Caterpillar Nematodes in Scottish Gardens

“Caterpillar nematodes only affect the pest caterpillars you're targeting and leave butterfly caterpillars you want to protect alone”

Caterpillar nematode products are not selective by species. They are insect-specific rather than pest-specific, meaning they infect susceptible caterpillars generally, not only the named target pest. Any caterpillar of a susceptible species present on treated foliage or soil at the time of application is at risk, including butterfly larvae a gardener might otherwise want to leave alone.

Our read Products are usually sold and marketed against a named pest (cabbage caterpillars, box tree caterpillar), which can read as species-specific targeting. The RHS caution applies to the whole group the nematode infects, not just the pest on the label.

What to do: Apply only where a specific, identified caterpillar problem exists, not as a routine or preventative spray across a bed with mixed planting, and avoid treating areas where you know desirable species (such as small tortoiseshell or peacock caterpillars on nettles) are feeding.

“Caterpillar nematodes work fine applied to dry foliage on a sunny day, since it's a living organism that finds its own way to the pest”

Nematodes should be applied during cool, dull or damp weather, and the longer treated caterpillars and foliage stay wet, the greater the chance the treatment works. Nematodes travel through a water film to reach their host, so foliage that dries quickly in sun and wind sharply cuts the number that survive to make contact.

What to do: Apply in the evening or during overcast, damp weather rather than full sun, and water the treated area afterwards if rain isn't forecast, to keep foliage and soil surface wet for as long as possible after application.

In Scotland: Scotland's cooler, damper summer conditions work in the nematode's favour compared with drier parts of the UK, but a spell of dry, sunny weather still calls for follow-up watering to keep foliage wet.

“A nematode treatment for codling moth will stop caterpillars tunnelling into this year's apples”

The nematode application for codling moth is timed to September to October, sprayed onto trunk, branches and the soil beneath, specifically after caterpillars have already left the fruit to overwinter in bark crevices and leaf litter. The treatment targets that overwintering generation, not the caterpillars currently feeding inside fruit, and it only provides some control of codling moth.

Our read The claim persists because the product is sold under the codling moth pest itself, which reads as season-long protection rather than a narrow autumn window aimed at next year's population.

What to do: Treat as a between-seasons measure applied in autumn once fruit has been picked and caterpillars have exited, not as a rescue treatment for fruit already showing entry holes, and don't expect it to prevent moths flying in from neighbouring gardens the following year.

“Caterpillar nematodes can be bought in advance and kept in the shed until you're ready to use them, the same as a bottle of pesticide”

Nematode products are living organisms that should be used as soon as possible after purchase or delivery; some formulations have a longer shelf life than others, but none can be stockpiled indefinitely. This is unlike a dry chemical pesticide, which tolerates storage on a shelf without losing effectiveness.

What to do: Order nematodes to arrive close to when the target caterpillars are actually present and conditions are cool and damp, rather than buying ahead of the growing season, and check the specific product's stated shelf life and storage temperature on arrival rather than assuming it matches other garden products.