Plum Moth is a minor pest that affects plums and damsons in Scotland. Here is what it looks like, how to spot the damage, and which controls have been tested.
What does it look like?
The caterpillar inside the fruit is the identifying sign, not the adult moth: pale pink with a brown head, up to 12mm long, found feeding around the stone with light brown excrement pellets nearby.
Adults: Rarely what a gardener actually spots first; the caterpillar-in-fruit damage is what gives this pest away. Adults are small, mottled grey-brown moths (see photo).
Larvae: Pale pink caterpillar, up to 12mm long, with a brown head. Feeds inside the ripening fruit, around the stone, leaving light brown excrement pellets close by.
What causes it
A warm summer is the one real trigger worth knowing: it can push some caterpillars to pupate early and produce a second generation late in the season, on top of the usual single generation.
Is this pest active now?
Adults: late May to July, mostly mid-June to mid-July
Larvae (the damaging stage): inside fruit from early summer until late summer, then overwinteringKeeping a plant alive and intact through the cold months, whether left outdoors, moved under cover, or stored dormant. Full definition → as cocoons under bark
Symptoms
Affected fruit ripens early and is often slightly misshapen, with light brown excrement pellets near the stone; fruit that ripens first usually has the highest proportion of caterpillars, while later-ripening fruit is much less affected. Two look-alikes are easy to confuse this with: pocket plum, a separate fungal disease, and gumming, a physiological response to changing water availability, neither of which is plum moth. Plum sawfly causes different damage again: its larvae tunnel into young fruitlets in June, which then fall from the tree early, unlike plum moth’s caterpillars, which develop inside fruit that stays on the tree until ripe. The cultivarsA plant variety that arose or was bred under cultivation and is kept true by deliberate propagation, rather than a naturally occurring wild species. Full definition → Czar and Victoria are more susceptible to plum sawfly specifically. A resinous gum around the stone is a physiological disorder, not moth damage.
Life cycle
Usually one generation a year. Adults emerge from late May, mostly mid-June to mid-July. Newly hatched caterpillars tunnel into fruit and feed around the stone until late summer, then emerge to overwinter in silk cocoons spun under loose bark or other concealed places. A warm summer can trigger a second generation late in the season.
Why this matters in Scotland
A second generation follows specifically from a warm summer, so Scotland’s cooler, shorter summers likely make that additional generation on Scottish plums and damsons less common than further south — though no source states a second-generation frequency for Scotland specifically, so treat this as reasoning from the stated trigger rather than a separate finding.
Controls: what actually works
All the methods below are backed by real evidence, not just repeated folklore.
Tolerate the loss of some fruit
Often only a small proportion of fruits are affected, even though the earliest-ripening, most visibly-damaged fruit can make it look worse than it is at first.
Encourage natural predators
Birds, hedgehogs and ground beetles are predators that help keep plum moth numbers down.
Pheromone traps for monitoring, hung from early May
Traps alone rarely control plum moth; their main value is monitoring activity, though on an isolated tree they may catch enough males to reduce the females’ mating success and cut the number of fertile eggs laid.
Hang in the tree in early May. Adding bird netting around the trap reduces the risk of birds getting caught on the sticky sheet.
Biological control: pathogenic nematodes watered onto trunk, branches and soil
A mixed-species nematode product sold as "Fruit and Vegetable Protection" may provide some control. Applied to the trunk, branches and soil under the tree in September and October, once caterpillars have left the fruit; not while they’re still feeding inside it.
There are real limits: this treatment gives no protection the following year against female moths flying in from nearby gardens, so it may not be worthwhile where plums are widely grown locally. Nematodes can affect non-target animals and should be used with care, only where there’s a specific problem to treat.
This is the same "Fruit and Vegetable Protection" product used against cabbage caterpillars, covered on this site’s Caterpillar Nematodes page.
Natural predators of plum moth
These 3 creatures eat plum moth, and each has its own page here on how to get more of them in your garden: what to plant, or what to order and when.
Myths about plum moth
5 common claims about plum moth, checked: 4 busted1 disputed0 untested0 hold up
“Hanging a pheromone trap in the tree is enough on its own to control plum moth.”
A pheromone trap hung in the tree from early May catches adult male moths, and on an isolated tree that can reduce how many females mate and how many fertile eggs get laid. But the traps alone rarely control the pest; their main practical value is monitoring how active the moths are that season, not stopping the caterpillars getting into the fruit.
Our read The gap between "monitoring tool" and "control method" is easy to blur once a trap is marketed as a plum moth trap, but the same source that recommends the trap is explicit that it rarely controls the pest on its own.
What to do: Hang a trap in early May to see how bad the moth pressure is that year, add bird netting round it so birds don't get caught on the sticky sheet, and pair it with tolerating some fruit loss or a biological nematode treatment rather than relying on the trap alone.
1 source
- RHS, Plum MothPheromone traps hung in the tree from early May help monitor moth activity but alone rarely control plum moth, though on an isolated tree they may catch enough males to reduce fertile egg-laying.
“Grease bands or a corrugated cardboard trap wrapped round the trunk protect plums from plum moth.”
