Stem borers
Caterpillars and grubs that tunnel inside stems and shoots, causing wilting, dead growing tips and broken stems.
Also known as: Shoot borers, Stalk borers
Symptoms and identification
Shoots wilt suddenly while the rest of the plant looks healthy, and the growing tip dies. A small hole with sawdust-like droppings (frass) shows on the stem, and cutting it open reveals a tunnel with a caterpillar or grub inside. Stems may snap at the tunnel.
Causes
Moths or beetles lay eggs on or near stems, and the young larvae bore inside where sprays cannot reach them. Many kinds exist and each has its preferred plants and season.
Organic treatment
Cut out and destroy wilted shoots below the tunnel, or slit the stem lengthways and remove the borer. Inject Bacillus thuringiensis (Bt) or a beneficial nematode product into entry holes.
Prevention
Inspect stems for entry holes and frass, remove and destroy crop stalks after harvest, and keep weeds and grasses down where adults shelter. Use fine netting on small crops.
Chemical treatment (use with care)
Systemic insecticides labelled for borers on that crop may help if applied when the larvae are young. Follow the label.