Some bugs are poisonous and you may be surprised to learn which ones can cause the most harm. According to a popular pest and lawn service company(8), “Caterpillars are especially dangerous to dogs. Monarch butterfly caterpillars eat milkweed, which contains a substance that can poison dogs and harm their hearts. Other caterpillars like the Slug, Asp and Gypsy moth varieties appear hairy or spiny and can also be toxic to pets, causing intestinal problems and painful internal reactions.” Also, venomous spiders like black widows and brown recluses can cause dogs to become seriously ill.