Neither is more effective across the board: a systemic treatment works better against insects feeding inside the tree and on trees too large to spray, while a topical spray works better against leaf feeding caterpillars on a tree small enough to cover. The deciding factors are where the pest feeds, how big the tree is, and how quickly control is needed.

Why does where the pest feeds decide the method?

A systemic insecticide is carried through the tree's vascular system, so it reaches insects that feed from within, while a topical spray acts only where it lands on leaves and bark. Boring insects feed on a tree's vascular tissue, out of reach of a spray, whereas caterpillars feed on the foliage a spray can coat.

When is a systemic treatment more effective?

Against boring insects, and on trees too large for spraying to be an option. Injected into shallow holes at the root flare, the Multi Insect Killer Tree Injection Kit spreads through the tree in about 72 hours, while Once A Year Insecticidal Drench w/Merit, applied at the base as a soil drench, takes about 3 weeks to take hold and then protects for a full year.

When is a topical spray more effective?

Against caterpillars feeding on the leaves of a tree the spray can reach. Late in the afternoon or on a cloudy day, since its active ingredient breaks down in sunlight, Safer Caterpillar Killer Concentrate is mixed with water and sprayed into the foliage, and caterpillars that eat it die within a few days.

Frequently Asked Questions

Does a topical spray keep protecting the tree?

Only for a limited time. The caterpillar killer may need reapplying every 7 to 10 days, and a spinosad spray every 3 to 4 weeks.

Is a systemic treatment still effective once borers are established?

Less so. Control becomes extremely difficult once borers are inside, though an injection can slow further damage and keep other borers from entering.

Does a systemic treatment work on every tree?

Not on every tree. Injections into conifers may be slow to take up or not taken up at all.

How many trees does one bottle of caterpillar spray cover?

Two to three large shade trees. One 16 oz bottle makes about 16 gallons of spray.