LEAFMINERS
General Description
Leafminers are so named because of their habit of mining between the upper and lower surfaces of leaves. They are one of the most serious pests of commercial flower crops, especially chrysanthemums, as well as bedding plants. With very few exceptions, azalea and holly for example, they are not a serious problem on woody ornamentals.
Leafminers may be the larvae (maggots) of flies, caterpillars of moths or grubs of beetles. The most damaging leafminers in Florida are the serpentine group which leave winding trails in the leaves.
The important dipterous leafminers are:
The three important lepidopterous leafminers are:
Comparatively few beetles have developed leafmining habits, however
is an economic pest in some areas of Florida.
Biology
All leafminers have complete metamorphosis (egg, larva, pupa and adult). The entire life cycle of the vegetable leafminer can be completed in 21 to 28 days and even more rapidly if conditions are favorable. Adult females insert their eggs into the leaf tissue. Eggs hatch in one to three days into laterally compressed maggots. Larval development requires three to four days during which the characteristic mine is made. The mature larva usually cuts a hole at the end of its mine, emerges and drops to the soil to pupate. The tiny, yellow puparium, which is approximately 1/16 inch long, can be found on the soil around the host plant. Adults emerge in seven to 14 days. There are many generations per year.
Host Plants
Serpentine leafminers attack a number of plants inside greenhouses, under shade cloth and outdoors. Host plants include ageratum, aster, calendula, chrysanthemum, dahlia, gerbera, gypsophila, marigold, petunia, snapdragon, and zinnia.
Blotch leafminers, Amauromyza spp., attack chrysanthemum and other flowering plants. Phytomyza spp. attack many species of holly. Seven species of Phytomyza are recorded in Florida, of which four feed as leafminers on Ilex spp.
The azalea, palm and magnolia leafminers are host specific.
Seasonal Abundance
The dipterous leafminers, Liriomyza spp. and Amauromyza spp. occur throughout the year, but peak numbers coincide with warm, dry weather. Phytomyza vomitoriae, the primary leafminer pest of the Phytomyza spp. attacks Yaupon holly and is a cool-season pest. The azalea leafminer is also a cool-season pest. The palm and oak leafminers occur primarily in the summer and fall months.
Damage
Leafminer larvae tunnel through the leaves, feeding on the parenchymal cells between the upper and lower epidermal leaf surfaces. Liriomyza spp. produce a serpentine mine. These mines appear in the upper leaf surface three to five days after oviposition. In addition to the mines, leaves may appear stippled due to the numerous feeding punctures made by the female fly's ovipositor. Adult flies feed on the sap exuding from these wounds. Feeding punctures occur particularly along the margin or at the leaf apex. Larva feed singly, but mines may be numerous and cross each other. Amauromyza spp. produce a blotch or blister mine which appears as a broad brownish spot on the leaf. One to several larvae may inhabit a single blotch mine. Phytomyza spp. attack holly, producing large unsightly blotches which can cover much of the foliage in heavy infestations. Some mines begin as a serpentine trail and end in a blotch mine.
Serpentine mine |
Feeding punctures |
Blotch or blister mine |
The feeding tunnels and oviposition punctures of dipterous miners are unsightly and objectionable in crops destined for market. Depending on the severity of the attack, feeding will reduce assimilation by the plant. This can lead to desiccation and premature leaf loss. If sufficient leaves are attacked, crop production is severely reduced. Seedling plants may be totally destroyed. The wounds in foliage also provide an entry site for fungal and bacterial plant pathogens. Some species have been incriminated as vectors of plant viruses.
Azalea leafminers are leafminers only for the first half of their larval life. Upon hatching, the young larvae enter the leaf and feed as leafminers creating blisters on the underside of the leaves. Mid-sized larvae migrate to the upper leaf surface and by means of silk pull the leaf over their bodies and chew holes in the leaf. The larvae may also tie newly expanding leaves together at the tip of a shoot and feed in the same manner. The larvae are then known as leaftiers. After they emerge, the leaf tips or margins fold over. Injured leaves usually turn yellow and drop.
Palm leafminer larvae are gregarious and create longitudinal mines that are covered with brownish frass pellets webbed together with silk. Oak leafminers attack several species of oaks and usually make a blotch or blister mine. Magnolia leafminers also make blister-like mines, usually at the margin of the leaves. In cases of severe infestation, the leaves turn brown as if scorched by fire.
Sampling
Leafminer populations can be monitored by several methods. Observations of susceptible plants will indicate the pest presence. Adults can be caught using sweep nets. Dipterous leafminer adults are attracted to yellow sticky cards or cups placed over the top of the plant canopy. Cards are available commercially or can be constructed by cutting cardboard into 3 x 5 inch pieces, painted bright yellow and then covered with Tac Trap or STP oil treatment. Also, Solo yellow plastic drinking cups coated with the above adhesives and stapled on stakes above the plant canopy are commonly used. One of these devices every 1,000 square feet throughout susceptible host plants should be sufficient for monitoring adult flies. The number of flies captured on a series of cards or cups in the nursery during a 24-hour period indicates the relative abundance of leafminers. Trapping should be repeated at least weekly. Early detection of adult leafminers using these monitoring techniques can lead to improved population management.
Yellow sticky card
Biological Control
Natural enemies of leafminers include several species of parasitic wasps which attack and destroy the larvae in the mines.
Cultural Control
An integrated approach to dipterous leafminer control is essential. When ornamental crops are not present in fields, leafminer populations can be found on broadleaf weeds. The destruction of weed hosts near nursery areas at least four weeks before placing plant containers in blocks would eliminate potential pests or delay their appearance if they must migrate from distant areas.
Removal or destruction of crop residues immediately after plant removal helps reduce the leafminer population. Burning crop residues is very effective in preventing emergence of any living larvae and pupae in leaves.
Chemical Control
For the most current insecticide recommendations to control these pests, please contact the local county office of the Cooperative Extension Service. If you have access to the World Wide Web you can consult the University of Florida's Insect Management Guide at:
http://edis.ifas.ufl.edu/TOPIC_GUIDE_Insect_Management_Guide
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