The larvae of butterflies and moths are one of the largest groups of pests to attack ornamental crops. All caterpillars have biting mouthparts and no part of the plant is entirely safe from attack. Those that gnaw away at roots (swift moth) or tunnel inside stems (leopard moth) will weaken plants. Those that eat foliage can strip a plant, skeletonising leaves.
On protected pot plants and bedding, the most common caterpillars are those of the silver Y and angle shades moths. Various tortrix moths can cause widespread damage during the growing season, while brown-tail moth is a problem for some local authorities because of the allergic reaction caused by the caterpillars' hairs. Winter moths attack fruit and ornamental trees and shrubs, and can defoliate small trees. Growers buying in plants should look out for notifiable caterpillar pests.
Control strategies mainly focus on larval stages. Burrowing caterpillars and tortrix moth larvae that knit leaves together with silk webbing are more difficult to control.
How to recognise it
Caterpillars come in many colours, with variable markings and levels of hairiness. Newly emerged, they can look very different to mature ones of the same species. They have three pairs of jointed legs behind the head capsule and two-to-five pairs of non-jointed legs towards the rear of the body. Adults can be difficult to identify because most are nocturnal.
- Angle shades moth (Phlogophora meticulosa). Caterpillar: green or brown, with V-shaped bands down each side, 35-50mm long and feeds at night on a range of ornamental plants. Adult: mottled fawn brown with forewings bearing characteristic darker triangular mark on their outside edge.
- Carnation tortrix moth (Cacoecimorpha pronubana).Caterpillar: green or yellow, about 20mm long, particular pest on glasshouse crops and evergreens such as euonymus and privet. Adult: brown forewings, orange hindwings.
- Lackey moth (Malacasoma neustria). Caterpillar: distinctive blue-grey, striped orange and white, and grows to 50mm long. Produces characteristic tent from webbing in which communities of caterpillars feed, that can be mistaken for brown-tail moth tents. Adult: dull-brown tortrix moth.
- Light-brown apple moth (Epiphyas posvittanus). Caterpillar: bright to olive green with a darker central stripe and two side stripes, up to 20mm long. Light-brown head. Adult: tortrix moth with brown forewings, darker at the tip.
- Silver Y moth (Autographa gamma). Caterpillar: bright to dark green, 30-40mm long, attacks glasshouse crops. Adult: mottled brown with dark forewings bearing characteristic small white "Y" marking.
- Small ermine moth (Yponomeuta species). Caterpillar: dark spots on grey-green background. Willow, euonymus and hawthorn are favourite hosts. Adult: tortrix moth. White wings with black spots. Eggs are covered by scales that protect the caterpillars over winter.
- Geranium bronze butterfly (Cacyreus marshalli). Caterpillar: yellow or green with many white hairs. Feeds and develops in tunnels inside the plant. Adult: bronze coloured. Forewings edged with broken white band, while hindwings have a white margin.
- Oak processionary moth (Thaumetopoea processionea). Caterpillar: brown body and dark head turning grey with a single dark stripe and white line either side along their back. Long hairs also develop from red/orange pinacula (wart-like growths). Develop in a large larval nest on oak, hornbeam, hazel, beech, sweet chestnut and birch. Adult: wingspan 30-32mm with grey speckled forewings and darker colourings.
Notifiable pests include the Mediterranean climbing cutworm (Spodoptera littoralis), Gypsy moth (Lymantria dispar) and South American tomato moth (Tuta absoluta). Details can be found on the UK Plant Health Risk Register at https://secure.fera.defra.gov.uk/phiw/riskRegister/.
Eggs of caterpillar pests are usually laid on the undersides of leaves, but sometimes on the upper surface or on bark, singly or in groups. Tortrix moth caterpillars spin leaves together, from which they can feed more safely. Pupation can take place on the plant or on the soil with several generations a year, especially under protection. Pests usually overwinter as a pupa, or as the egg in some species. Adults feed on nectar. The winter moth is unusual in that adults are active during autumn and winter.
Symptoms include leaf-feeding damage, ranging from holes to complete defoliation, leaf webbing (tortrix moths and winter moths) and frass deposits.
Treatment: biological control
The parasitic wasp Trichogramma can control carnation tortrix caterpillars on protected crops. Females lay eggs inside pest eggs, working best at temperatures of 23-25 degsC. Macrolophus and lacewings predate on eggs and young caterpillars.
Treatment: cultural control
Pheromone traps are available for monitoring adults of several species. Installing a range of different traps in problem areas at the beginning of the season can help with identification and effective spray applications.
Control weeds including charlock and wild mignonette (cabbage white); creeping thistle, dandelion, docks and couch (swift moth); dandelion, docks and sorrel (tiger moth) and goat willow (brown-tail moth).
Treatment: chemical control
Active ingredient Bacillus thuringiensis var. kurstaki
IRAC code 11
Formulation Dipel DF (Interfarm)
Action(s) A bioinsecticide specific to caterpillars.
Active ingredient Cypermethrin
IRAC code 3
Formulation Toppel 100* (United Phosphorus)
Action(s) Contact, synthetic pyrethroid with long residual activity.
Active ingredient Deltamethrin
IRAC code 3
Formulation Various including Decis* (Bayer CropScience)
Action(s) Contact synthetic pyrethroid with long residual activity.
Active ingredient Diflubenzuron
IRAC code 15
Formulation Dimilin Flo (Certis)
Action(s) Selective insecticide. Best control achieved by applying at egg hatch in April.
Active ingredient Fenoxycarb
IRAC code 7B
Formulation Insegar WG (Syngenta)
Action(s) Insect growth regulator for summer fruit tortrix moths.
Active ingredient Indoxacarb
IRAC code 22A
Formulation Steward* (DuPont)
Action(s) Acts by ingestion and contact with larval and egg control.
Active ingredient Methoxyfenozide
IRAC code 18A
Formulation Runner (Landseer)
Action(s) Moulting accelerator insecticide for larvae of codling/tortrix/winter moths.
Active ingredients Pyrethrins
IRAC code 3
Formulations Pyrethrum 5EC (Agropharm), Spruzit (Certis)
Action(s) Short-term, knock-down product. Predators can be re-introduced after seven days.
Active ingredient Spinosad
IRAC code 5
Formulation Conserve (Fargro), Tracer* (Landseer)
Action(s) Selective insecticide with good control on a range of moths.
Fully updated by Dove Associates
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Incompatible with biological controls.
Compatible with biological controls.
Use plant protection products safely. Always read the label and product information before use.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.