When it comes to insects landing on a spiders web, many forces come into action.
Buzzing insects generate large amounts of electrical charge that makes them easily become trapped by spider webs. Webs are generally neutral or negatively charged, and are attracted by the positive charge on insects such as bees or flies, which increases the chance for a flying insect to immediately stick to the web. Directly before an insect comes into contact with a spider web, the web distorts slightly towards the insect, although this will only occur with charged insects (Berkeley.)
Any insect that flies close enough to a spiderweb can be trapped by a spiderweb with a neutral charge.