Armyworms
Spodoptera frugiperda & Mythimna unipuncta
also called Fall armyworm, True armyworm
Caterpillars that arrive in numbers and strip a lawn or a row overnight, marching on when they have finished. Catch them small — a grown armyworm shrugs off almost everything.
How to know it
A caterpillar 30–40 mm long, olive to near-black, with pale stripes along the sides and a dark head. The fall armyworm carries an upside-down pale Y on the face and four raised black dots in a square on the second-to-last segment. What names them is the behavior: dozens or hundreds together, moving across the ground in a front.
What you'll see
- A patch of lawn browns off in a day or two, and the turf is grazed to the crown rather than dug up.
- Birds working one part of the lawn hard, and small green-brown caterpillars visible when you part the grass.
- Rows of vegetables stripped from one end, with the damage advancing along the row day by day.
- Whorl leaves of sweetcorn shredded and packed with wet frass.
- Caterpillars crossing paths and drives in the morning, all traveling the same way.
Where it comes from
The fall armyworm overwinters only in the far south and flies north each year, arriving anywhere from midsummer to autumn depending on the season and often making headlines when it does. Moths lay eggs in furry masses on leaves, walls and fenceposts; the caterpillars feed for two to three weeks, growing slowly at first and then very fast, and pupate in the soil.
Outbreak years follow warm springs and favorable winds carrying the moth flight north. Lush, well-watered lawns and late-planted sweetcorn are the first things hit; drought-stressed turf tends to be passed over.
Dealing with it
- Act while the caterpillars are under 15 mm — Bt and spinosad both work then and neither touches a large one.
- Soak a suspect lawn patch with soapy water; the caterpillars come to the surface within minutes and can be swept up.
- Water the lawn well afterwards and feed lightly — grazed turf regrows from the crown if the roots were not touched.
- Hand-pick along a vegetable row at dusk, when they come out to feed.
- Leave the birds to it where damage is light; a flock working the lawn is doing the job for you.
Keeping it out
- Scout weekly from midsummer, parting the turf at the edge of any patch that looks off-color.
- Mow a little higher and avoid pushing lush growth with nitrogen in late summer.
- Sow sweetcorn early rather than late so it is past the vulnerable whorl stage when the flight arrives.
- Keep the flowering herbs going for the tachinid flies and parasitic wasps that follow the outbreak.
- Clear thick thatch, which shelters both eggs and small caterpillars.
What it goes for
Lawns and grasses first, then sweetcorn, and from there almost any vegetable in the row.
Who hunts it for you
- BatsChiroptera A single bat takes hundreds of moths and mosquitoes in a night, and the moths are the parents of your cutworms, corn earworms and hornworms. Put up a bat box on a warm south wall, leave a night-scented corner unlit, and never spray after dark. Grow to recruit it: Evening Primrose,Honeysuckle,Nicotiana,Summer Jasmine
- Garden birdsPasseriformes Both things are true: birds strip your currants and a brood of blue tits takes several thousand caterpillars out of one fruit tree in a fortnight. Net what you must and welcome them everywhere else — a nest box near the apples is worth more than a spray. Grow to recruit it: Echinacea,Elderberry,English holly,Goldenrod,Rudbeckia,Sunflower
- Ground beetleCarabidae Fast, glossy night hunters that patrol the soil surface for slugs, cutworms, and root-fly larvae. Give them daytime cover — thick perennial edges, stepping stones, a log — and skip the tillage that wrecks their burrows. Grow to recruit it: Lavender,Mint,Thyme
- Spined soldier bugPodisus maculiventris A predatory stink bug — same shield outline as the pest, but with a sharp spine at each shoulder and a beak it uses to spear caterpillars and beetle grubs. Learn the spines before you squash a shield bug, because this one is on your side. Grow to recruit it: Dill,Fennel,Sweet alyssum,Yarrow
- Tachinid flyTachinidae Bristly, house-fly-lookalike parasitoids that lay eggs on caterpillars, beetle grubs, and earwigs — the maggot consumes the host from within. Adults sip nectar from shallow umbels and daisies. Grow to recruit it: Cilantro,Dill,Feverfew,Parsley,Sweet alyssum
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com