Bollworms, dreaded caterpillars defoliating vegetables

Large bollworm on a cabbage leaf with excrements

Typical of many insects (and even more so for butterflies), adult bollworms won’t cause any damage in the garden or vegetable patch. However, caterpillars and larva that emerge from their eggs can devastate nearby plants.

Bollworm: key facts

Branch: Invertebrates
Class: Insects
Order: Lepidoptera
Family: Noctuidae

Description

Bollworms are average-sized butterflies, but their body is massive compared to most. Wings take on different colors and patterns depending on the species; a common point is that the color is discrete and often dark (brown, gray). A characteristic that makes these butterflies easy to identify is that their wings form a “roof” when they’re at rest.

Adult bollworm on a flowerThe caterpillars of bollworm species also have a very different appearance depending on the species. Most don’t have any hairs, have a dark and bland-colored skin, and they have 5 or less pairs of feet. Both the adult butterfly and its larva are night-goers, so you’ll only rarely see them in the daytime. Some caterpillars have chosen this as a defense mechanism, hiding underground in the daytime and coming out at night. This helps them evade the more numerous daytime predators. When they feel attacked, bollworm larvae may react in different ways:

  • They might curl up.
  • Some jut out the front of their body to the side.
  • Others flick their body up and down like springs to jump away.

The main bollworm species you can find in the garden and vegetable patch are:

  • the cabbage moths (Mamestra brassicae and oleracea) and angle shades (Phlogophora meticulosa) will devour leaves;
  • and earth-roving caterpillars such as the yellow underwing moth (Noctua pronuba), the knot grass moth (Acronicta rumicis), turnip moth (Agrotis segetum) and the ipsilon dart (Agrotis ipsilon) easily travel between plants.

Did you know ? Bollworms are so numerous that about 1 in 5 butterfly species is classified as such, making the family account for around 20% of the Lepidoptera genus.

Biology

As is the rule for all members of the Lepidoptera species, a bollworm’s lifecycle counts 4 major stages:

  • egg;
  • larva (caterpillar);
  • nymph or cocoon;
  • and finally, butterfly.

Single bollworm crawling along cement

Eggs are usually lain on the surface of leaves, either one at a time or in clusters. They’re never more than 1/25th of an inch in size (1 mm), but they come in different colors: brownish-black, purple, transparent, etc. When they hatch, the caterpillars are between one and two inches long (25 to 50 mm). A caterpillar goes through a series of molts, shedding its skin to grow, until it reaches the nymph stage. A single cycle lasts at least 10 days, but if the weather is cold, it may take the insect several weeks to mature.

Impact of bollworm larvae in the garden

Symptoms include holes in leavesThe oldest caterpillars are the ones that damage gardens the most. They not only eat leaves from the rim, their powerful mouths can even snip off the leaf stem and leave a plant completely bare if there are too many of them. Infestations become clear when you notice holes on leaves or on fruits. Some species of bollworm larva (commonly called “cutworms” although they aren’t truly worms) will only attack roots and root crowns of ornamental plants, and will impact harvests for vegetable patch plants.

Treating against bollworm

Even though the damage they inflict seems impressive, on their own, bollworm caterpillars are rarely fatal to plants. Indeed, since nature always tries to even out the odds, many beneficial animals join in to help control overpopulation:

  • Treating against bollworm involves other predatorsBirds such as finches, sparrows and tits all love caterpillars and cutworms.
  • As for larvae that live underground (like cutworms) and those that move around at night, effective predators include hedgehogs, ground beetles and rove beetles: they’ll get the job done for you!
  • Bats, for their part, willingly hunt adult bollworm butterflies: this helps hinder their spread.
  • Lastly, a great many beneficial insects such as nematodes, parasite wasps and flies, stink bugs, green lacewing larvae and hoverflies will naturally control pests and keep them in check.

Nonetheless, it may happen that your plants are overrun for a time. In this case, there are different steps you can take to control the invasion, and this depends on which part of the plant is under attack.

Leaves are being targeted

Spraying a solution that has Thuringe’s bacillus mixed in helps fight off bollworm caterpillars without hurting any of its natural predators. This treatment is particularly effective on young larvae. Elder leaf decoction also is known to be effective. Another possibility is to rely on a pyrethrum-based treatment, but this should only be a last recourse, because it would also destroy a number of beneficial insects along with the bollworm.

Roots are under attack

Here again, Bacillus thuringensis is also very effective: spray the ground itself with the product along the rows of your vegetable patch, and even on the soil between rows. A specific nematode called Steinernema carpocapsae comes in handy here as well, it acts as a parasite that feeds off bollworm caterpillars.

And of course when you’re working through the soil, turning it over perhaps, pick off any cutworms you come across. In this manner, running the hoe along your cultures is a good mechanical way to fight against bollworms. Another solution is to set up a series of traps: every foot or two, bury a few slices of the following: carrot, red beet, and/or potato. Check on them several times a week, and pick off any bollworm larvae you find feeding on them.


Image credits (edits Gaspard Lorthiois): Pixabay: Merja Partanen, Angela Yuriko Smith