Large green sphinx moth caterpillar (family Sphingidae; exact species and host plant not confirmed) on a twig, with diagonal pale side stripes and a horn at the tail end.
Summary
Hornworms are the large caterpillars of sphinx (hawk) moths. They are easy to recognize by their size, diagonal side stripes and the small horn on the tail. In the Sonoran Desert, several kinds feed on landscape and garden plants, and they often appear in numbers after monsoon rains bring a flush of new leaves. A few caterpillars can strip a lot of foliage on a small plant in days, but on established trees and shrubs the damage is usually cosmetic and the plant leafs out again.
Key symptoms
- Large green (sometimes brown or patterned) caterpillars with a tail horn.
- Leaves chewed from the edges or stripped down to the midrib, often on the newest growth.
- Dark pellet-shaped droppings (frass) on leaves or the ground below.
- Caterpillars blend in well; frass and bare stems are often noticed first.
Likely causes, ranked
- Seasonal sphinx moth activity. Moths lay eggs on host plants; larvae feed and grow quickly, then drop to the soil to pupate.
- Flushes of tender new growth. Post-monsoon and spring growth attracts egg-laying moths.
- Few natural enemies nearby. Birds, predatory insects and parasitic wasps normally keep numbers in check.
Look-alikes and how to tell them apart
- Other caterpillars: smaller, often hairy or spiny, and lack the tail horn.
- Leaf-cutter bee damage: neat, round notches cut from leaf edges, with no droppings.
- Grasshopper feeding: ragged chewing on many plant types, with the grasshoppers easy to see.
What to check on site
- Identify the host plant and how much of it has been eaten.
- Look on stems and leaf undersides for caterpillars and check the ground for frass.
- Look for white rice-like cocoons on a caterpillar's back; those are parasitic wasps, which are already controlling it.
- Check whether the plant is a young or high-value plant where defoliation matters.
Treatment
- Hand-pick caterpillars and relocate or dispose of them; this is usually all that is needed on trees and shrubs.
- Leave parasitized caterpillars (with white cocoons) in place so the wasps can emerge and keep working.
- Bt (Bacillus thuringiensis) products are a low-impact option for young caterpillars on heavily attacked plants; use exactly per the label.
- Avoid broad-spectrum insecticides, which kill the beneficial insects and pollinators that normally keep hornworms in check.
- Water deeply so a defoliated plant can recover and push new leaves.
What to expect and how to monitor
- Established trees and shrubs usually re-leaf within weeks after the caterpillars finish feeding.
- Caterpillars may return after the next flush of growth; check new growth every few days in late summer.
- Young plants that are stripped repeatedly may need more protection or monitoring.
Worried about a tree? Text Justin, ISA Certified Arborist, at (520) 451-8790 or email Justin@TimberxTrees.com. TIMBERx serves Tucson, Oro Valley, Marana, Vail, Sahuarita, Green Valley and Benson.
