Mustard sawfly
Dark, slug-like larvae that skeletonise mustard seedlings from the margins in — voracious at dawn and dusk, and worst on the young crop.
- Pest type
- Defoliating (sawfly) larva
- Attacks
- Seedlings & young leaves
- Worst in
- Early crop; cool dawn/dusk
- Main risk
- Skeletonised seedlings; resowing
Overview
The mustard sawfly is an early-season defoliator of mustard and rapeseed. The damaging stage is the larva — a dark, greenish-black, wrinkled, slug-like grub with several pairs of legs — which is unusually voracious for its size and feeds mainly in the cooler parts of the day, at dawn and dusk, hiding lower down in bright midday sun.
The larvae bite holes in the leaves and eat them from the margins inward, skeletonising and stripping the tender seedlings and young leaves. On the young crop this can be severe enough to thin the stand and even require resowing, while on older plants heavy defoliation reduces vigour. Because the larvae feed openly (if mostly at dawn/dusk), the pest is easier to manage than a borer — handpicking, timing observation to dawn/dusk, and timely sprays all work.
How to identify it
- Dark greenish-black, wrinkled, slug-like larvae (with several pairs of legs) on the leaves, curling up and dropping when disturbed.
- Leaves bitten full of holes and eaten from the margins inward, skeletonised in a heavy attack.
- Larvae feeding actively at dawn and dusk and hiding lower down or on the soil during bright midday.
- Damage concentrated on seedlings and the young crop.
The damage it does
- Holed, skeletonised and stripped seedlings and young leaves, reducing growth.
- Severe stand loss on the young crop that can require gap-filling or resowing.
- Reduced vigour of older plants under heavy defoliation.
- Fast damage because the larva is unusually voracious for its size.
When to act
- Watch the seedling and young-crop stage closely — this is when the sawfly does its worst.
- Inspect at dawn or dusk, when the larvae are actively feeding on the leaves and easiest to find.
- Act promptly on the young crop before the stand is thinned.
Organic & cultural control
- Handpick and destroy the slug-like larvae at dawn or dusk when they are feeding on the leaves — effective on small plots.
- A light irrigation can dislodge larvae and force them from the seedlings.
- Sow at the recommended time for quick, uniform establishment so seedlings outgrow early damage, and destroy crop residue after harvest.
- Conserve natural enemies by avoiding unnecessary broad-spectrum sprays.
Chemical control
- Where seedlings are heavily attacked, use a CIB&RC-registered insecticide for the crop, applied to the foliage — an evening application catches the dawn/dusk-active larvae well.
- Because the larvae feed openly on the leaves, good foliar coverage gives effective control.
- Rotate insecticide groups and observe the pre-harvest interval on the seed crop.
- Confirm the current recommendation and dose for your area with your local KVK.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
Dark slug-like grubs are eating holes in my mustard seedlings. What are they?
Those are mustard sawfly larvae — dark, greenish-black, wrinkled, slug-like grubs that are very voracious and feed mainly at dawn and dusk, biting holes and skeletonising the seedlings from the margins in. They hide lower down in bright midday sun. Inspect and handpick at dawn/dusk, and protect the young crop before the stand is thinned.
Why can’t I find the larvae during the day?
Because the mustard sawfly larva feeds mainly in the cooler parts of the day — at dawn and dusk — and hides lower down on the plant or on the soil during bright midday sun. So inspect and handpick, or time an evening spray, when they are active on the leaves at dawn or dusk.
How serious is it for a young mustard crop?
It can be serious. The larva is unusually voracious and concentrates on seedlings and young leaves, skeletonising and stripping them, and a heavy attack can thin the stand enough to need gap-filling or resowing. Sow on time for quick establishment, watch closely at dawn/dusk, and act promptly on the young crop.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.