Hello Betsy, I am including the worksheet you submitted in another question here so our volunteer has everything they need in one spot. Thanks!

Dear Betsy,

Thank you for reaching out to the UVM Master Gardener Program with your question.

The damage to your basil is consistent with damage from the four-lined plant bug. The insect has piercing, sucking mouth parts so when it feeds on the leaves it results in small almost angular spots. The spots can appear almost clear or brown but later appear more black. There can be drop out of tissue leaving holes in the leaves. Unclear if the areas on the leaves that appear chewed are a result of confluence of spots and tissue drop out or if there is another process. The four-lined plant bug is quick and difficult to detect so the fact that you have not seen it is not unusual. The attached resource has nice photos of both the nymph and adult stage of the insect.

Damage from the four-lined plant bug occurs in May and June. There is only one generation each year. Both nymphs and adults can feed on the plant but they are only active for about a month. You could use row covers to protect the plants. If you can tolerate the damage the basil should recover. Usually treatment is not necessary. Adults lay eggs on the plant stems so you should remove damaged plants in the fall to remove eggs. Remove from your landscape, bury, or compost the damaged plant material.

Some of the “chewed" areas on the leaves could represent slug damage. The fact that the affected plants are partly shaded may suggest more moisture around those plants which would favor slugs. The slugs do their damage at night so you could inspect the plants with a flashlight at night to look for slugs

Selected Resources:

Fourlined plant bugs | UMN Extension

managingslugssnails.pdf (oregonstate.edu)