Nettle caterpillars

on the fronds (the big palm leaves) and on banana leaves, look for pale, thin, see-through patches — like little windows — where the caterpillars have scraped one side of the leaf away. As they grow the windows join into ragged holes, and then whole leaflets are stripped down to the bare ribs and midrib, like a leaf skeleton, leaving the tree looking dry, scorched and brown. It usually starts on the older, lower fronds and spreads across a patch of neighbouring trees.

turn a frond over and look on the underside. Each one is small — from shorter than your thumbnail up to about the length of your thumb (1–2.5 cm) — flat and sluglike, with no legs you can see, so it seems to glide rather than crawl. Most are bright green or yellow with stripes or spots, and they carry rows of stiff, bristly spines. They sit crowded side by side in clusters, not alone.

Brush bare skin against the spines and you get an instant burning, nettle-like rash (that is how the pest got its name). An ordinary crawling caterpillar never does this.

with bagworms (which drag a little bag of leaf-bits around and dangle from a thread) or with ordinary loopers and leaf-worms (which have clear legs, loop or crawl along, have smooth bodies and do not sting). Nettle caterpillars are the ones that look legless and sluglike, huddle together in clusters, and sting.

hard, oval, brown cases about the size of a small bean or seed, glued onto the fronds, leaf bases or trunk. This is where each caterpillar seals itself in to turn into a moth — and where the next generation comes from.

nettle caterpillars build up in the warm, humid months when the canopy is full and the bunches or nuts are filling — the mid- to late season. Warm, damp weather (about 22–32°C with high humidity) is when the colonies grow fastest, so step up your checks in that window.

wear gloves and long sleeves — the spines sting the instant you touch them, even by accident.

across the block so you cover all four corners and the middle, not just the row by the path. Make about 10 stops along the way.

(and the banana leaves) and check the undersides, where the colonies start. Look for the pale window-pane scraping, the flat sluglike caterpillars packed in clusters, and any hard, seed-like cocoons on the fronds, leaf bases or trunk.

— because they feed in groups. Across your 10 palms, note how many fronds carry a cluster and roughly how many caterpillars are on each, so you can tell whether the numbers are rising from one week to the next.

A few colonies left alone this month become brown, skeletonised blocks next month.

tiny parasitic wasps and flies — whose young feed on the caterpillars — normally keep nettle caterpillars in check on their own. Blanket-spraying the whole plantation kills these helpers and is the main reason outbreaks flare up, so do not spray the whole block. Keep scouting instead, and deal with colonies while they are still small and on only a few trees.

while the attack is still on a few fronds, prune off the fronds carrying the clusters and the hard cocoons, and burn them — with your gloves on, because the spines still sting. Taking out the colonies and cocoons early stops the next, bigger wave from hatching.

where colonies are spreading, spray Bt (Bacillus thuringiensis — a natural soil bacterium that sickens only caterpillars and is harmless to you, your bees, birds and the helpful wasps) onto the undersides of the infested fronds, where the caterpillars sit. Spray while they are still small and young — big caterpillars are far harder to kill — and spray in the cool of the day (early morning or late afternoon), wetting the leaf undersides well.

a pyrethroid is a fast-acting contact poison (it kills the caterpillar when the spray lands on it). Use it only to spot-treat the outbreak patch — never the whole plantation — because it also kills the natural enemies and makes the next outbreak worse. When you do spray: cover your skin, eyes, nose and mouth; spray in the cool of the day with the wind at your back; keep it off food, water, children and bees; and wash yourself and your clothes straight afterwards. Respect the pre-harvest interval (the number of days you must wait after spraying before the fruit or nuts are safe to harvest). And to stop the pest getting used to the poison (resistance), rotate to a different chemical group each time rather than reaching for the same one again and again.

if colonies keep multiplying and whole fronds are being skeletonised across the block in spite of your spot-treating, stop any broad spraying (it is feeding the outbreak by wiping out the helpers), cut and burn the worst-hit fronds to pull out the caterpillars and cocoons, and seek advice on protecting or bringing back natural enemies. When a tree is already stripped to bare ribs, those fronds — and this season's bunches or nuts on it — are lost: protect the central growing point (the spear or heart of the palm), put your effort into saving the trees that are still green, and rebuild the helper population so the block can recover next season.