The chafers are beetles of the Scarabaeidae family. The larvae of both species live in the soil and feed on the roots of many plants. They are a serious pest that can cause substantial damage. The adults are not harmful.
Larval feeding on the roots can lead to yellowing and wilting of plants. Damage can be severe on young plants and especially in grassland and turf. There, secondary damage is often caused by birds and small mammals digging for the grubs to eat them.
Adult Amphimallon sostitialis are 14-18 mm long and yellowish chestnut brown. Amphimallon majalis is somewhat smaller; about 13-15 mm long and reddish-brown in colour.
The adult beetles fly in the evening, generally in June and July and land on trees to mate. During the night the females move to the ground to lay their eggs. Eggs are shiny and white. The eggs hatch after about four weeks and the larvae start to feed on roots. The larvae are white-coloured, C-shaped grubs, with a yellow-brown head, and up to 30 mm long when fully grown. Larval development takes two years, fully fed larvae pupate in an earthen cell and the adult beetles emerge within 2-4 weeks.