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UC Pest Management Guidelines


Tobacco hornworm larva.

Tomato

Hornworms

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

(Reviewed 1/08, updated 1/08)

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 on blossoms, leaves, and fruit. At high populations they can extensively defoliate plants and scar the fruit. They are rarely a problem in the warmer interior valleys unless natural enemies are disrupted, in which case, they can do serious damage. They are mostly problems in garden situations.

MANAGEMENT

In commercial tomato 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 tomatoes. Hornworm eggs are attacked by Trichogramma parasites and the larvae by Hyposoter exiguae. Trichogramma released for control of tomato fruitworm will also attack hornworm eggs.

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.

Organically Acceptable Methods
Biological and cultural controls as well as Bacillus thuringiensis sprays are acceptable for use on an organically certified crop.

Monitoring and Treatment Decisions
Treat hornworms only if they are causing extensive foliage damage, or if they are feeding on fruit. Hornworm damage can be assessed as part of the sampling guidelines and thresholds listed under beet armyworm. 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. In addition, hornworm feeding produces larger, deeper cavities than those caused by beet armyworm. Consider spot-treating sections of a field where hornworm damage is found because it is rare for an entire field to become infested.

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.B2
  COMMENTS: This material is highly effective against hornworms and does not destroy the natural enemies of tomato pests. It will also control loopers and to some extent beet armyworms and tomato fruitworms.
 
B. ESFENVALERATE*^
  (Asana XL) 0.66 EC 2.9–5.8 fl oz 12 1
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: Do not exceed 0.5 lb a.i./acre/season. Do not use this product if leafminers are present because it is destructive of their parasites.
 
C. CARBARYL*^
  (Sevin) 80S 1.5–2.5 lb 12 3
  (XLR Plus) 1–2 qt 12 3
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Do not use if psyllids are in the field as carbamates tend to promote development of their populations.
 
 
**  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.
^ Do not apply when bees are present.
# 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: Tomato
UC ANR Publication 3470
Insects and Mites
F. G. Zalom, Entomology, UC Davis
J. T. Trumble, Entomology, UC Riverside
C. F. Fouche, UC Cooperative Extension, San Joaquin County
C. G. Summers, Entomology, UC Davis/Kearney Agricultural Center, Parlier
Acknowledgments for contributions to the insects and mites section:
N. C. Toscano, Entomology, UC Riverside

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