Plants affected
Bay (Laurus nobilis)
Symptoms
Bay tree leaves thicken and curl at the margins and turn yellow. The discoloured areas later dry up and become brown.
Cause
Most of the damage is caused by the nymphal stages of an insect, the bay sucker that sucks
sap from the young leaves. The winged adults are greenish
brown and about 2mm long, and they overwinter in sheltered
places.
In April to May the adult suckers emerge and start to feed, causing the leaf margins to start curling under. In May the females lay eggs under the curling leaf margins. The young nymphs are grey, flattened wingless insects, whose bodies are covered with a white fluffy material. Like the adults, these also suck sap and increase the leaf curling. Two or three overlapping generations occur between May and September.
Control
For light infestations pick off and burn or otherwise dispose of infested leaves. Badly affected shoots can be pruned out in winter. This will encourage new growth in the spring and may remove some overwintering adults.
If an insecticide is necessary and the plant is small enough to be sprayed, it can be treated with the systemic insecticide thiacloprid (Provado Ultimate Bug Killer Ready To Use) when signs of new damage are first seen. If the leaves are being used for culinary purposes, a minimum 14-day interval must be left between spraying and using the leaves.