Caterpillars And Chewing Pests

Anything that leaves an actual hole in a leaf has jaws. That separates this whole group from mites, thrips and aphids, which pierce and suck and never remove tissue. It is overwhelmingly an outdoor problem, or an indoor problem on plants that spent time outside.

What you are seeing#

Ragged holes with chewed, uneven edges. Sometimes a leaf is eaten back from the margin, sometimes it is perforated through the middle, and in a bad week whole fan leaves go missing and leave bare stems.

Look for frass. Caterpillar droppings are dark green or black pellets, much larger than the dust-fine specks thrips leave, and they collect on leaves below the feeding and in the crotches of branches. Frass is usually the first thing you notice, before you find the animal.

The caterpillars themselves are green and match the plant almost perfectly. Check along the midrib on the underside of leaves and inside the tops, at night with a flashlight if you can, since most feed after dark.

The serious version is in the bud. Corn borers and budworms tunnel directly into a cola and eat it from the inside. The outside signs are a small entry hole, frass packed around it, and a single sugar leaf in the middle of a bud that has gone yellow or brown while everything around it is fine. One dying leaf in an otherwise healthy cola is a caterpillar until proven otherwise.

Slugs and snails leave large holes on lower leaves plus dried slime trails. Grasshoppers strip leaf margins in big bites. Leaf miners are different again: they tunnel between the leaf surfaces and leave pale winding trails rather than open holes.

Why it happens#

A moth lays eggs on the plant. That is the entire mechanism, and it is why this is an outdoor problem: moths have to reach the plant. A sealed tent with a screened intake essentially never gets caterpillars unless something came in on a plant that was outside.

Risk climbs through late summer and into fall, which lines up exactly with flowering for outdoor plants, and dense indica-style colas give larvae somewhere protected to feed.

Slugs and snails come from the ground and climb at night in damp conditions. Grasshoppers arrive in late summer from surrounding grass.

How to fix it#

1. Hand pick tonight. Go out after dark with a flashlight and pull off everything you find. For a few plants this is genuinely the most effective control there is.
2. Open every suspicious bud. Any entry hole, packed frass or single browning sugar leaf gets opened. Remove the caterpillar, cut out the damaged tissue back to clean bud, and throw the damaged part away.
3. Spray Bt (Bacillus thuringiensis var. kurstaki). This is the standard treatment. It is a bacterium that only affects caterpillars, it does not harm people, pets or beneficial insects, and it is safe to use up to harvest. Spray late in the day because sunlight breaks it down.
4. Repeat every 5 to 7 days while moths are active. Bt only works on larvae that eat it, so it does nothing to eggs. New hatches need new applications.
5. Spinosad is the alternative if Bt is not available, on the same interval, though it is harder on beneficials.
6. For slugs and snails, use iron phosphate bait on the ground around the pot and keep the area clear of damp debris. Do not use metaldehyde around pets.
7. Next season, net the plants with fine insect mesh through late summer. Stopping the moth from landing is the only control that works before the damage starts.

How to tell it apart#

Versus thrips and spider mites. No contest once you look closely. Sucking pests remove chlorophyll and leave the leaf intact: thrips give silvery irregular scarring, mites give a fine scatter of pale dots and eventually webbing. Neither ever removes tissue. A hole you can see light through is chewing damage.

Versus bud rot. These arrive together more often than not, because a tunnel full of frass is where rot starts. Bud rot on its own is gray fuzzy mold with a brown crumbling core and no entry hole and no frass. If you find a hole and pellets, a caterpillar came first and the mold followed. Treat both.

Versus physical damage. Torn leaves from handling, fan blades or clips have clean straight tears and no frass anywhere.

How long until it looks better#

Hand picking works immediately. Bt takes 2 to 4 days to kill a caterpillar, and it stops feeding well before it dies, so damage halts sooner than the bodies disappear.

Chewed leaves never repair. Judge progress on whether new holes and fresh frass appear, not on the leaves you already found.

Bud damage does not recover at all. The tissue you cut out is gone, and the rest of that cola needs checking daily until harvest for rot starting in the cavity. If a plant is within a week or two of being ready and caterpillars are in the buds, harvesting early is usually the better trade than losing colas to mold.