Larva can have both positive and negative economic impacts, as they can be a vital food source for some while also being agricultural pests that damage crops. Some larva are edible and provide a source of protein and nutrients for humans and animals. 

On the other hand, larva can consume leaves and other plant parts, potentially damaging crops and reducing yields. In the context of the manner in which the question was asked, the negative economic impact is actually what the examiner intended. Thus, option D is the correct answer.