Insect of the Month - Leafminers

 
  Hort Snacks - August 2017
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 Causal Agent: Chrysanthemum leafminer – Liriomyza trifolii; vegetable leafminer – Liriomyza sativae; pea leafminer – Liriomyza huidobrensis; birch leafminer – Fenusa pusilla, Profenusa thomsoni; aspen serpentine leafminer – Phyllocnistic populiella; Phyllonorycter nr. salicifoliella and P. nr. nipigon; lilac leafminer – Gracillaria syringella; cottonwood leafmining beetle – Zeugophora acutellaris, Z. abnormis; European alder leafminer – Fenusa dohrnii; plus other species

Crops Affected:
Edible crops – beets, spinach, Swiss chard, tomato, cole crops, cucurbits, peas, beans, lettuce, etc.
Ornamental crops - aster, begonia, dahlia, impatiens, lily, marigold, petunia, and verbena, greenhouse ornamental crops (e.g. chrysanthemum, gerbera), woody ornamental trees and shrubs (e.g. birch, poplar, aspen, oak, willow, alder, lilac, etc.)

Life Cycle:

  • Adult insects are flies, moths or beetles which lay their eggs on/in the undersides of host leaves (depending on the species)
  • Larvae tunnel into the leaves to feed between the upper and lower leaf surfaces (epidermal layers), forming “mines”
    • Larvae develop fairly rapidly, depending on the species and the temperature
    • Larvae typically complete development by exiting the leaf and dropping to the soil to pupate
  • Damage is largely cosmetic
    • Yields of edible plants are typically not affected unless severe defoliation occurs
    • Damage is typically insufficient in ornamental species to cause lasting harm to large woody plants
  • Populations can increase rapidly, particularly in protected environments, depending on the species
    • For woody ornamental hosts, flies emerge in spring and often lay eggs as leaves are starting to emerge from buds
  • The presence of the various pest species will vary by region, with some being more prevalent in some areas and others not being present at all
Symptoms:
  • Mines are visible, twisting back and forth across the leaves of host plants
    • In some host species, mines become pockets of spaces, within which larvae can be observed feeding
    • Some mines resemble blotches
    • Differences between the appearance of mines can be indicative of the pest species
  • Leaves may appear brown and dried out, as the leaf tissues die off as mines are enlarged
Leafmining damage on birch & cutleaf weeping birch – note: visible larvae
Leafmining damage on spinach leaves
Photos by Robert Spencer
Photo Courtesy: Whitney Cranshaw, Colorado State University, Bugwood.org

Monitoring:
  • In greenhouses, yellow sticky traps can be used to monitor adult fly populations
  • Watch fields for signs of early mining and be prepared for preventative protection in advance or
    to stop later generations from developing
Management:
  • Ensure that plants have adequate water and nutrition to produce strong, healthy growth
  • Rotate to non-host crops on a regular basis
  • Covering edible crops can restrict access to the plants by adults
  • Resistant or tolerant varieties can be available for some host crops
  • Control weeds, to prevent population carryover between crops
  • Early removal of mined leaves can interrupt the life cycle of the pests
  • Parasitic wasps can be effective biological controls some species, particularly in protected crops (e.g. greenhouse)
  • The use of registered insecticides can be used to control adult leafminers
    • Resistance can develop quickly in this pest – use caution and rotate chemical groups
    • Control options may be limited in some crops
  • It is very difficult to control leafminers in woody ornamentals, other than through the use of systemic insecticides applied early in the spring
    • There are very few options available in the present day
  • Strict sanitation programs (in protected environments) can help to keep populations in check
 
 
 
 
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For more information about the content of this document, contact Robert Spencer.
This information published to the web on July 28, 2017.