How to Manage Pests

UC Pest Management Guidelines

Cucurbits

Leafminers

Scientific Names:
Vegetable leafminer: Liriomyza sativa
Serpentine leafminer: Liriomyza trifolii, and other Liriomyza spp.

(Reviewed 12/09, updated 6/12)

In this Guideline:


DESCRIPTION OF THE PESTS

Liromyzid leafminer adults are small (0.06 inch or 1.5 mm) black and yellow flies having a bright yellow scutellum, a triangular spot on the top rear of the thorax. In the San Joaquin Valley, it is important to distinguish between L. trifolii and L. sativae, which can occur in the same areas, because L. trifolii is much more resistant to most chemical controls. Using a strong hand lens, observe the compound eye of the adult. The eye of L. trifolii is nearly completely surrounded by yellow with only a small portion of black touching the rear of the eye. Liriomyza sativa has approximately one-fourth to one-third of the eye bordered in black at the rear. The top of the thorax of L. sativa is shiny black whereas on L. trifolii it is more dull dark gray colored. The larvae of leafminers are tiny bright yellow maggots about 2 mm long when they emerge from the leaves to pupate. Pupae look like tiny brown grains of rice. There can be many generations a year.

Females insert the white, oval eggs into the leaf tissue by puncturing the leaf at the tip and along the edges. Numerous punctures are made, but only a small percentage contain eggs: the majority serve as feeding sites for the adults who lap up the exudate.

DAMAGE

Larvae mine between upper and lower leaf surfaces, creating winding, whitish tunnels that are initially narrow, but then widen as the larvae grow. They may cause leaves to dry, resulting in sunburning of fruit and reduction in yield and fruit quality. In severe infestations, the leafmining may reduce yields or cause plant death. The most serious infestations usually appear late in the season.

MANAGEMENT

Biological control, unless destroyed by treatment for other pests, is often adequate to control leafminers. Good field sanitation can also greatly reduce the numbers of this pest. Evaluate levels of parasitism before making treatment decisions. Also, avoid summer plantings, if possible, in the San Joaquin Valley where leafminers usually do not reach damaging levels until mid-July to early August.

Biological Control

Because of numerous parasites, leafminers are generally not serious pests, but can be sporadic in their attacks. The destruction of beneficials by frequent applications of organophosphates, carbamates, and pyrethroids applied to control other pests can result in leafminer outbreaks. Of the numerous parasites that attack the leafminer in California, the most abundant is the parasitic eulophid wasp, Solenotus intermedius, but Diglyphus spp. and Chrysocharis spp. are probably the most important for controlling leafminers.

Cultural Control

Cutting forage crops and deep plowing after harvesting crops aid greatly in reducing leafminer numbers.

In desert production areas, exclude leafminers by applying row covers (plastic and spun-bonded materials) at planting and gradually removing them at first bloom or earlier if needed. Row covers are not recommended for the San Joaquin Valley.

  • Do not remove the entire plastic row cover at one time because a drastic reduction in humidity will shock plants and can lead to collapse. Instead vent the covers and remove them gradually. Covers made of spun-bonded materials do not need venting because hot air is able to escape.
  • Remove row covers if the air temperature underneath reaches 104° F before bloom.
  • Remove row covers before plants grow high enough to touch hot plastic.

Plants that are not stressed for moisture can better tolerate this pest. In the San Joaquin Valley, plant in early spring and avoid summer plantings because leafminers often occur in damaging numbers beginning in mid-July.

Organically Acceptable Methods

Biological and cultural controls are acceptable to use in an organically certified crop.

Monitoring and Treatment Decisions

Start monitoring leaves for mines during the vegetative growth stage. Early season infestations are common, but in most cases are controlled by natural enemies. However, if leafminer populations build to high levels and parasitism is low, an insecticide application may be necessary when seedlings have four to five leaves. In desert areas treatment may be required 2 to 4 weeks after planting and then repeated as needed. Use 3-by-5-inch yellow sticky cards to monitor adults moving into the fields from surrounding crops being harvested. Plastic trays can be used to monitor pupating larvae emerging from the leaves by placing the trays under the plant to catch the larvae as they drop to the ground. No economic threshold values are available yet, however.

If parasitism approaches 50% or more, the chances of the leafminer population being kept below economic levels are excellent. Once larvae have entered the leaf, they are difficult to control with insecticides.

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.
 
A. ABAMECTIN*
  (Agri-Mek 0.15 EC) 8–16 fl oz 12 7
  MODE OF ACTION GROUP NUMBER1: 6
  COMMENTS: Controls serpentine leafminer, L. trifolii. Apply by ground only. In heavy infestations, apply a second treatment 7–10 days following the first. Under severe, continuous pressure, a second set of treatments may be needed 3 weeks following the first set. Highly toxic to honey bees.
 
B. CYROMAZINE
  (Trigard) 2.66 oz 12 0
  MODE OF ACTION GROUP NUMBER1: 17
  COMMENTS: Apply as a foliar spray when leafminers first appear. Do not make more than 6 applications/growing season.
 
CAUTION: Be aware that the following chemicals are harsh on predators and parasites, especially at high label rates and should not be used if beneficials are present.
 
C. ESFENVALERATE*
  (Asana XL) 5.8–9.6 oz 12 3
  MODE OF ACTION GROUP NUMBER1: 3
  COMMENTS: Repeat as necessary to maintain control. Do not exceed 0.25 lb a.i./acre/season. Provides only sporadic control of L. trifolii. Highly toxic to honey bees.
 
D. OXAMYL*
  (Vydate L) 2–4 pt 48 1
  MODE OF ACTION GROUP NUMBER1: 1A
  COMMENTS: Labeled for use on cucumber, cantaloupe, honeydew melon, watermelon, squash, and pumpkin only. Apply low rate for light infestations. Not effective on L. trifolii. Highly toxic to honey bees.
 
E. DIMETHOATE 2.67 EC 0.75–1.5 pt 48 3
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Labeled for use on melons and watermelons only. Pest has developed tolerance to this material in some areas; not effective on L. trifolii. Highly toxic to honey bees.
 
F. DIAZINON*
  (Diazinon 50W) Label rates 72 3
  (Diazinon AG500) Label rates 72 3
  MODE OF ACTION GROUP NUMBER1: 1B
  COMMENTS: Labeled for use on melons and watermelons only. May not be as effective as other materials because pest has developed tolerance in some areas. Highly toxic to honey bees.
 
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 REI exceeds the PHI. 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: Cucurbits
UC ANR Publication 3445

Insects and Mites

  • E. T. Natwick, UC Cooperative Extension, Imperial County
  • J. J. Stapleton, UC IPM Program, Kearney Agricultultural Center, Parlier
  • C. S. Stoddard, UC Cooperative Extension, Merced & Madera counties
Acknowledgment for contributions to Insects and Mites:
  • R. L. Coviello, UC Cooperative Extension, Fresno County
  • L. D. Godfrey, Entomology, UC Davis
  • C. B. Fouche, UC Cooperative Extension, San Joaquin County
  • C. G. Summers, Entomology, UC Davis/Kearney Agricultural Center, Parlier
  • J. B. LeBoeuf, AgriData Sensing, Inc., Fresno
  • M. Murray, UC Cooperative Extension, Colusa/Glenn counties

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