Also called the Grecian laurel, sweetbay or Laurus nobilis, the bay laurel is best known for its leathery, aromatic bay leaves that are used to season food. Unfortunately, chewing insects can ravage your bay laurel, damaging the leaves and making them unusable. The most common insects that infest bay laurels include oleander scales, greenhouse thrips and laurel psyllids or bay suckers. But keep in mind that other types of insects may chew on your bay laurel's leaves as well.
Study the damage to the leaves on your bay laurel to identify the insect. If you have bay suckers, also called trioza alacris or jumping plant lice, the leaves will curl at the edges and turn yellow, then brown. Oleander scales and laurel psyllids suck on the plant sap and excrete a sticky liquid called honeydew that can sometimes facilitate a fungal infection of black sooty mould. Greenhouse thrips bleach the leaves and leave behind black excrement on the undersides of the leaves.
Look at the insect itself to make the identification, if you're lucky enough to see the bug in action. Use a magnifying glass to study the insect. The adult greenhouse thrip has a black body with pale-coloured wings, while the laurel psyllid or jumping plant lice is only 2mm in length and looks like a tiny cicada with wings. Oleander scales are tiny insects with strawlike mouthparts and often immobile, wingless and elongated or flattened.
Identify the insect on your bay laurel leaves by its eggs or larvae. Greenhouse thrips have larvae with white to yellowish bodies; bay suckers or laurel psyllids lay nearly microscopic eggs on the underside of the leaves; and immature scales are wingless, immobile and typically have heads that are indistinguishable from their bodies.
Consider other potential culprits, such as beet armyworms, diamondback moths, blister beetles, cabbage loopers and multiple species of grasshoppers. Beet armyworms are 1 ¼ inches long and bright green with dark stripes, while cabbage loopers are light green with white or yellowish stripes. Diamondback moths are grey, 1/3-inch long with yellow diamond-shaped spots on their backs, while their larvae are yellowish-green with fine black hairs on their bodies. Blister beetles have prominent body segments, are long and narrow, and are grey, black or striped.
Control oleander scales using horticultural oils or introducing natural predatory insects and parasites. Although psyllids usually go away naturally, introduce natural predatory insects like lady beetles, lacewings or parasitic wasps to feed on the psyllid population, or treat your bay laurel with a systemic insecticide, neem oil, insecticidal soap or horticultural oil. Good cultural practices and applications of narrow-range horticultural oils can help control greenhouse thrips.
Don't consume or cook with any bay laurel leaves to which you've applied insecticides or horticultural oils. Always follow the instructions carefully on the chemical label when you're using insecticidal products. Also, try to choose the least toxic products for use on your plants.