Treatment Plans
Eliminate sawfly larvae
Why are these false caterpillars devouring your leaves?
The problem
These are sawfly larvae, rather than caterpillars. Adult sawflies resemble small harmless wasps, named for the female's saw-like egg-laying organ. The real problem is their extremely voracious false caterpillars. They gather in groups and can devour a plant's leaves remarkably quickly, sometimes leaving only veins. Unlike true caterpillars, they have 6 to 9 pairs of abdominal prolegs, compared with a maximum of 5 in butterfly and moth caterpillars.
Because these are not true caterpillars, standard Bacillus thuringiensis (Bt) treatments are completely ineffective.
Their life cycle
The full cycle can last 4 to 6 weeks in favorable conditions. In the northern-hemisphere seasonal calendar described here, adults appear in spring, between April and June. Females cut into leaves or young stems to lay up to 300 eggs. Depending on climate, 2 to 3 generations may occur annually, explaining severe attacks from local spring to autumn.
| Stage | Estimated duration | Detail |
|---|---|---|
| Egg laying | A few days | Eggs are inserted directly into host plant tissues. |
| Larvae | 10–15 days | Intense feeding and visible damage. Larvae devour foliage in groups. |
| Pupae | Several weeks to several months | The final generation drops to the ground, burrows in and overwinters as pupae where winter occurs. |
| Adults | Short | Emerge in local spring, mate and restart the cycle. |
What to do now
- Remove all visible larvae by hand. Wear gloves if preferred, but they are harmless to touch. Crush them or drown them in soapy water.
- Use a strong stream of water to dislodge remaining larvae, particularly those difficult to reach.
- For persistent infestation, spray soft soap solution (diluted to 1–2%, or approximately 15 to 30 mL (1 to 2 US tablespoons) per 1 L (about 1.06 US qt) of lukewarm water). This contact treatment works on young larvae. For severe infestation, spinosad is the most effective larvicide against sawflies.
- Always test the product on one small leaf 24 hours before treating the whole plant to avoid scorching.
- Cut off and discard the worst-affected leaves, where eggs may remain.
Prevention
- Inspect leaf undersides once or twice a week from local spring to autumn.
- Lightly hoe the soil surface in autumn where a cold winter follows, exposing overwintering pupae to cold and predators.
- Encourage biodiversity: birds, spiders and certain wasps are natural predators of sawflies.
Common problems
- Sawflies · False caterpillars that chew scalloped edges into leaves.
FAQ
How can I reliably distinguish a sawfly larva from a true caterpillar?
The most reliable feature is the number of prolegs (sucker-like structures on the abdomen). A true caterpillar has **5 pairs or fewer**. A sawfly larva has **6 to 9 pairs**. Sawfly larvae also tend to rear into an S shape when disturbed.
Are adult sawflies dangerous to plants?
No. Adults are generally harmless, feeding on pollen or nectar. Their only potential damage is the small incisions females make to lay eggs in leaves or stems.
Can sawfly larvae kill my plant?
A light infestation on a healthy, established plant mainly causes cosmetic damage, and the plant recovers. However, severe, repeated attacks can weaken it considerably and make it more susceptible to disease.
Does caterpillar treatment (Bacillus thuringiensis) work?
No, absolutely not. **Bacillus thuringiensis (Bt)** is a bacterium that specifically targets Lepidoptera (butterflies, moths and their caterpillars). Sawflies are Hymenoptera, so this treatment is **completely ineffective** against their larvae.
When should I be most vigilant?
Increase monitoring from spring to autumn where these local seasons apply. In the northern-hemisphere calendar described here, the first adults appear around **April–May**, with several generations possible until September.
Are these larvae dangerous to people or pets?
No. Sawfly larvae are completely **harmless** to people and animals. They neither sting nor have irritating hairs. You can handle them without concern.
Can soft soap harm my plant or other insects?
Yes. Soft soap is a non-selective contact insecticide. It can kill beneficial insects such as ladybugs if they are hit. It can also scorch sensitive foliage. Always **test a small area** and use sparingly, only when hand removal is insufficient.