Grease bands and sticky trunk barriers work against moths whose females are wingless and have to climb the trunk to reach the tree and lay eggs, such as winter moth and mottled umber moth. Plum moth females are fully winged and fly straight to the fruit in midsummer, so a barrier on the trunk never intercepts them.
What to do: Skip the grease band for plum moth specifically. It still has a role against wingless winter moths on apple and other trees, but for plum moth use a pheromone trap for monitoring, encourage predators, or apply a biological nematode treatment instead.
1 source
- RHS, Grease Bands and Tree Barrier GluesSticky trunk barriers give no protection against codling moth or plum moth, because those species have winged females active in midsummer rather than the wingless, trunk-climbing females that grease bands are designed to stop.
“Plums or damsons dropping early in June means plum moth has got into the crop.”
Fruit carrying a plum moth caterpillar stays on the tree and ripens, usually slightly early and misshapen, right through to when it's picked; the caterpillar develops inside mature fruit rather than causing it to fall. Early June fruitlet drop is the signature of a different pest, plum sawfly, whose larvae tunnel into young fruitlets and cause them to fall off the tree weeks before ripening.
Our read The two pests get confused because both leave a caterpillar-like grub inside a stone fruit, but the timing and the fruit's fate on the tree tell them apart cleanly.
What to do: If fruit is falling in June while still small and green, look for a sawfly larva and don't treat it as plum moth. Save plum moth suspicion for fruit that ripens normally on the tree but looks early-ripe and slightly misshapen, with a pink caterpillar and brown excrement pellets near the stone.
1 source
- RHS, Plum MothPlum moth caterpillars feed inside fruit that ripens and stays on the tree, while plum sawfly larvae tunnel into young fruitlets that then fall from the tree early in June, a distinctly different symptom from plum moth damage.
“Victoria and Czar plums are especially at risk from plum moth.”
Victoria and Czar are documented as more susceptible to a different pest, plum sawfly, not plum moth. No variety-by-variety susceptibility to plum moth specifically has been published; the two pests get merged in gardener conversation because both produce a grub inside the fruit, and the sawfly susceptibility note for these two well-known varieties gets carried over to plum moth by mistake.
Our read Marked busted rather than unproven because there is a specific, sourced claim in circulation, and it traces to a real finding about a different pest rather than to nothing at all.
What to do: Don't pick or avoid plum varieties based on plum moth risk specifically; there's no published variety data to act on. Watch Czar and Victoria trees for sawfly's early fruitlet drop in June instead, since that's the pest they're actually flagged as more susceptible to.
1 source
- RHS, Plum MothThe plum cultivars Czar and Victoria seem more susceptible than others to plum sawfly, a claim made specifically about sawfly rather than about plum moth.
“Once you cut the caterpillar out, the rest of a plum moth-affected plum is safe to eat.”
University extension guidance on garden produce with internal caterpillar feeding is more cautious than the trim-and-eat habit many gardeners fall back on: once a caterpillar has broken the skin and tunnelled inside, it treats the fruit as carrying a foodborne-illness risk and recommends throwing it away rather than cutting round the damage and eating the rest.
Our read This guidance covers caterpillar-in-fruit damage generally rather than plum moth specifically, since no plum moth source addresses food safety directly, but plum moth caterpillars breaking the skin to tunnel to the stone is exactly the kind of internal feeding the guidance is written about.
What to do: Treat visibly affected fruit as compost or bin material rather than trimming and eating it. If you want to keep a share of the crop, pick fruit before any hole or excrement pellets are visible and accept some losses, which is also the recommended stance on plum moth generally.
1 source
- University of Wisconsin-Madison Division of Extension, To Eat or Not to Eat? Less-than-Perfect Garden ProduceWhen caterpillars break the skin of a fruit there is potential for foodborne illness, and fruits and vegetables with caterpillar feeding damage should be thrown away rather than salvaged.
Frequently asked questions
What should I do first if I find plum moth in my garden?
Start with tolerate the loss of some fruit. After that, encourage natural predators. Work through the rest in order of how well-evidenced they are if the first two aren’t enough on their own.
Will plum moth kill my plants?
They rarely kill established plants. The damage is usually cosmetic on outer leaves, and natural predators often bring numbers under control without you doing anything.
When are plum moth most active in Scotland?
Adults are active late May to July, mostly mid-June to mid-July; larvae do the main damage inside fruit from early summer until late summer, then overwintering as cocoons under bark. Scottish timings may lag behind UK-generic advice, so use the Frost Date Calculator to adjust for your region.
Should I just leave plum moth alone?
In most gardens, yes. plum moth are food for birds, wasps and predatory insects, and a healthy garden carries some without trouble. Tolerance is also RHS's official position now, for that same reason.
Sources
2 sources, recorded with what each was used for
- RHS1: identification, symptoms, look-alikes (pocket plum, gumming, plum sawfly), life cycle, active-by-month timing, tolerance guidance, pheromone traps, biological control, natural predators
- RHS1: corroborating the "Fruit and Vegetable Protection" nematode product and its Steinernema species, cross-referenced against the existing Caterpillar Nematodes wildlife entry