Western growers battle on many fronts

The name sounds dangerous: Psyllid. Like a Cylon or Cyborg in a sci-fi movie. As such, it is a very fitting name, because the psyllids can be a deadly bunch.

PHOTO BY MIKE LEWIS.
This potato psyllid is an example of a group of insects that is increasingly destructive because lax regulation and inspection allow them to spread from country to country.

Not often does a single insect species threaten to destroy entire crop industries in several nations, but the Asian citrus psyllid is doing just that. And other psyllid species have become or are becoming pests on other fronts. There’s a potato psyllid, a carrot psyllid, a pistachio psyllid, an apricot psyllid, an avocado psyllid, an apple psyllid—the list just goes on and on. The insect family can also be a deadly pest to native and ornamental plants in the U.S. Oddly enough, their notoriety comes more from their ability to vector plant diseases than for actual physical damage from feeding.

Mark Hoddle says that there are a couple of major reasons that the psyllid group is finding its name in the news lately. First, these insects, which are part of the Heteroptera or sucking insects along with whiteflies and aphids, are well-equipped to transmit disease. They have needle-like mouthparts that are plunged deep into plant tissue, and when they do that they can inflict physical damage. But more importantly, they can pick up or inject disease organisms such as bacteria into the plant.

“There have always been pest species,” Hoddle says of the psyllids, “but recently they have become more problematic.” Which illustrates the second reason they are in the agricultural news more often. This group has greatly benefitted from the current world free trade philosophy and the inability of trade regulations and inspections to prevent all of them from crossing borders undetected. The countries where they disembark are likely to have no natural enemies to attack the pests or barriers to the diseases they carry and can spread.

Hoddle, who is director of the Center for Invasive Species Research (CISR) at the University of California, Riverside, and an extension specialist in biological control, notes that the Asian citrus psyllid, Diaphorina citri, is the poster child for this scenario. Because of the psyllids’ ability to penetrate deep into plant tissue, a single insect that finds the huanglongbing bacteria in a tree can carry the disease into many other trees. The insect itself does minimal damage, but the bacteria cause the wasting of the trees’ fruit. Hordes of insects are not needed to inflict extensive damage; a few psyllids can wipe out an orchard’s production, and the only recourse is to destroy the trees.

As a result, citrus production in Asia has basically been eliminated, and since the discovery of the Asian citrus psyllid in Florida in 2005, that state’s citrus production has declined drastically. The pest has also been detected in Mexico, South America, the Caribbeanand the Middle East as well as Texas and other Southeastern states, and in some of those venues the huanglongbing bacteria is also present. Infestations of the psyllids have established themselves in three counties in California. The bacteria is not present yet, and the California Citrus Research Board is working with the CISR, other University of California scientists and the California Department of Food and Agriculture to prevent the spread of psyllids as well as prevent introduction of the bacteria, but Hoddle notes that it is very likely that huanglongbing will be brought into the state either through smuggled plants from another country or via migration of infected psyllids from Mexico.

PHOTO BY WILLIAM M. BROWN JR.
This is the physical damage done to a citrus tree by the Asian citrus psyllid, but the real damage is done by the bacteria the insect carries.

“Now that we have the Asian citrus psyllid in California, they will be able to move the bacteria out of that silent reservoir,” Hoddle says. Once both the insects and the disease are present it is very difficult to stop them. The CISR and other researchers, learning from disastrous infestations in Asia and Florida, are working on ways to contain the spread of the disease once it arrives.

Another major psyllid pest in the U.S., which is spreading overseas, is the potato psyllid—which can also affect tomato and capsicum plants. This insect, Bactericera cockerelli, inflicts damage by feeding on plant tissue, but in some cases damage is made worse by the transfer of pathogens from one plant to another. In California, the potato psyllid creates unusual amounts of honeydew on pepper plants, which contaminates the fruit. This pest now occurs from Texas to Nebraska, and recently established in New Zealand.

Hoddle is also working on four other psyllid species in the genus Trioza that affect avocado trees. The feeding of these insects induces galls on leaves or causes leaf roll, resulting in a decrease in fruit production. One or more of these avocado psyllids are currently a problem from South America to Mexico, and the danger is that insect immigration or illegal movement of infected trees will bring them into Florida, Texas and the prime avocado growing state of California. He adds that another danger to avocados is the laurel wilt fungus, carried by the invasive redbay ambrosia beetle, which is currently devastating native and horticultural laurels throughout the southeastern U.S. but can also be harmful to avocado trees.

