Most monarchs die before they grow up because of five main monarch caterpillar threats. Predators and parasites attack them on the plant. Pesticides and disease poison them from within. Extreme heat kills them on hot days. Together these dangers wipe out more than 97% of all eggs and caterpillars. Each one calls for a different plan in your garden.
Monarch caterpillar predators cause the most visible losses you'll see. Fire ants are the worst in southern states. Pocius 2021 found 0% survival at Texas sites with heavy fire ant activity. Paper wasps, yellow jackets, and stink bugs also attack your caterpillars at every stage. I lost six caterpillars to paper wasps in one summer before I figured out what was going on. You can spot wasp damage by torn body parts left on the leaf.
Parasites are harder to find than monarch caterpillar predators but kill just as many. Tachinid flies lay tiny white eggs on the caterpillar's skin. The fly larvae burrow inside and feed from within. You'll know you have a problem when you see white strings hanging from the chrysalis. Those are fly larvae coming out to pupate. I now check every caterpillar for small white dots on the body. You can sometimes pick these off with tweezers before they hatch.
The pesticide problem hits your monarchs in ways you might not expect. Pesticides killing monarchs don't just come from direct spray on your milkweed. Chemicals like clothianidin drift from treated fields and settle on your plants from far away. Doses as low as 0.5 to 5.0 parts per billion cause 50% mortality in first instar caterpillars. Even tiny amounts that don't kill can stop your pupae from forming right. You end up with butterflies that have bad wings and can't fly.
Heat is a growing threat that you should take more care with each year. James 2024 found that temps at 107.6°F (42°C) kill 80 to 90% of exposed caterpillars. Your caterpillars have no way to cool down on their own. They depend on shade and air flow to make it through hot days.
Predator Control
- Fire ants: Treat mounds within 30 feet of your milkweed using bait products that won't get on your plants.
- Wasps and birds: Put mesh popup cages over your milkweed to block flying predators while letting air through.
- Stink bugs: Pick them off your plants by hand in the morning when they move slow and drop them in soapy water.
Pesticide Buffer Zones
- Distance: Keep your milkweed at least 100 feet (30 meters) from any treated lawns, gardens, or crop fields.
- Neighbors: Ask the people near you to skip spraying on windy days and try organic pest control instead.
- Clean plants: Grow your milkweed from seed or buy from nurseries that promise their plants are chemical-free.
Heat Protection Steps
- Shade: Plant your milkweed where it gets afternoon shade during the hottest months of the year.
- Misting: Spray your milkweed with water during heat waves to cool the leaf surfaces where your caterpillars rest.
- Indoor rescue: Move your caterpillars inside when forecasts call for temps above 100°F (38°C) for days in a row.
You can't stop every threat, but cutting down the biggest killers in your yard makes a real difference. Figure out which dangers hit your area hardest and focus your work there. A few targeted steps will protect far more of your caterpillars than trying to fix everything at once.
Read the full article: Monarch Caterpillar: Stages, Care, Facts
