How to Manage Pests

UC Pest Management Guidelines

Tomato

Hornworms

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

(Reviewed 12/13, updated 12/13)

In this Guideline:


Description of the Pests

Hornworm eggs are laid singly on leaves. Eggs are round to oval, 1.5 mm in diameter, and white to light green.

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 per acre** R.E.I.‡ P.H.I.‡
(example trade name)   (hours) (days)

  Calculate impact of pesticide on air quality
The following materials 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. CHLORANTRANILIPROLE
  (Coragen) 3.5–5 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 28
  COMMENTS: Can be applied as foliar spray or by drip chemigation. Read label for treatment intervals.
 
B. METHOXYFENOZIDE
  (Intrepid 2F) 8–16 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 18A
  COMMENTS: Low toxicity to beneficials. Apply at the beginning of egg hatch. When traps indicate moth flights have begun, sample leaves for eggs. Treat when eggs are first detected.
 
C. FLUBENDIAMIDE
  (Synapse WG) 2–3 oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 28
  COMMENTS: Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
D. SPINETORAM
  (Radiant SC) 5–10 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Apply as a foliar spray.
 
E. SPINOSAD
  (Entrust)# 1.25–2.5 fl oz 4 1
  (Success) 4–8 fl oz 4 1
  MODE-OF-ACTION GROUP NUMBER1: 5
  COMMENTS: Use higher rate for larger larvae and heavy infestations. Best control is achieved when aimed at newly hatched larvae and coverage is thorough. Less toxic to natural enemies than many other choices. For resistance management, do not apply more than 0.45 lb a.i./acre per season.
 
F. BACILLUS THURINGIENSIS spp. KURSTAKI#
  (various products) Label rates 4 0
  MODE-OF-ACTION GROUP NUMBER1: 11A
  COMMENTS: This material is most effective against newly hatched larvae, so proper treatment timing is essential. This material is also somewhat effective on other lepidopteran pests.
 
G. NOVALURON
  (Rimon 0.83EC) 9–12 fl oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 1A
  COMMENTS: Apply at egg hatch to the second instar. Use higher rates when larvae are large or foliage canopy is tall or dense.
 
H. EMAMECTIN BENZOATE*
  (Proclaim) 2.4–4.8 oz 12 7
  MODE-OF-ACTION GROUP NUMBER1: 6
 
I. INDOXACARB
  (Avaunt) 3.5 oz 12 3
  MODE-OF-ACTION GROUP NUMBER1: 22
 
J. ESFENVALERATE*
  (Asana XL) 5.8–9.6 fl oz 12 1
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Use only under heavy pest pressure and close to harvest. Some resistance has been documented. May cause outbreaks of Liriomyza spp. Leafminers and tomato russet mite. In some areas where tomatoes are grown, resistance to this material is a problem. Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
K. FENPROPATHRIN*
  (Danitol 2.4EC) 10.66 fl oz 24 3
  MODE-OF-ACTION GROUP NUMBER1: 3A
  COMMENTS: Use only under heavy pest pressure and close to harvest. May cause outbreaks of Liriomyza spp. leafminers and tomato russet mites.
 
L. CARBARYL
  (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. Do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
 
** 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: Tomato
UC ANR Publication 3470

Insects and Mites

E. T. Natwick, UC Cooperative Extension, Imperial County
C. S. Stoddard, UC Cooperative Extension, Merced and Madera counties
F. G. Zalom, Entomology, UC Davis
J. T. Trumble, Entomology, UC Riverside
G. Miyao, UC Cooperative Extension, Solano and Yolo counties
J. J. Stapleton, UC IPM Program and Kearney Agricultural Center, Parlier (false chinch bug)
Acknowledgments for contributions to Insects and Mites:
C. G. Summers, Entomology, UC Davis and Kearney Agricultural Center, Parlier
C. F. Fouche, UC Cooperative Extension, San Joaquin County
N. C. Toscano, Entomology, UC Riverside

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