Pod borer

What you are looking at

A round hole bored into a pod, a boll or a fruit, cut cleanly rather than chewed, and often a caterpillar with its head and shoulders pushed inside and the rest of its body hanging out in the open.

Open the pod and the seeds are gone or half eaten, with damp dark frass packed around them.

The caterpillar is up to about 40 mm long and comes in almost any colour. Pale green, straw, pink, brown and nearly black all occur in the same field, with pale stripes running the length of the body and short stiff hairs standing on small dark bumps. Colour identifies nothing here.

Look along the row and you will find several damaged pods on one plant rather than one pod eaten out, which is the thing that separates this insect from most caterpillars.

One caterpillar, a dozen ruined pods

A leaf-eating caterpillar eats a leaf and the plant grows another. This one eats the part you are growing the crop for, and it does not finish what it starts.

A larva bores into a pod, eats some of the seed, leaves, and bores into the next. Over its development it damages far more pods than it could eat, so the loss runs well ahead of the number of insects present. Ten caterpillars in a row of chickpea is not ten damaged pods.

There is also only ever one of them in a pod, however many eggs were laid on the plant, because they eat each other on meeting. That is worth knowing when you open pods to assess a crop: the count of caterpillars is not a count of what was laid.

A single female lays several hundred eggs and sometimes over a thousand, one at a time, on buds, flowers and growing tips. She is drawn to plants in flower, which is why the fortnight either side of flowering decides the crop.

Why spraying is the wrong instinct

By the time the damage is visible the caterpillar is inside the pod, where no spray reaches it. That alone makes chemistry a poor tool here.

The larger problem is resistance. This insect carries documented resistance to pyrethroids, organophosphates and carbamates across India, China, Australia and much of Africa, in some populations at levels that make the labelled rate pointless. Everything about the insect speeds that up. It runs several generations a year, each female lays hundreds of eggs, the host list is long enough that a population never runs out of somewhere to breed, and the moths fly far enough to mix resistance genes across a whole region.

Spraying a resistant population still kills the Trichogramma wasps that parasitise the eggs and the Campoletis and Bracon wasps that attack the caterpillars. The pest survives and its natural enemies do not, so the next generation arrives into a crop with nothing holding it back.

The things that do work are Bt and the nucleopolyhedrovirus, and both share the same constraint as everything else: they have to reach a small caterpillar while it is still out in the open. Timing rather than product is what decides whether they work.

Everything turns on the flowering window

Moths lay on plants in flower. On chickpea and pigeon pea that gives a window of roughly a fortnight either side of flowering when the eggs arrive and the young caterpillars are feeding on the surface where they can be reached.

Inspect twice a week through it. A crop looked at after podding has already taken its losses.

The other lever is the sowing date. Populations build through the season, so a crop that pods before the moths peak escapes much of what a later crop meets. Local advisory services publish the peak flight, and a single pheromone trap will find it where they do not.

Longer term, the rotation matters more than anything applied. Cotton, chickpea, pigeon pea, tomato, maize, sorghum, okra and sunflower are all hosts, so a rotation between two of them lets numbers run straight from one crop into the next. Cultivating after harvest turns the overwintering pupae up for the birds.

What to do about it

Inspect twice a week from first flower, and act while the caterpillars are small and outside the pods.

Pick them off by hand where the plot is small enough, in the evening, every few days.

Put bird perches through the crop before flowering, one every ten metres or so.

Sow early enough that podding happens before the moths peak, where the season allows it.

Plant marigold as a trap crop and pull the heads off with the eggs on them.

Spray Bt or the nucleopolyhedrovirus at egg hatch rather than at first damage.

Take damaged pods off the plant and out of the plot.

Cultivate after harvest to expose the pupae.

Put a non-host crop between susceptible ones.

Leave the parasitic wasps alone, and treat a broad-spectrum insecticide as the thing that makes next season worse.

What it gets mistaken for

Corn earworm is the same animal for practical purposes and a different species, confined to the Americas.

Fall armyworm strips leaves and feeds in the maize whorl rather than boring into pods.

Pink bollworm lives its whole larval life inside a cotton boll and leaves a tiny exit hole.

Pea and bean weevil notches the leaf edges and never goes near a pod.

Fruit fly leaves a sting and soft rot rather than a hole you could put a pencil in.