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UC Pest Management Guidelines


Mature tobacco hornworm larva.

Eggplant

Hornworms

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

(Reviewed 4/10, updated 4/10)

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 midsummer, 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/Acre** R.E.I.+ P.H.I.+
(trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
 
A. BACILLUS THURINGIENSIS spp. KURSTAKI#
  (various products) Label rates 4 0
  MODE OF ACTION GROUP NUMBER1: 11
  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)# 1.25–2.5 oz 4 1
  (Success) 4–8 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 5-7 days after treatment. Do not exceed 29 fl oz of Success or 9 oz of Entrust/acre/crop.
 
** See label for dilution rates.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until harvest can take place. In some cases the R.E.I. exceeds the P.H.I. The longer of these two intervals is the minimum time that must elapse before harvest may take place.
* Permit required from county agricultural commissioner for purchase or use.
# 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). For additional information, see their Web site at http://www.irac-online.org/.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Eggplant
UC ANR Publication 3475
Insects and Mites
R. H. Molinar, UC Cooperative Extension Fresno County
J. L. Aguiar, UC Cooperative Extension Riverside County
M. J. Jimenez, UC Cooperative Extension Tulare County
P. B. Goodell, UC IPM, Kearney Agricultural Center, Parlier

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