The common earwig is very widely distributed and abundant on apple trees. Most trees have a resident earwig population. Earwigs are omnivorous feeding on plant material including apple fruit and a wide range of insects and mites.
- Earwigs mate in late autumn and the female then excavates an underground nest in which the pair overwinter.
- Eggs are laid in the late winter and early spring, the first batch typically containing 30‑40 eggs.
- The female then ejects the male from the nest as males eat the eggs.
- By late spring most males have died. Some females lay more than one batch of eggs but all eggs are fertilised before oviposition.
- Females display maternal care for their young.
- The young larvae leave the nest after moulting to the second instar. There are four larval instars.
- Larvae from the first batch of eggs become adult around mid-July and those from the second batch in September.
A phenological model has been developed in the Netherlands to predict earwig life stages. Earwigs from the first egg batch reach the adult stage after 880 day degrees above a threshold temperature of 6°C measured from 1 January. Orchard populations are highest between July and September. Earwigs forage at night and shelter by day so populations in orchards are often underestimated. They tend to shelter in narrow crevices.
Earwigs are important predators of many pests of apple and pear. They feed on aphids (particularly woolly aphid), apple and pear suckers, Blastobasis caterpillars, codling and tortrix moth eggs and larvae, scale insects and spider mites.
- Reports from European funded research suggest that thiacloprid (Calypso), spinosad (Tracer), indoxacarb (Steward) and flonicamid (Mainman) could harm earwigs.
- To test these claims, AHDB funded Project TF 196 to investigate the effects of a range of commonly used products on different earwig life-stages.
- In the project’s first year, laboratory tests suggested that earwigs were far more sensitive to chlorpyrifos than to the other products tested. Spinosad (Tracer) and thiacloprid (Calypso) were also harmful. Methoxyfenozide (Runner) reduced the growth of nymphs. Indoxacarb (Steward), spirodiclofen (Envidor) and acetamiprid (Gazelle) were only moderately toxic. Abamectin (Agrimec), chlorantraniliprole (Coragen) and flonicamid (Mainman) were not harmful in the laboratory experiment. This initial study did not take into account the repeated exposure that would occur in a commercial orchard.
- In the second year, the effects of products applied in spring or summer in a Discovery orchard were assessed. In trees treated with chlorpyrifos, numbers of earwigs fell over time. Significantly fewer earwigs were found in areas treated with thiaclorpid (Calypso) and flonicamid (Mainman). It was thought that thiacloprid may have a direct effect on the earwigs.
- Conventional spray programmes were also compared to ‘earwig-safe’ programmes on two commercial farms. Numbers of male earwigs were unaffected by either spray programme, but significantly more females and nymphs were found on trees treated by the ‘earwig-safe’ programme.
- At present, growers will be forced to continue to use some of the products harmful to earwigs, to control a range of other pests. AHDB has commissioned Project TF 220 to further research the effects of these products on earwig nymphs and adults, and to assess the effects of different spray timings.
Unfortunately, earwigs feed on plant material including the tender leaves in shoot tips (causing characteristic marking along either side of the main vein) and on the flesh of ripening apple.
- It is believed that they mainly cause secondary damage by excavating pre-existing damage. This can be serious on soft skinned varieties like Discovery. Up to 20% of fruits may be damaged but injury is usually much less.
- AHDB funded research (Project TF 185) used a form of ‘genetic fingerprinting’ to identify the species whose remains were found in the guts of earwigs caught in orchards, and to see whether they had a preference for eating any orchard pests. The prinicpal contents were found to be lichen and fungi, but the DNA of apple pest species including rosy apple aphid and apple leaf midge was also detected. No apple DNA was found in gut samples, confirming that the earwigs had not been feeding directly on the fruit.
- Overall therefore, the benefits of earwigs outweigh their disadvantages as pests.
- They should be encouraged by the provision of artificial refuges and the use of pesticides that are harmful to them should be avoided from June to September.