Caterpillars on Geraniums — Which Ones and How to Stop the Damage

Caterpillars chewing through geranium leaves at night can cause surprising damage very quickly, particularly on container pelargoniums where the entire plant is accessible from a single pot. Moth caterpillars are the usual culprits — they feed nocturnally and hide in the compost or under lower leaves during the day, which is why damage appears to materialise overnight without an obvious cause. Identification and targeted removal are the most practical responses.

Which Caterpillars Attack Geraniums?

In the UK and much of Europe, the most common culprits are caterpillars of various noctuid moths, commonly called cutworms or the larvae of angle-shades moth (Phlogophora meticulosa) and similar species. These are fat, greenish or brownish caterpillars, 2–4cm long, that curl into a C-shape when disturbed. They consume leaves rapidly and can move from pot to pot. Some species also eat stems at the base — a symptom that looks like slug damage but occurs higher on the plant.

Finding the Caterpillars

Search at night with a torch when caterpillars are actively feeding. Check the undersides of leaves, inside curled or touching leaves, and in the top layer of compost around the plant's base. During the day, they are often concealed in the compost — dig gently into the top 3–5 cm and look for the caterpillars curled just below the surface. Large, older caterpillars may also hide under the pot rim or in gaps in baskets. Pick them off by hand and dispose of them away from the garden.

Biological Control with Bacillus thuringiensis

Bacillus thuringiensis (Bt) var. kurstaki is a naturally occurring soil bacterium available as a spray that kills caterpillars selectively without harming other insects, birds, or mammals. The caterpillar must eat treated foliage to be affected, so coverage of the leaf undersides is important. Bt breaks down in sunlight within a few days, so it needs reapplying weekly while caterpillars are active. It is one of the best organic options for caterpillar control on ornamentals and is approved for use in organic growing systems.

Physical Barriers and Deterrents

Fine insect-proof mesh draped over outdoor geraniums prevents adult moths from laying eggs on the foliage. This is practical for plants in borders but more awkward for hanging baskets and tiered containers. Sticky copper tape around pot rims provides some deterrence but is less effective against moths than against slugs. Checking plants in the evening and removing any newly hatched caterpillars before they grow large is the most time-efficient prevention method if moth populations in your garden are reliably high each summer.

Chemical Controls

Pesticides containing deltamethrin or lambda-cyhalothrin are effective against caterpillars on ornamental plants and provide a rapid knockdown. They also affect a wide range of other insects, including beneficial ones, so they should be used only when biological and physical methods have failed. Always apply in the evening when bees are less active and follow all label directions. Avoid spraying open flowers. Pyrethrins (organic pyrethrum) offer a compromise — effective against caterpillars but breaking down quickly and having a lower environmental persistence than synthetic pyrethroids.