Scientists seeking the off-season whereabouts of a group of insects called psyllids have found that what the insects eat suggests where they ate it. Such analysis may enable researchers to pinpoint areas from which psyllid pests colonize agricultural fields.
A study
published this week in Environmental Entomology describes how the vegetative reservoirs from which psyllids infiltrate crops can be inferred via DNA analysis that identifies plant matter in psyllid gut contents. The process could enable managers to predict which crops are at risk, to date an iffy proposition.
Psyllids, sometimes known as “jumping plant lice,” are small, plant-feeding insects within the same suborder (Sternorrhyncha) or the order Hemiptera as aphids, scales, and whiteflies. Psyllids feed by penetrating plant phloem and sucking the sap that flows through it. About 45 of the 3,800 psyllid species are serious pests. Many of them hit crops with a double whammy, by attacking plants themselves and also spreading disease pathogens.
The new study was conducted at several institutions in the Pacific Northwest, where psyllids threaten potatoes and orchards, and in Florida, where the cause damage to citrus crops. Gut content analysis has been used typically to identify the dietary history of predator insects and, to some extent, of those that chew plants. But it had not before been done for sap suckers. “In our study,” says researcher W. Rodney Cooper, Ph.D., of the U.S. Department of Agriculture’s Yakima Agricultural Research Laboratory, “we used gut content analysis to basically track the landscape movements of phloem-feeding insects. As far as we know, this is a novel use for gut content analysis.”
Most psyllids feed on one plant species or a closely related group of them. As seasons change and host plants disappear, however, they move about a diversity of other species that serve as “whistle stops” where they may shelter or grab some nutrients or water. Methods of tracking the insects and their presence in plants have been restricted mostly to small-scale sampling, not sufficient to create a clear picture of psyllid ecology. In particular, scientists know little about the movements between agricultural and non-agricultural plant hosts of three psyllids that spread diseases of pears, potatoes and citrus, respectively.
Sampling given areas for insects can provide what the authors call a “snapshot” of their distribution across different habitats but not much information on their movements. Tracking movement of larger animals requires monitoring, generally accomplished with markers, such as banding birds. Although insects have been individually tagged and monitored, the process can be difficult, and marking is more often done on a mass, catch-as-catch-can scale, such as dusting an area containing insects with dye. The new research offers a different way to monitor insect movements.
The ability to find where psyllids go when not in crop fields has been made possible by DNA sequencing, which permits identification of a species by a short strand of DNA from a given part of an organism’s genetic material. The DNA molecule is sequenced by sorting out the order of its chemical building blocks, or bases, the letters of the genetic code. The arrangement of the sequence indicates the type of genetic information it carries.
Each species has a DNA sequence that works like a barcode at the checkout at a supermarket. Species can be identified by comparing the barcodes. Researchers conducting the study used technology that allows rapid copying of an immense number of the DNA fragments to be sequenced, which is a far more efficient than earlier methods. It permits identification of a large number of plant species, more readily than techniques used in previous studies.
Sequencing of animal genes is relatively standardized, using mitochondrial DNA as a marker, or barcode. With plants, the process generally employs chloroplast genes, which were used in the psyllid study, along with what is called “spacer DNA,” which contains the sequence but does not express a genetic function. Found between genes, separating them, spacer DNA varies more between species that are distantly related than between closely related species. The psyllid researchers focused on two chloroplast genes and spacer DNA.
“Most importantly for our study is that sequences of these spacer regions are unique to different plant species, providing a molecular barcode (or fingerprint) to identify what plants the psyllids had previously fed upon,” says Cooper.
W. Rodney Cooper, Ph.D., research entomologist at the U.S. Department of Agriculture’s Yakima Agricultural Research Laboratory collects psyllids in the early spring in Wapato, Washington. (Photo credit: Pauline Anderson)
Researchers involved in the study—from the U.S. Department of Agriculture, Washington State University, Oregon State University, and University of Florida—collected five species of psyllids for the study. All moved extensively through the landscape, searching for feeding plants as well as those in which they could overwinter. The lifecycles of the species differed considerably. Three—the pear psylla (Cacopsylla pyricola), potato psyllid (Bactericera cockerelli), and Asian citrus psyllid (Diaphorina citri)—are pests that spread plant diseases and whose mysterious roving around the landscape has made them difficult to control.
The mystery, however, is dissipating as plant genetic material from psyllids is scrutinized. The study showed, for example, that pear psyllas collected in a Washington State orchard contained genetic signs of hops. Given that Washington is the nation’s hop production leader, this was not especially surprising. At the same time, since the nearest commercial hops operation was several kilometers away, it appeared the tiny winged insects—a tenth of an inch long—traveled considerable distance in their wanderings. Gut content from Asian citrus psyllids in the study showed extensive movement between citrus and non-citrus shrubs.
“The feeding by psyllids on a diversity of non-host species shows how psyllids taste and sample green plants as they move across the landscape in search of suitable host or shelter plants,” Cooper says. “The adult psyllids likely get water and perhaps some nutrients that allow them to survive periods when host plants are not available. ”
The use of non-host plants by psyllids that reproduce once a year, whose hosts are not available outside of breeding season, is obvious, says Cooper, “but the use of non-host plant ‘whistle stops’ by psyllid pests that produce multiple broods, like pear psylla or potato psyllid, has been overlooked. Our findings give a new perspective on the landscape movements of psyllids.”
Future research will attempt to identify non-crop sources of psyllids that damage pears and potatoes. Included in the focus of research will be attempts to locate where psyllids pick up pathogens and patterns of host-plant use.