Management options should be carefully considered since some treatments have the potential to be more damaging than the pests themselves.
By Noel Dickinson, BS in Agriculture, UH Hilo, 2014.
Vegetable Leafminer: Liriomyza sativae (Blanchard)
We’ve all seen them, white trails meandering across leaves in your garden. Most of us know leafminers are the culprits, but what is a leafminer? When these trails are observed they are usually empty, as if they’ve been traced by some phantom pest.
In mid-September, I observed what I believed to be leafminer damage on the foliage of hydroponic tomato plants in the hydroponic greenhouse, at the University of Hawaii at Hilo Farm Laboratory in Panaewa. I came to the immediate conclusion that the damage was done by leafminers because of the conspicuous grey, serpentine lines on the leaves of the plants, a symptom specific to all leafminer species.
After doing a literature search, I discovered that the feeding of larvae deposited by small flies was the cause of the foliage damage. I returned to the greenhouse the next morning and observed the top surface of the tomato leaves, since this is where the adult, female leafminer fly lands and deposits eggs using its sharp ovipositor. I found an impossibly small, 1 mm long, fly on the surface of a tomato leaf. The fly appeared to be shiny, black, and vibrant yellow. When disturbed, its flight pattern was jerky and unpredictable, but the fly stayed within range of the tomato plants, always landing on leaves.
Light green larvae were also observed actively mining leaves. Pupae, approximately 1.5 mm in length, were present on both the top surface of some leaves as well as the hydroponic table tops. Pupae, larvae, and flies were collected for further identification.
Liriomyza sativae belongs to the class Insecta, the order Diptera, and the family Agromyzidae. With yellow and black coloring, L. sativae is easily misidentified as Liriomyza trifolii, the American serpentine leafminer. L. sativae is distinguished by a prominent yellow triangle between its wings as well as a shiny black mesonotum; as opposed to L. trifolii’s matte appearance with a thorax covered in bristles–good luck trying to observe them in their natural setting, they’re tiny and fast!
Adult females typically produce 200 to 300 eggs during their lifespan, depositing 30 to 40 eggs per day. After a single egg is inserted beneath the surface of a leaf, it will hatch in approximately three days. Oviposition starts at sunrise; females are most active by mid-morning. You can observe leaves being mined in the morning as well, since this is when larvae are most active–they can be seen at the ends of fresh trails on leaves.
L. sativae larvae are initially colorless, as they mine, they transition from greenish to yellow. Larvae exhibit three active instars or larval stages, each completed in two to three days. During the first instar, the larva burrows into the mesophyll tissue of a leaf.
The larva continues to feed on mesophyll tissue in its second instar. Mining is concentrated toward the surface of the leaf by the end of the third instar. A slit, from which the mature larva will escape, is cut in the upper surface of the leaf. Once the larva emerges from the mine, it either drops to the soil where it will burrow down approximately 2 cm and form a pupa or the larva pupates on the leaf’s surface.
Distinctly segmented, oval shaped pupae are an average of 1.5 mm in length, with reddish brown color. Adult L. sativae emerge during early morning hours within 5 to 12 days of pupa formation -depending on temperature. Adults are able to mate the day after emergence.
L. sativae is prevalent in most temperate zones; it cannot survive cold winters. During cool months flies are uncommon, conversely they thrive during summer months.
L. sativae is not host specific, instead it is able to complete its life cycle on a range of plants; it favors the plant families Cucurbitaceae, Leguminosae, Solanaceae, and Brassicaceae. Broadleaf weeds serve as hosts until preferred vegetable crops become available–so get rid of those weeds in your garden!
L. sativae, like all leafminer species, target foliage and young tender stems. Adult females puncture leaves during oviposition and feeding, causing foliage to have a stippled appearance. Adult males do not damage plant material, but feed from punctures made by females. Larvae do the most obvious damage. As larvae feed, they move forward through leaf mesophyll tissue, leaving behind conspicuous serpentine trails of frass and damaged tissue which, when observed, is comparable to grey meandering trails.
The mines of L. sativae and L. trifolii are nearly indistinguishable.
Individual mines are of little consequence, since a lightly damaged leaf is still valuable, in terms of its photosynthetic potential. Entire leaves may be mined if larval population is high. Over-mined leaves drop prematurely, causing fruit to be scorched by exposure to direct sunlight. Yield and fruit size are subsequently decreased by defoliation. Punctured leaves also make plants susceptible to pathogen infection as well as desiccation. Heavy infestation can potentially affect yield marketability.
All leafminers are pests. But even with this distinction, management options should be carefully considered since some treatments have the potential to be more damaging than the pests themselves.
In the hydroponics greenhouse, the leafminer infestation is being managed by the removal of affected leaves. The culled foliage is then stored in a covered trash can for approximately one week before being disposed since leafminers are able to complete their life cycles even if the leaf is detached from the plant.
This method is being implemented in order to prevent heavy infestation of the entire greenhouse, since mines have been observed on leafy greens being grown in proximity to the affected tomato plants. With no natural enemies present in the hydroponics greenhouse environment, the L. sativae population has the potential to increase exponentially if left unchecked.
Leafminers are notoriously hard to control. Insecticide use should be considered a last resort, since most treatments are foliar sprays which do not affect actively mining larvae protected within leaf tissue or larvae protected within a puparium.
Cultural practices like maintaining a weed-free growing environment, vigilant observation of growing foliage as well as not introducing leafminers to an unaffected area, and the utilization of sticky traps, are less invasive methods which prevent an infestation.
In the case of a greenhouse environment, preventative practices like inspection of plants prior to bringing them into the greenhouse will lower the chances of introducing leafminers and other pests as well.
So, now you know where those unattractive trails on your leaves are coming from, how will you deal with them?
Cranshaw, W.S., D.A. Leatherman, and J.R. Feucht. “Leafmining Insects.” Colorado State University Extension, 5 Aug. 2014. Web. 20 Nov. 2014.
Grafon-Cardwell, E.E., D.H. Headrick, K.E. Godfrey, J.N. Kabashima, B.A. Faber, V.F. Lazaneo, P.A. Mauk, and J.G. Morse. “Citrus Leafminer.” How to Manage Pests: UC Pest Management Guidelines. University of California Statewide Integrated Pest Management Program, 14 May 2014. Web. 10 Nov. 2014.
Mau, Ronald, and Jayma Martin Kessing. “Liriomyza Sativae (Blanchard).” Crop Knowledge Master, 1 Apr. 2007. Web. 17 Nov. 2014.
Murphy, Graeme, Gillian Ferguson, and Jim Chaput. “Leafminers Attacking Greenhouse Crops.” Ontario Ministry of Agriculture, Food, and Rural Affairs, 1 July 2014. Web. 18 Nov. 2014.
Photo by Lyle J. Buss, University of Florida.
About the author: Noel Dickinson received her bachelor of science in agriculture, with specialization in tropical plant science and agroecology, from UH Hilo in 2014. She graduated with high honors and was awarded for excellence in service at Ag Seniors Night, Dec. 2014.