thumb|Eggs on underside of squash plant's leaf
200px|thumb|Squash bug eggs on the underside of yellow crookneck squash leaves
thumb|Nymphs of several instars, on squash
thumb|thumbtime=108|Squash bugs including a [[Sphecidae wasp investigating them and a feather-legged tachinid fly quickly depositing another egg on one of them.]]
thumb|thumbtime=108|Mating pair of squash bugs.
Anasa tristis is a species of bug in the family Coreidae. It is a major pest of squash and pumpkins, found throughout North America, and is a vector of the cucurbit yellow vine disease bacterium. These bugs can emit an unpleasant odor when disturbed. It is commonly known as the squash bug but shares this name with certain other species.
Description
The adult A. tristis is a greyish-brown, somewhat flattened insect reaching a length of about and a width of . There is often a row of alternate brown and gold spots along the margin of the abdomen. Adults survive for three or four months.
Life cycle
In the southern part of its range, the adult female A. tristis lays two or three batches of about 18 eggs, but in the northern part of the range it just lays a single batch. The eggs are oval, somewhat flattened and bronze in colour, and are deposited on the underside of the leaves of the host plant. They may be clustered close together or more widely dispersed but are often regularly arranged. The eggs hatch after 7-9 days into nymphs which have five instars. The first instar nymphs are green and about in length. Each successive instar is larger and less hairy and grey. The fifth instar is grey, with developing wing pads and about in length. The complete nymphal stage lasts about 33 days.
Damage
Anasa tristis is a true bug that feeds by sucking sap, mainly from the leaves, but sometimes also the fruit. Historically, at least as far back as 1902, some gardeners believed that A. tristis had toxic saliva, however more recent research from 1993 suggests the process of harming plants does not involve any toxins. What happens is that the insects physically damage the xylem and leaves of the plant, which causes them to wilt, darken in colour and die. The heavier the infestation, the greater the damage to the plant. Sometimes one plant or part of a plant can be heavily attacked while surrounding plants are untouched.