Two leafmining fliesLiriomyza huidobrensis and L. langeiare
pests of many vegetable and flower crops, including peas, beans, melons,
onions, tomatoes, potatoes, celery, garlic, lettuce, chrysanthemums,
and carnations. Their larvae tunnel inside leaves and other plant parts
as they feed, leaving winding trails called mines that are visible through
the leaf surface.
During outbreaks, these insects can cause severe damage, resulting
in substantial economic losses. Originally present only in the western
United States and South America, they have now invaded Europe, the Middle
East, and Asia.
L. huidobrensis and L. langei are so similar in form
and structure that they were once considered to be one species. But
ARS molecular biologist Sonja
Scheffer and biological sciences technician Matthew Lewis used DNA sequence
data from mitochondrial and nuclear genes to show that there are actually
two species. DNA data also showed that the invasive leafminers causing
extensive crop damage around the world are L. huidobrensis, not
L. langei. Currently, the highly invasive L. huidobrensis
is not known to be present in the United States.
Using DNA sequence data is an expensive, time-consuming way to identify
species. Now Scheffer has refined a less expensive, faster molecular
method to differentiate the two pests. It uses polymerase chain reaction
combined with restriction fragment length polymorphism (PCR-RFLP) analysis.
This method can be used with adult, larval, or pupal leafminer specimens.
It was developed to provide researchers, pest managers, and quarantine
officers with a simple and quick molecular method to differentiate L.
langei from L. huidobrensis.
In early 2000, Scheffer confirmed the accuracy of PCR-RFLP by applying
it to 31 fly specimens for which DNA sequence data had clearly identified
the species. She tested 52 more specimens from recently introduced leafminer
populations in Sri Lanka, South Africa, and Canada and found all to
be L. huidobrensis.
PCR-RFLP can be performed by anyone with access to a laboratory that
has a PCR thermocycler and associated paraphernaliaan increasingly
common piece of equipment.
"The entire set of proceduresfrom DNA extraction to final
identificationcan be completed within a single working day,"
says Scheffer, "compared to several days and additional expense
for DNA sequencing."
But there's more opportunity for misidentification with this method
than with sequence data, because PCR-RFLP banding patterns may be shared
by multiple species. For this reason, it is appropriate to use a particular
PCR-RFLP test only with those species for which the test was developed.By
formerly with ARS.
is with the USDA-ARS Systematic
Entomology Laboratory, Bldg. 011A, 10300 Baltimore Ave., Beltsville,
MD 20705-5129; phone (301) 504-5317, fax (301) 504-6482.