Jute semilooper

ragged, chewed holes and torn edges on the young upper leaves. As numbers build the leaves get skeletonised — eaten away until only the veins are left — and whole plants can be stripped bare. The most costly sign is right at the top: the soft growing tip is bitten off and dies, and the plant answers by throwing out side branches instead of one tall, straight stem.

a smooth grass-green caterpillar, the same colour as the leaf, so you must look closely — up to about 4 cm long (roughly the length of your finger), with faint pale lines running down its back. It gives itself away by the way it moves: it arches the middle of its body into a loop, then reaches forward, as if measuring the leaf — this looping walk is why it is called a semilooper (a looping caterpillar). Look for them on the top leaves and around the growing tips, feeding most in the cool of morning and evening.

with the jute hairy caterpillar, which is brown, covered in hairs, crawls flat (no looping) and feeds in crowds on the undersides of the lower leaves. Your semilooper is smooth, green, moves alone, and loops as it walks — and it is the one that goes for the growing tips at the top.

start checking once the crop is growing tall and leafy (from about a month after sowing) and keep it up right through mid-season, while the plant is building the long stem you harvest for fibre — this is exactly when the tender tips are most at risk. Check more often in warm, muggy weather after rain, because humid heat is when the caterpillars multiply fastest.

cross it in a big "W" shape so you cover the four corners and the middle, not just the row next to the path. Make about 10 stops along the way.

look over the top leaves and the growing tip of a few plants, and turn some leaves to check underneath — you are hunting green looping caterpillars, fresh chewed holes, and any biting or wilting of the tender top shoot.

act now if you find about 2 to 3 caterpillars on a plant, or if you can already see the growing tips being bitten and the leaves noticeably stripped. Don't wait past that — tip damage is what costs you fibre length and grade.

sow at the recommended time and spacing so the whole crop grows fast and evenly and passes the tender young stage quickly, and keep the field weed-free. Weeds and odd late-sown plants give the moths a steady supply of soft leaves to feed and lay eggs on, which keeps the pest going from one patch to the next.

while numbers are still low, handpick the green caterpillars in the cool of the morning and drop them in soapy water — it costs nothing and works well in a small plot. And leave your natural helpers — birds, ants, and the tiny wasps that lay their eggs inside the caterpillars — to do their work by not spraying broad poisons early; they hold the pest down for free.

at threshold, while the caterpillars are still small, spray Bt (Bacillus thuringiensis, a natural germ that sickens only caterpillars and harms nothing else). Wet the top leaves and growing tips well, and spray in the cool of the day. It is safe for you, your family, bees and your natural helpers, so it is the first spray to try before any poison.

if the caterpillars keep building past threshold, use a registered insecticide (a shop-bought chemical spray cleared for use on the crop), aimed at the small caterpillars on the upper leaves. Protect yourself: cover your skin, eyes, nose and mouth, spray in the cool of the day with the wind behind you, and keep it off people, food, drinking water, children and bees. Wash yourself and your clothes straight afterwards. Because jute is cut for fibre, respect the pre-harvest interval — the set number of days you must wait between your last spray and cutting the crop. And rotate to a different chemical group each time, so the pest doesn't build resistance (stop being killed by that chemical).

if the growing tips are already widely bitten and the plants have branched, no spray will bring the fibre length back — accept the lower grade on that patch and spot-treat only the worst hotspots to save the rest. If the crop is close to fibre maturity, it is better to harvest early than to keep fighting. After harvest, clear away and destroy the crop trash, so hiding caterpillars and pupae don't carry the problem over into your next jute crop.