a number of low impact practices that help reduce the chances of major crop damage:
- Rotating cole crops in the garden, repeating specific locations on a 3-year or longer cycle.
- Because pupae overwinter in soil and plant debris, remove and dispose of dead or damaged plant matter during and after the season
- Interplanting, creating small patches or rows of specific crops and mixing them with other plants, reduces the risk of a single major infestation.
- Thinning overcrowded and weaker plants, creates a healthier crop that is easier to visually manage.
- Encouraging predators can help. Create a favorable environment for worm-eating birds by providing shelter, nesting material and water. Encourage, or at least don’t harm, predatory insects including certain beetles, yellowjackets, green lacewings, and parasitic wasps.
- Regular observation of plants is important. Watching for the moths can sound the alarm in time to head off major damage. Regularly checking plants for signs of infestation such as eggs, windowpanes and leaf holes, allows gardeners to intervene before too much damage is done.
- Hand-picking larvae off plants can be effective, especially on smaller plots. Drop them into a jar of soapy water and pour boiling water over them, or if, like me, you need an emotional release as you save your crop from invaders, squish them between your thumb and forefinger as you search for the next victim.These measures are minimally invasive and are smart practices for everyone. However, they are all focused on damage minimization, implicitly accepting a certain amount of destruction. A practice that is more focused on damage prevention is the use of floating row covers. They provide a mechanical barrier that allows sunlight and rain to reach plants but prevents moths from reaching them to lay their eggs. They require some up-front work to set up, but are insecticide free and eliminate the daily need to inspect and remove pests all summer long. They can be constructed simply and inexpensively and deserve consideration. Because we eat the roots and leaves of cabbage family plants, pollination is not necessary and row covers don’t harm yield. A short summary of floating row cover construction and installation is included at the end of this article.
If non-chemical measures don’t suit you or aren’t effective enough, there are multiple insecticide options available for controlling cabbage worms.
Bacillus Thuringiensis (Bt), is a naturally-occurring, spore-forming soil bacterium. It produces proteins that aggregate into crystals and are toxic when consumed by the larvae of many insect species. Bacillus thuringiensis subsp. kurstaki (Btk), and subsp. aizawai are the specific varieties that are used as pesticides for cabbage pests. Bt’s great benefit is its specificity. It is harmful only to the targeted larva and does not affect other insects. It must be sprayed on the portion of the plants, typically the leaves, where the larvae feed, and at the time when they are feeding. It doesn’t kill instantly but stops them from eating immediately and kills them in 1 to 2 days. It breaks down quickly in the environment, specifically in UV light, and must be reapplied every 3-5 days. It is an organic pesticide because it is found naturally in soils around the world. It has no known harmful effects on humans, although carefully following label directions, as with any insecticide, is important.
While Bt is probably the least toxic insecticide option available, there are other organic options. They include Spinosad, which doesn’t break down as fast as Bt, so doesn’t need to be reapplied as often. Its drawback is that it is not as selective and can cause harm to other insects that eat treated plant parts. Other organic insecticide active ingredients that are applicable to cabbage worms include pyrethrins and neem oil. Read label directions carefully to be sure you choose a product that fits your needs and preferences.
There are also synthetic insecticides appropriate for at least a part of our target larvae population. Active ingredients include permethrin, bifenthrin, cyfluthrin and esfanvalerate. Some Diamondback moth populations have developed resistance, so read labels carefully before making a selection. Advantages of synthetics include faster killing and longer viability in the environment. Drawbacks are higher toxicity generally and damage to beneficial insects. Pay special attention to the pre-harvest interval on the label. Harvest produce at the end of the interval to minimize residual chemicals in the produce you eat.
Floating Row Covers: A Non-Chemical Preventive Solution
Floating row covers are mentioned above as one of several cultural measures that can reduce cabbage worm damage. They require some time and effort to build, but overall are less work than picking and squashing and are more organic than spraying. And they work.
There are any number of ways to make floating row covers. A simple method is to use ¾” flexible PVC tubing to make a hoop to span the bed to be protected. Cut the tubing to a length that allows it to span the bed at a height that clears the plants. Create stakes out of ½” steel rebar, about 16” long. Drive them 8” deep into the ground and leave 8” showing above ground. Slide the tubing over the rebar on two sides of the bed, creating a radial hoop. Place hoops every few feet along your row or bed. Then cover the set of hoops with an insect resistant spun polyester fabric and anchor the fabric to the hoops and the ground to keep the insects out. It can be fastened to the hoops by making clips out of old garden hose slit lengthwise and slid around the hoops on top and sides. Secure the fabric to the ground with dirt, rocks/bricks, or longer pieces of wood, rebar or other material with enough weight to hold the fabric down. The proper fabric lets light and water through while keeping insects out. It also moderates temperature, shading plants on hot days and holding some heat in cooler weather.
Once set up, row covers spare gardeners the daily inspection chore, preserve prettier produce, and eliminate the risk of a guest finding a caterpillar in their food. There is plenty of guidance about construction of floating row covers on the web. Here is a link that I found helpful.
And a Hail Mary
Finally, a plausible but personally unproven method that may be of interest. Cabbage moths are territorial. When a moth sees that another of its species has beaten it to its intended target, it looks for another place to lay its eggs. No less a professional than the master vegetable gardener at a major area historical attraction, hangs imported cabbage moth decoys over her cole crops and claims good success at preventing infestations. Here is a link to an article with step by step instructions. It is simple, cheap, organic and worth a try.
It’s About Balance
If you are a backyard gardener aiming for low environmental impact and minimal chemical solutions to gardening issues, there are plenty of non-chemical ways to preserve your cabbage family crops. If you are more inclined to take the chemical-based route, they exist too. Or, as my mom once said to my sister, referring to the cabbage worm she found in the cole slaw, “It’s mostly cabbage anyway; what are you worried about?”
“The pest caterpillars of cole crops in Virginia,”www.pubs.ext.vt.edu.pdf
“Imported Cabbageworm in Home Gardens,” pubs.ext.vt.edu/ENTO-253.pdf
“Cabbage Looper,” pubs.ext.vt.edu//3104-1544/ENTO-244NP.pdf
“Diamondback Moth in Virginia,” pubs.ext.vt.edu//444-007
NC State Cooperative Extension: Controlling Caterpilars in Vegetable Gardens, Charlotte Glen, https://pender.ces.ncsu.edu/2013/09/controlling-caterpillars-in-vegetable-gardens/
NC State Cooperative Extension: Good Bug/Bad Bug, Charlotte Glen, https://pender.ces.ncsu.edu/2011/05/good-bug-bad-bug/