How to Manage Pests

UC Pest Management Guidelines

Grape

Black Widow Spider

Scientific name:
Lactrodectus hesperus

(Reviewed 6/06, updated 4/14)

In this Guideline:


Description of the Pest

The typical adult female black widow has a shiny black body, slender black legs, and a red or orange mark in the shape of an hourglass on the underside of the large, round abdomen. The body, excluding legs is 5/16 to 5/8 inches long.

The adult male black widow is one-half to two-thirds the length of the female, has a smaller abdomen, and is seldom recognized as a black widow. The topside of its abdomen is greenish gray with a pattern of cream-colored areas and one light-colored band going lengthwise down the middle. The hourglass mark on the underside of the abdomen typically is yellow or yellow-orange and broad in the middle. The legs are banded with alternating light and dark areas.

Like males, young female black widow spiders are patterned on the top side. In the early stages they resemble males, but gradually acquire the typical female coloration with each molt. In intermediate stages they have tan or cream-colored, olive gray, and orange markings on the topside of the abdomen, a yellowish orange hourglass mark on the underside and banded legs.

The egg sacs are mostly spherical, about 1/2 inch long and 5/8 inch in diameter, creamy yellow to light tan in color, opaque, and tough and paperlike on the surface. A female may produce several egg sacs during her lifetime, which can be 2 years. Tiny, young black widows, which are nearly white in color, emerge from the egg sac and remain close together during the first days after emergence, often preying on each other. Soon afterwards, the spiderlings disperse to new locations by 'ballooning' on light silken thread and infest new areas. Webbing produced by black widow spiders is very strong compared to other spider webbing.

Damage

Generally spiders play a beneficial predatory role in a vineyard and are not thought of as pests. However, in the southern San Joaquin Valley (Kern and Tulare counties) and the Coachella Valley, black widow spiders can be a problem in table grape vineyards because of quarantine issues in crops to be exported to other countries and because of the public's fear of black widows.

Management

In table grape vineyards, preventive treatment may be justified in crops destined for exportation. A delayed dormant treatment of chlorpyrifos (Lorsban) aimed at grape mealybug will also control black widow spider populations. Treatment at this time is very effective because the leaves are off the vine, allowing for good coverage of the trunk; however, unless the coverage is complete throughout the vineyard, the spider populations can increase before harvest.

If a mealybug treatment is not planned, an in-season treatment for caterpillars with fenpropathrin (Danitol) will control black widows. Also, because there are always protected areas in the vineyard where insecticide coverage is poor (e.g., cement irrigation pipe stands, trellis poles, and cross-supports), an in-season application is often required to keep these spiders out of the grape cluster. Treatments of fenpropathrin (Danitol) will suppress black widow populations, but again, full coverage is important.

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 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.
 
DELAYED DORMANT
 
A. CHLORPYRIFOS*
  (Lorsban Advanced) Label rates 24 35
  MODE-OF-ACTION GROUP NUMBER1: 1B
  . . . PLUS . . . (optional)
  NARROW RANGE OIL
  (Superior, Supreme) 1–2 gal 4 0
  MODE OF ACTION: Contact including smothering and barrier effects.
  COMMENTS: Direct sprays at the vine from the crown down to the ground being sure to obtain thorough coverage of all above-ground plant parts, especially the trunk and cordons. Most effective when applied during warm weather (60°F or higher). Apply in 150 to 200 gal water/acre. Use allowed under a Special Local Needs registration (SLN CA-080009). Do not apply chlorpyrifos more than once a year or apply it after budbreak. Avoid drift and runoff into surface water.
 
GROWING SEASON
 
A. FENPROPATHRIN*
  (Danitol 2.4EC) 5.33–10.66 oz 24 21
  MODE-OF-ACTION GROUP NUMBER1: 3
  COMMENTS: Direct the spray towards fruit clusters. Coverage is very important. Use sufficient water to get the insecticide throughout the vine. To protect honey bees, apply only during late evening, night, or early morning when bees are not present. Disruptive to other beneficial insects.
 
** Apply with enough water to provide complete coverage.
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 to harvest. In some cases the R.E.I. exceeds the P.H.I. The longer of two intervals is the minimum time that must elapse before harvest.
* Permit required from county agricultural commissioner for purchase or use.
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: Grape
UC ANR Publication 3448

Insects and Mites

W. J. Bentley, UC IPM Program, Kearney Agricultural Research Center, Parlier
L. G. Varela, UC IPM Program, Sonoma County
F. G. Zalom, Entomology, UC Davis
R. J. Smith, UC Cooperative Extension, Sonoma County
A. H. Purcell, Environmental Science, Policy and Management, UC Berkeley
P. A. Phillips, UC IPM Program, Ventura County
D. R. Haviland, UC IPM Program, Kern County
K. M. Daane, Kearney Agricultural Research Center, Parlier
M. C. Battany, UC Cooperative Extension, San Luis Obispo County
Acknowledgment for contributions to Insects and Mites:
J. Granett, Entomology, UC Davis

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