How to Manage Pests

UC Pest Management Guidelines

Eggplant

Hornworms

Scientific Names: Tobacco hornworm: Manduca sexta
Tomato hornworm: Manduca quinquemaculata

(Reviewed 4/10, updated 4/10, pesticides updated 5/16, corrected 10/16)

In this Guideline:


Description of the Pests

Hornworm eggs are laid singly on leaves. While both species of hornworms have a large horn on the posterior end of the body, the tobacco hornworm has seven diagonal stripes on each side of the body in contrast to the tomato hornworm, which has eight chevron-shaped stripes. Larvae feed for 3 or 4 weeks, then burrow into the soil to pupate. The adult moth is a strong flier with a wingspan up to 5 inches (12 cm). Development takes about 2 months in summer; the winter is passed in the pupal stage. There are two generations a year in most areas; larvae are usually most common in mid-summer, but there may be a small population peak in late summer. Infestations tend to be more severe in warm inland areas.

Damage

Hornworms feed primarily on leaves but will eat blossoms and stems when the leaves are gone and even burrow into fruit. At high populations they can extensively defoliate plants and scar the fruit. Infestations tend to be spotty, and if left unchecked can cause considerable damage.

MANAGEMENT

In commercial fields, natural enemies, crop rotation, and discing after harvest play a key role in keeping hornworm populations below damaging levels. Conserve natural enemies by not treating with disruptive pesticides, especially early in the season before fruit begin to mature.

Biological Control

There are several important naturally occurring parasites that help control hornworms in eggplants. Hornworm eggs are attacked by Trichogramma parasites and the larvae by Hyposoter exiguae.

Cultural Control

Discing after harvest destroys pupae in the soil. Rotations with crops that are not attacked by hornworms will also help to keep population levels low in individual fields. In small fields, handpicking hornworms off plants is effective but as in tomatoes, it can be difficult to find the worms, which are camouflaged by the foliage.

Organically Acceptable Methods

Biological and cultural controls as well as sprays of Bacillus thuringiensis or the Entrust formulation of spinosad are acceptable for use on an organically certified crop.

Monitoring and Treatment Decisions

It is rare for an entire field to become infested. Consider spot-treating sections of a field where hornworm damage is found. It is not uncommon to find various locations in a 20-acre field, each with 10-40 plants being stripped of leaves. Look for hornworm larvae on plants that have severe foliar damage as you sample to determine if damage is the result of hornworm or armyworm activity. On fruit, hornworm feeding produces larger, deeper cavities than those caused by beet armyworm.

Common name Amount per acre REI‡ PHI‡
(Example trade name)   (hours) (days)

UPDATED: 5/16
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Not all registered pesticides are listed. Always read the label of the product being used.
 
A. BACILLUS THURINGIENSIS spp. KURSTAKI#
  (various products) Label rates 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: This material is highly effective against hornworms; especially when applied early to young caterpillars. It will also control loopers and to some extent beet armyworms.
 
B. SPINOSAD
  (Entrust)# 1–2 oz 4 1
  (Success) 3–6 oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Use higher rate for larger worms and heavy infestations. Best control is achieved when aimed at newly hatched larvae and coverage is thorough. More broad-spectrum than Bt but has very low toxicity to humans, vertebrates, and the adults of many natural enemies. Can remain toxic to larval stages (especially syrphid fly) for 5 to 7 days after treatment. Do not exceed 29 fl oz of Success or 9 oz of Entrust/acre per crop. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
C. TEBUFENOZIDE
  (Confirm) See label 4 7
  MODE-OF-ACTION GROUP NUMBER1: 18
 
D. METHOXYFENOZIDE
  (Intrepid 2F) See label 4 1
  MODE-OF-ACTION GROUP NUMBER1: 18
 
E. NOVALURON
  (Rimon) 9–12 fl oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 15
 
** See label for dilution rates.
Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment until harvest can take place. In some cases the REI exceeds the PHI The longer of these two intervals is the minimum time that must elapse before harvest may take place.
# Acceptable for use on organically grown produce.
1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Eggplant
UC ANR Publication 3475

Insects and Mites

J. L. Aguiar, UC Cooperative Extension, Riverside County
M. J. Jimenez, UC Cooperative Extension, Tulare County
P. B. Goodell, UC IPM, Kearney Agricultural Research and Extension Center, Parlier

Acknowledgments for contributions to Insects and Mites:
R. H. Molinar, UC Cooperative Extension, Fresno County

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