Many pest insects cause the most damage in the larval stage, particularly those that eat plants. Maggots are legless. There are harmful and beneficial species. Closely related to spiders. For example, a poison that works on contact or as a stomach poison.
Some species attack roots or leaves of plants, others are parasites on animals or insects.
The tops may die back in winter or in drought, but the roots or rhizomes persist to resume growing in favourable conditions. Toxicity is one consideration in assessing the hazard in handling a particular pesticide.