We believe these creatures are southern armyworm larvae, which means they are caterpillars, not worms! Southern armyworms, or Spodoptera eridania, are found throughout Central and South America, the Caribbean, and primarily in the southeastern United States, but populations have been reported as far west as California. Southern armyworms have a wide range of host plants including fruit, vegetable, field, and ornamental crops like zinnias!
Young southern armyworms are usually green or greenish-black, but mature caterpillars are black with red heads, like the specimens our reader found. Now that we have identified these “worms” we can move on to answer our reader’s other questions. Are these southern armyworms good or bad for her plants?
This answer isn’t as straightforward as an identification. In short, these caterpillars aren’t great for her zinnia plant. Since they feed in groups, they can quickly skeletonize the leaves of a plant. On the other hand, many plants can bounce back from the damage that caterpillars cause. There are a few natural ways our reader can protect her plants from these voracious eaters. The first is by manually removing the caterpillars and releasing them far from her garden. Another method is to introduce predators into her garden such as braconid wasps or Trichogramma wasps. These wasps lay their eggs inside of armyworm eggs, and the wasp larvae then eat the armyworm larvae. Finally, our reader can install barriers of floating row covers to keep these caterpillars away from her plants, especially in the spring and fall. Our reader can try any combination of these tips to protect her zinnia plant.
In conclusion, a reader asked us about the worms she noticed on her zinnia plant. We believe the creatures are southern armyworm caterpillars. We have offered a few ways to protect her plants from these voracious eaters.