Skip to content

Plant bugs

Also known as Capsid bugs, Mirid bugs, Miridae, Tarnished plant bug (Lygus lineolaris), Four-lined plant bug (Poecilocapsus lineatus)

PestDepends
Four-lined plant bug adult with its four black stripes Photo: WanderingMogwai · CC BY-SA 4.0

In short

Plant bugs (capsids or mirids) are active, flattened true bugs; many pierce buds, shoot tips and young leaves, leaving sunken spots, holes or distorted, blackened growth. The family is huge, however, and some members are predators used in biological control, so identify a bug before acting.

How it looks

Most are under 12 mm long. A family feature is the cuneus, a small triangular section near the tip of each forewing. The four-lined plant bug has yellow (sometimes bright green) forewings with four distinct black stripes; its nymphs are bright red to yellow. The tarnished plant bug is a mottled, strong-flying bug whose nymphs are green or pale yellow and grow quickly.

Signs

Four-lined plant bug: many small (about 1.5 mm) round, sunken spots on the top leaves that turn black or translucent and later drop out, leaving holes; the pattern can look like a leaf-spot disease. Tarnished plant bug: distorted foliage, lost flower buds, stunted bushy growth and blackened, dying shoot tips.

Life cycle

The four-lined plant bug overwinters as eggs in slits in shoots, nymphs hatch in late spring and develop in about 30 days, and there is one generation a year. The tarnished plant bug overwinters as adults in leaf litter and has two to five generations a year, each taking about 25 days in summer.

What favours it

Outdoor gardens with leaf litter and herbaceous plants. In the tropics, related mirids (mosquito bugs) are pests of tea, cacao and cotton.

Prevention

  • Clear leaf litter and plant debris where adults overwinter.
  • In winter, prune out shoot tips carrying four-lined plant bug egg slits.
  • Check new growth regularly during the growing season.

Treatment

Start with the first step and move down only if the problem persists.

  1. Tolerate cosmetic damage; natural predators often keep numbers down.
  2. Spray insecticidal soap on nymphs when numbers warrant it; follow the product label.
  3. Stronger insecticides are a last resort; follow the product label.

Pros and cons

Pros

  • Some mirids are predators: Macrolophus pygmaeus is sold to control whiteflies, and Dicyphus hesperus feeds on whiteflies and spider mites.

Cons

  • Feeding scars leaves and can kill shoot tips and flower buds.
  • Some species transmit plant viruses.

Look-alikes

Fungal leaf spot
No bugs or nymphs present; four-lined plant bug spots are small, even, sunken dots that later drop out as holes.
Leafhoppers
Wedge-shaped, jump readily and cause fine white stippling rather than sunken spots.

Affected plants

The four-lined plant bug feeds on about 250 plant species, mostly herbaceous, including oregano; the tarnished plant bug attacks many ornamental flowers, fruit and vegetables.

Sources

  1. Tarnished and four-lined plant bugs — Missouri Botanical Garden
  2. Miridae — Wikipedia
  3. Indoor plants: sap-feeding insects — RHS

Written in our own words from these sources. Check a product label before using any treatment.