By Julius Barcelona
It would be reasonable to assume that you, dear reader, have encountered the telltale signs of a leaf miner infestation: the white scars snaking around the leaves of our crops.
Leaf miners are a collective common name for any species of insect whose larvae grow inside of and feed on plant leaves, leaving their signature feeding trails. A number of insects are leaf miners; species of moths, sawflies, flies, and even a few species of beetle have leaf mining larvae. In the Philippines, the most commonly identified leaf miner is Liriomyza trifolii, which has a wide range of host plants such as cucumbers, melons, and tomatoes.
L. trifolii is native to the Caribbean and the southeastern United States, and is an invasive species in the Philippines. Some studies show another species, Liriomyza huidobrensis, affecting potato crops in Benguet. Regardless of species, leaf miners in the Philippines have similar feeding and reproductive habits, and similar control methods work against them. Thus this writer will henceforth refer to them simply as leaf miners.
Leaf miners are quite small, measuring less than two millimeters in length for most adults. Their bodies tend to be fairly yellow, with some dark brown or grey areas. Adults inject their eggs below the surface of the leaf, and when the eggs hatch, the larva will burrow throughout or “mine” the inside of the leaf, leaving their distinctive trails of damage. At the end of the larval phase they exit the leaf and drop into the soil to pupate and upon emerging as adults prepare to mate and continue their life cycle.
Like many other pests discussed here, leaf miners are not readily controlled with pesticidal methods. Common synthetic pesticides for leaf miners include novaluron, cyromazine, or lambda-cyhalothrin, while biological pesticides include abamectin, azadirachtin, or spinosad. Using pesticidal control however is highly discouraged against leaf miners, as they quickly become resistant to these inputs. In fact, some studies have shown that the overall effect of using pesticides against leaf miners tends to have a far more detrimental effect to beneficial organisms rather than to the leaf miners themselves.
Leaf miners do have multiple biological controls in the form of parasitoid wasps, mostly from the Braconidae, Eulophidae, and Pteromalidae families. These wasps feed on leaf miner eggs, and keep the population of leaf miners stable in an area. This is also another reason why pesticidal controls are highly discouraged in controlling leaf miners, as these parasitoid wasps are more severely affected than the leaf miners themselves. However, this writer has not been able to find accounts of Philippine species that attack leaf miners as hosts, despite there being records of various parasitic wasps observed locally.
Another biological control, which this writer has personally used to great effect, is the use of entomopathogenic fungi. If you have access to rice farmers, allow them to leave the discarded rice straw in the field for a few days to allow a variety of wonderful bacteria and fungi to grow in the straw. Collect this inoculated straw then thickly and generously mulch around the base of your crops. When the leaf miner larvae drop out of the leaves and into the soil to pupate, they instead are met with a layer of thick mulch, populated by a host of parasitic fungi which then infest and kill the pupating larvae. A thick mulch of rice straw also provides a friendly environment for other beneficial insects to live in, such as spiders.
Cultural/mechanical control such as greenhousing is not particularly effective against leaf miners, again due to their small size. Like whiteflies, they can penetrate most fine nets and in doing so escape their predators, which makes greenhouses ideal environments for them to live.
Instead, consider combining the use of rice straw mulching inoculated with entomopathogenic with a trap or barrier garden planted to host plants such as garland chrysanthemum or celery. Leaf miners will happily lay their eggs in these host plants, only for their larvae to be met with death by fungi.