The psyllids or plant suckers are a group of

Sap is a fluid that circulates through a plant's vascular system, in a similar way to blood moving through our veins. Phloem sap carries the sugars produced in the leaves by photosynthesis down to roots and other storage organs, as well as carrying minerals and hormones. Xylem sap is watery and transports nutrients absorbed by the roots upwards to the rest of the plant. Some plants leak sap from wounds or pruning cuts, and this is known as bleeding.

Suckers are growths that appear from the root systems of many trees and shrubs such as lilac, cherries, Robinia. On grafted plants such as roses, suckers can grow from below the graft union, on the rootstock material. Suckers are usually removed by gently pulling off when young.

sucker

• Bay tree leaves thicken, curl downwards at the margins and turn yellow; often only half of the leaf is affected
• The discoloured areas later dry up and become brown 
• Small greyish white insects may be seen underneath or near the curled leaf margins. The nymphs usually producing a white waxy material
• The long term health of the plant is usually unaffected

Bay sucker can affect the appearance of bay but does not usually reduce the vigour of bay plants and so control is not necessary. Plant suckers can be a part the biodiversity a heathy garden supports.  

• Often suckers do not affect the growth or vigour of plants and so can be tolerated
• Encourage predators and other natural enemies of suckers, in the garden, such as birds, ladybirds, wasps and ground beetles
• If necessary distorted leaves can be removed from the plant however, this may cause more damage than the insect

Curled thickened bay leaves are often caused by the nymphal stages of the bay sucker when they suck sap from the young leaves during the summer.

• The winged adults are greenish brown and about 2mm long (less than 1/8in), and they overwinter in sheltered places
• In April to May, adult bay suckers emerge and start to feed, causing the leaf margins to begin curling over. Eggs are laid under the curling leaf margins
• The nymphs are grey, flattened wingless insects, whose bodies are covered with a white fluffy material
• Like the adults, the nymphs suck sap and increase the leaf curling and discoloration
• Two or three overlapping generations occur between May and September