Common stalk borer
Papaipema nebris
One plant in a row suddenly wilts and dies with a small hole in the stem. A caterpillar tunnelled in from the weedy edge and is inside it now.
How to know it
A caterpillar up to 40 mm, purple-brown with a broad white stripe down the back and several white stripes along the sides — the side stripes are interrupted by a purple band on the front third, which is diagnostic while it is young. Older caterpillars lose the markings and turn plain creamy gray. It is inside the stem, so what you actually find is a round hole with wet frass around it.
What you'll see
- A single plant wilting from the top down while its neighbors are fine.
- A small round hole in the stem near the base with damp sawdust-like frass around it.
- The stem hollow and packed with frass when split lengthways.
- A leaf whorl or growing tip dead while the rest of the plant stands.
- Damage concentrated on the row nearest a weedy edge or a grassy verge.
Where it comes from
One generation a year. Eggs are laid in autumn on grasses and weeds — giant ragweed and thick grass especially — and hatch in spring. The young caterpillar bores into a grass or weed stem, outgrows it, and then walks to something bigger, which is usually your tomato or dahlia. It leaves in late summer to pupate in the soil.
Gardens bordered by long grass, ragweed and weedy verges, in the midwest and east. Damage appears in June and July as the caterpillars move out of the weeds and into the garden.
Dealing with it
- Split the stem with a knife above the hole, find the caterpillar, and remove it — the plant often survives if you do it early.
- Pull and bin a plant that has already collapsed, with the caterpillar still inside.
- Cut off and bin a wilted tip or side shoot before the caterpillar works further down.
- Mound soil over the wounded stem of a tomato so it can root above the damage.
- No spray reaches a caterpillar inside a stem, so do not try.
Keeping it out
- Mow or clear the weedy border and the grassy verge in early autumn, before the eggs are laid there.
- Keep a mown or bare strip between the garden and any rough ground.
- Remove giant ragweed specifically, which is the favorite egg-laying site.
- Do not mow the surrounding weeds in June, which drives the caterpillars straight into the crop.
- Grow a wide row rather than a single line of a valued plant, so losing one matters less.
What it goes for
Tomatoes, peppers, potatoes, sweetcorn, dahlias, delphiniums, asters and rhubarb — over two hundred plants.
Who hunts it for you
- 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
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com