So this week is National Moth Week. Much of the info you’ll see online is about how to attract nice moths to your garden, how some moths are pollinators, and other interesting moth facts. And while most moths are perfectly nice creatures, there are some that you definitely DON’T want in your garden. These are like the lepidopterans that your moth-er warned you about. They’re insects only a moth-er (or entomologist) could love. (Note: Lepidoptera is the order of insects that includes moths and butterflies).
So I thought I’d take an opportunity to talk about a few of the top moth pests in the vegetable garden and what you can do about them.
The Tomato/Tobacco Hornworms
First up are the Tomato Hornworm (Manduca quinquemaculata) and it’s cousin the Tobacco Hornworm (M. sexta). Despite the resemblance to each other in photos and being mistaken as the same insect, these are two different species. Not only do they look similar, but they feed on plants that are closely related (tobacco and tomato are both in the Solanaceae family, and they feed on other plants, too). They are much easier to tell apart in their adult forms.
They can eat lots of leaves off of your tomato plant, and almost defoliate it if the plant is small or if they are in large numbers. Foliage loss can result in lower production and plant vigor. They can even cause some damage to the fruits.
Cultural control: Handpick caterpillars off of plants, till the soil after harvest to destroy pupae
Biological control: These juicy caterpillars can fall prey to a number of predators and parasitoids. Plant diversity, including flowers, encourages beneficial insects that may act as predators. If you see hornworms with white protrusions in its back, those are cocoons from a parasitoid wasp which consumes the hornworm from the inside. Leave those caterpillars in the garden so the wasps will hatch. Once infested, hornworms typically slow/stop feeding, though you can move it to a less desirable spot on the plant if preferred.
Organic/low-impact chemical control: insecticidal soap, Bt (Bacillus thuringiensis) or Spinosad
Conventional chemical control: carbaryl, permethrin, bifenthrin
The Cole Caterpillar group
During the summer months, holes start to appear in many of the cole crop plants like cabbage, broccoli, kale, cauliflower, etc. Sometimes the damage can result in total defoliation or destruction of the edible parts (as in the leafy parts of kale or collards, or the heading leaves of cabbage).
There are three different common species of lepidopterans that cause cole crises. First up is the Cabbage Looper, which is the green inch-worm like caterpillar of the cabbage looper moth Trichoplusia ni.
Second is the Diamondback Moth, sometimes called cabbage moth (Plutella xylostella), whose smaller, light green larvae also love to feed on cole crops.
One of the more common and destructive cole crop pests, the Imported Cabbage Worm (Pieris rapae), has to get special mention here, even though the larvae of this pest turns not into a moth…but into a butterfly! The adult form of these caterpillars is a small, bluish-white butterfly with a black spot on it’s wings that you might see fluttering around your garden on a hot day in the early summer.
Cultural control: remove/destroy crop residue after harvest to reduce overwintering sites, hand pick caterpillars from plants, eliminate weeds in the cole/mustard family that are alternative hosts, such as wild mustard and shepherd’s purse
Biological control: Plant diversity, including flowers, encourages beneficial insects that may act as predators.
Organic/low-impact controls: Bt (Bacillus thuringiensis), spinosad, pyrethrin
Conventional control: carbaryl, permethrin, bifenthrin
Squash vine borer
The menace to many a-squash grower, the Squash Vine Borer (Melittia cucurbitae) is a hidden pest that you don’t often find until it is almost too late. The feature headline photo for this article is from my colleague Gary Lesoing’s own squash and pumpkin patch. This is an interesting one, because the adult form is a clear wing moth that resembles a wasp more than a moth. Mother borer lays her eggs at the base of a tasty looking squash plant, and borer junior hatches out and bores into the stem to feed. You can sometimes split the stem above the damage and extract the intruder to stop damage. Mounding soil up over the damage may initiate new adventitious roots on the stem that can provide nourishment above the damage. You can use yellow sticky traps to detect the adults as they begin searching for places to lay eggs so you know when to apply controls.
Cultural control: plant vine crops that aren’t attractive to SVB – butternut squash, cucumbers, and melons. Late/second plantings of summer squash/zucchini made in July will mature after adults have finished laying eggs and mature fast enough for a harvest. Quickly remove infected/killed plants. Rotate crops to different areas. Use row cover to exclude the adult moths, keeping in mind to remove them once flowering starts so that bees may pollinate the flowers. Keep in mind that adults will emerge from the soil, so row covers will not be effective if cucurbit crops were planted in the same place as the year before.
Conventional chemical control: Carbaryl, bifenthrin, permethrin. Apply at the base of plants when crops start to vine or when adults are detected.
While not covered in this article, other moth pests in the vegetable garden include Corn Earworm, Cutworms, and Zebra caterpillars. It is also interesting to note that some butterfly larvae, like the Black Swallowtail, feed on members of the parsley family such as parsley, fennel, and dill. This is great if you love butterflies, but not so great if you are trying to grow these plants.
Read and follow all insecticide labels carefully before purchasing and using any insecticide products. Make special care to note if the product will control the pest you wish to control, can be used on the plant you wish to protect.
Sources: I found a treasure trove of great garden insect at University of Minnesota Extension, so it provided great source material for this article.
Caterpillar pests of cole crops (UMN)
Tomato hornworms in home gardens (UMN)
Squash vine borer management in home gardens (UMN)
All photos are creative commons from Wikimedia Commons, except the photo of the Diamondback Moth larva.