虫 insizzect

Insect is middle plus a line at the bottom, which for the purposes of this mnemonic, we'll call a "stinger." So you can say,
Put some insects in the middle of your mouth and CHEW. Later use their stinger as a toothpick. MMMM!!! ごちそうさまでーす!

crybaby ★★☆☆☆
泣 (cry) + 虫 (insizzect) = 泣き虫 (crybaby)

weakling ★★☆☆☆ KUNKUN
弱 (weak) + 虫 (insizzect) = 弱虫 (weakling)

weakling, gayrod

cavity ★★☆☆☆ KUNKUN
虫 (insizzect) + 歯 (tooth) = 虫歯 (cavity)

a cavity (literally, 'bug tooth,' because parents would tell their kids, 'If you get a cavity, bugs will crawl in the hole and nest and lay eggs.')

insect ★☆☆☆☆ NP
昆 (insect) + 虫 (insizzect) = 昆虫 (insect)

insect, (a more scientific term than 'むし')

parasite ★☆☆☆☆
寄 (get close) + 生 (life / birth) + 虫 (insizzect) = 寄生虫 (parasite)

parasite (literally, 'close living insect')

poisonous insect ★☆☆☆☆
害 (damage) + 虫 (insizzect) = 害虫 (poisonous insect)