Gadenin日本語Get the appSwallowtail caterpillars: the one that actually costs you the plant
Japanese pepper sits in the citrus family, and swallowtail butterflies use that whole family as a nursery. Females lay single pale, spherical eggs on the soft new growth, usually on the upper surface near the shoot tip. The small caterpillars are dark and knobbly and look convincingly like bird droppings, which is why nobody notices them. In their last stage they turn smooth and green with dark bands, reach two inches (5 cm), and eat astonishingly fast.
On a mature shrub this is cosmetic damage and you can leave them be. On a young plant of two or three feet (60 to 90 cm) a handful of large caterpillars will defoliate the whole thing in under a week, and a plant stripped bare twice in one season may not come back. Check the new growth twice a week from late spring through summer, looking at the tips rather than the whole plant.
Hand removal is the only control worth using here. Sprays reach the caterpillars poorly among fine leaflets, and anything strong enough to work also kills the predators keeping mites and scale in check. Pick them off into a tub and move them elsewhere if you would rather not kill them; a single mature shrub in a corner of the garden can feed them without harm.
| Symptom | Cause | First action | When to worry |
|---|---|---|---|
| New shoots eaten back, black droppings on leaves below | Swallowtail caterpillars | Inspect tips, pick off by hand twice weekly | Immediately on plants under three feet (90 cm) |
| Sticky leaves, black sooty coating, ants running the stems | Scale insects | Rub off with a cloth, prune out worst stems | If the coating covers most of the canopy |
| Fine pale stippling on leaves, dull bronze cast, webbing | Spider mites in hot dry air | Spray the undersides with water, mulch and shade the roots | If leaves start dropping in mid summer |
| Sudden wilt, leaves green and hanging, soil damp | Root rot or root disturbance | Stop watering, check for recent digging | Same day, but expect to lose the plant |
| Sawdust at the base, one branch dying alone | Boring grubs in the trunk or main branch | Cut out the affected branch below the tunnel | If the tunnel is in the main trunk |
| Leaves yellow between green veins, no pest visible | Alkaline soil, iron locked up | Mulch with leaf mold, use rainwater | Only if growth also slows |
Scale insects and the sticky year
Scale is the slow problem. The adults are small oval brown or gray bumps clamped to the undersides of leaves and along the thin stems, and they look so much like part of the bark that they are routinely mistaken for it. The giveaway is stickiness: the insects excrete honeydew, the leaves below turn tacky, a black sooty mold grows on that stickiness, and ants patrol the stems to farm them.
Scrape a suspected bump with a fingernail. If it lifts off and leaves a damp patch, it is an insect. Once you know they are there, the practical treatment on a shrub this size is mechanical: rub the stems and leaf undersides firmly with a damp cloth, and prune out any stem that is thickly encrusted. Do this in late winter when the plant is bare and every stem is reachable, and repeat in early summer when the newly hatched crawlers are moving and vulnerable.
The sooty mold itself does no direct damage but blocks light, and it disappears on its own over a season once the insects producing the honeydew are gone. Do not scrub the leaves to remove it; deal with the insects and let the mold weather off.
Mites, aphids and hot dry summers
Spider mites arrive in hot, dry, still air, and a plant in a pot against a sunny wall is the classic victim. The first sign is a faint pale stippling across the leaflets, as though someone had gone over them with a very fine pin, followed by a dull bronze cast and, when it is bad, fine webbing in the leaf joints. Turn a leaf over and look at the underside against the light; the mites themselves are barely visible dots.
Humidity is the cure. Spray the undersides thoroughly with plain water every few days, mulch the root zone to keep it cool, and move a pot out of reflected heat. Mite outbreaks almost always follow a broad-spectrum spray that killed the predatory mites first, so the best long-term control is not spraying at all.
Aphids cluster on the softest spring shoot tips and curl them. On an established shrub they are not worth treating and disappear as predators build up. On a young plant, squash the colonies between finger and thumb every few days for a fortnight in spring and the problem passes. Ants running up and down the stems mean either aphids or scale somewhere above.
Sudden wilt: the one that is usually your own fault
A plant that wilts abruptly with its leaves still green and firmly attached is the most alarming thing this shrub does, and it is rarely an infectious disease. The shallow, brittle roots are extremely intolerant of damage and of standing water, and both produce the same symptom. Ask yourself two questions: has anyone dug, forked, hoed or edged within three feet (90 cm) of the trunk in the last few months, and has the soil been saturated for more than a couple of days.
If the soil is wet, stop watering immediately, pull the mulch back from the crown so the surface can dry, and if it is in a pot tip out any saucer and raise the pot on feet. If the cause was digging, there is nothing to do but shade the plant, keep the soil merely damp, and wait. Do not feed a wilting plant and do not move it, both of which are the standard reflexes and both of which finish it off.
Recovery, when it happens, shows as new buds pushing from older wood some weeks later, often after the original leaves have all dropped. Give a plant until the following spring before writing it off. A plant that shows no bud movement by early summer of the next year is dead, and scratching the bark with a thumbnail will show brown rather than green underneath.
Never replant a new Japanese pepper into the exact spot where one died of root rot. Choose a position at least a few feet away, on higher, better-drained ground, and improve the drainage before you plant rather than after.
Keeping a pest record that is actually useful
Pests on this plant are seasonal to within a week or two each year, so dates are worth far more than descriptions. Write down the first date you find caterpillar eggs or droppings, the first date you see crawlers or stickiness from scale, and the first date the leaves look stippled. Next year those dates let you look a fortnight early, when problems take five minutes to solve rather than a month.
Record which plant, since in a group of shrubs the same one is often hit first every year, usually because of where it stands. Record what you did and whether it worked, in one line. And record any work done near the roots, with the date, so that a wilt eight months later can be traced back rather than blamed on a mystery disease.