Four-lined plant bug
Poecilocapsus lineatus
Neat round sunken spots on mint and basil that look exactly like a fungal leaf spot. Four weeks of damage a year from a bug you will hardly ever see.
How to know it
A bright yellow-green bug 7 mm long with four crisp black stripes running down the wing cases — unmistakable, and gone by the time you look for it. The nymph is bright red-orange with black wing pads. Both drop and hide the instant the plant is disturbed, which is why the damage is far better known than the insect.
What you'll see
- Round, sunken, dark spots two to three millimeters across, in neat rows following where the bug fed.
- Spots turning translucent, then brown, and eventually dropping out to leave holes.
- Damage confined to the top few centimeters of new growth.
- Everything appearing over three or four weeks in late spring and then stopping abruptly.
- Mint, basil and oregano hit hardest, with woody plants nearby untouched.
Where it comes from
One generation a year, and a short one. Eggs are laid in slits in the stems in summer and overwinter there; the nymphs hatch in late spring, feed for about a month, become adults, lay, and are gone by midsummer. Everything you see is caused inside four to six weeks.
Late spring in the eastern and central states, on soft new herbaceous growth. Beds that were not cut back in autumn carry the overwintering eggs and get it worse.
Dealing with it
- Wait it out. The whole business lasts a month and the plants grow out of it.
- Cut affected mint, oregano and lemon balm hard back once the damage stops; the regrowth is clean.
- Knock the nymphs into soapy water in the early morning if you can catch them.
- Spray insecticidal soap onto nymphs only, and only if a young plant is being set back.
- Do not treat it as a leaf spot; a fungicide will do nothing at all here.
Keeping it out
- Cut herbaceous perennials and herbs down in autumn and remove the stems, which is where the eggs are.
- Check new growth in May and squash the red nymphs while they are still few.
- Keep the herb bed open and weeded so nymphs have nowhere to drop to.
- Grow a spare mint or basil plant for the month, and cut the damaged one back.
- Encourage the general predators; nothing specific hunts it but the damage window is short anyway.
What it goes for
Mint, basil, oregano, lemon balm and most soft herbs; also chrysanthemum, echinacea, shasta daisy, forsythia and dahlia.
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
- SpidersAraneae Webs, ambushes, and night patrols make spiders the garden’s steadiest generalist predators, taking whatever flies or crawls past. Mulch, ground cover, and undisturbed corners keep the population dense. Grow to recruit it: Lavender,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com