For lists of troublesome psyllid species, go to the CISR Web site at www.cisr.ucr.edu. Another site that shows an extensive list of these pests is www.psyllids.org, maintained by a University of British Columbia researcher.

Hoddle sees a trend here, and it isn’t pleasant to contemplate. There is an increasing danger from invasive species all across the world, and the psyllids are a prime example of just how damaging they can be. With the movement of people and vegetative materials across borders becoming more and more common, the danger will likely only get worse. He notes that the problem also occurs in horticultural plants. The red gum lerp psyllid, for example, was only a harmless insect in its native Australia, but when it hit California’s eucalyptus trees it was devastating.

It is “predictable” that these types of agricultural and horticultural outbreaks will occur in the future, Hoddle says. Researchers are well aware of damaging species that are wreaking havoc in other countries and are on the lookout for invasions into the U.S. But that isn’t the only danger. “The wild card in all these psyllid invasions are the species we don’t know about,” he says. The red gum lerp psyllid caught everyone in the U.S. by surprise, for example, and caused millions of dollars in damage before it could be analyzed and managed.

Control in agricultural crops is possible in many cases, but in many cases it will be more difficult than with horticultural infestations because of the large acreages involved. On the other hand, most psyllids have a narrow host range, which often focuses infestations and makes them worse but also could increase efficacy of biological, chemical or cultural controls. The Asian citrus psyllid has caused worldwide awareness of the group’s threat, and researchers from Brazil to Florida, and now California, are working on solutions.

“The answer to the question is, what level of control do you need?” Hoddle says. The Asian citrus psyllid, for example, likely has no effective natural enemies in California, but a combination of insecticide applications and the introduction of biological controls may allow California citrus growers to live with the pest even once the huanglongbing bacteria is established. His group is looking at an introduced parasitoid insect that would bring some level of control, and when that is combined with careful applications of insecticides, growers may be able to keep their trees healthy.

PHOTO BY MIKE LEWIS.
These potato psyllids seem frail, but they can cause damage both through feeding and through vectoring deadly diseases.

He points out that the same insecticides that work for aphids and whiteflies will likely be effective on psyllids. Hoddle says that will probably consist of a two-tiered approach that will team systemic products with foliar-applied products. The systemic will be crucial, because it will repel the psyllid so that it will not be able to inject the deadly bacteria into plant tissue. Of course, this will increase the cost of growing citrus fruit.

UC Riverside is now evaluating currently available insecticides for the best Asian citrus psyllid control practices. There are problems, of course. One is that different citrus species and varieties have different reactions to the insecticides, and efficacy is affected. One of the primary goals is to be able to treat citrus nursery stock so that young trees can be planted without the disease. Plant breeding to find varieties more resistant to the diseases that psyllids carry is another promising area.

Another view of the psyllid situation comes from Diana Percy, a researcher at the University of British Columbia Botanical Garden and Centre for Plant Research. She points out that the psyllids are less well-known than their relatives, the aphids, whiteflies and scales, because they are in general less damaging. They rarely cause physical damage—the glaring exception is the red gum lerp psyllid—and their roles as disease vectors are the reason they are becoming known.

Percy, who also maintains the psyllids.org web site, notes that some members of the group are actually beneficial. For example, an invasive melaleuca tree in the Florida Everglades is being successfully controlled by an introduced psyllid from the tree’s native Australia. She notes that after the potato psyllid and the Asian citrus psyllid, the apple/pear psyllid is the most damaging to U.S. crops. She says that orchards are particularly nice targets for psyllids, which are relatively sedentary insects that can build up troublesome populations in a monoculture of trees.

“I suspect that the introduction of more mixed-species growth practices may help to reduce the buildup of psyllid populations , though of course this may reduce efficiency and production,” Percy says. This could help reduce damage, because these insects are very host-specific.

Hoddle foresees that psyllids and other invasive pests could become even more problematic, since the factors involved in their invasions are not changing for the better. Unless better plant inspections and quarantines can reduce psyllid invasions, these insects are well-adapted to take advantage of their new host countries. More disasters, such as a potential one from the avocado psyllid, could be in our future.

Don Dale is a freelance writer and a frequent contributor. He resides in Altadena, Calif.