Another article on Cracked, another post of creepy bugs.
Malaria apparently makes human hosts more attractive to mosquitoes. Humans carrying malaria at all are more attractive than non-malarial humans but the most attractive are those who carry malaria in its contagious or transmittable state. How about a disease that, if caught, makes a PC more attractive to the local beasties? In old school, this could be any monsters (as it fits with mythical underworld) and increase the rolls on wandering monster checks. A more realistic vein would be to attract one type (insects/rodents) or subtype (giant caterpillars/rats) of monster that is known to live in the area. The PC would emit a scent (perhaps perceptible by the PCs themselves, perhaps not) that attracts the monsters and the infected PC(s) would be preferentially targeted. Somewhat similar to a shrieker, I suppose.
Gall wasps form galls on certain plants (usually oak trees). The eggs are laid on an oak blossom (or other part of the tree) and they signal the formation of a gall, or ball-like outgrowth, that encases the eggs and protects and feeds the larvae when they hatch. This by itself is an idea for a forest (or cavern) encounter where the landscape has numerous gall growths on trees (or outcroppings of rocks) that, if disturbed, burst open and pour forth regular-to-giant-sized wasps. On top of the galls, there are a number of other wasps that specifically prey upon gall wasps. These wasps have a metallic appearance and an extra-long ovipositor (egg laying spike/tube) that is used to penetrate the gall and lay their own eggs within, which then pray upon the gall wasp larva. As a further counter to this, some gall wasps actually develop a sweet-smelling gall that attracts ants. The ants then serve as defense against the hyperparasitoid wasps. For this, the PCs would need to be seeking out the gall and hoping to kill the larva (or otherwise retrieve something from inside) and the giant ant guardians (or whatever creature it attracts) are just an extra layer of defense.
Baculovirus preys upon gypsy moth caterpillars. The virus infects the caterpillar through contact or ingestion and forces the caterpillar to climb up to the top of a tree or branch and hang upside down, all the while replicating within the caterpillar's tissues. Once the caterpillar is in position, the virus begins replicating digestive proteins that liquefy the caterpillar's body and rains its infectious tissue down on other caterpillars, spreading the virus further. The idea is somewhat similar to the infectious troll idea posted earlier. It works best as a way to spread disease to the PCs, perhaps the malaria-smell virus from above or a more demonic strain (literally demonic).
Xenos vesparum is an insect that infects wasps and literally controls them from the inside. This might work to make a wild giant wasp nest more dangerous. In a sandbox style game, the PCs encounter the wasps and steer clear. When X vesparum shows up, it specifically makes wasps antisocial and leads them away from the nest to mate with other X vesparum (who arrive in their own possessed wasps). This would allow a giant wasp attack away from the site of the giant wasp nest and, if PCs investigate the bodies (who knows? maybe it swallowed a jewel) they would find another large insect literally living inside the wasp. And maybe if the wasp stings a PC, one of the X vesparum eggs end up inside them ... mwahahahah!!!
The final insect on the list in the article is a wasp that preys upon a special butterfly caterpillar. The caterpillar smells just like an ant larva, so after feeding on leaves for a few weeks, it falls to the ground and foraging ants, thinking it is one of their larva, take it back to the nursery in the ant hill. They then feed and protect it while it finishes fattening up and turning into a pupa. The wasp is somehow able to tell which ant hills have the butterfly larva engineers a distraction to get inside and lay its eggs in the caterpillars. It enters the ant hill and, after getting attacked by the ants, sprays a pheromone that drives the ants to attack each other (ignoring the wasp). It lays its eggs in the caterpillars and leaves. The wasp egg hatches after the caterpillar forms a chrysalis (which also smells like an ant pupa, so it remains protected by the ants) and develops within while eating the caterpillar. Both the hatching butterflies and wasps then leave the ant hill without incident (perhaps still smelling like ants due to the chrysalises). I am not as certain how to use that idea.