The Other Pandemic
The Tamarixia wasp is smaller than a grain of rice but a key ally in the fight against Huanglongbing.
Photo credit UC Riverside
The disease is deadly, it’s spreading unchecked and officials are deeply concerned. But this story isn’t about Covid-19.
There is another pandemic sweeping through our region, and this one is lurking in our gardens. Huanglongbing, or HLB, is a bacterial infection that kills citrus trees. It is spread by an invasive insect called the Asian citrus psyllid. This tiny bug causes massive damage. It ravaged the citrus groves in Florida, and now it’s on Irvine’s doorstep.
In Southern California, where the majority of households has a vulnerable citrus tree in the backyard, officials are turning to an unlikely ally: the wasp. Tamarixia radiata is a tiny parasitic wasp that lays its eggs under the Asian citrus psyllids. The psyllids are killed when the wasp eggs hatch and eat their hosts while developing. Because the Tamarixia wasp is a natural predator of the Asian citrus psyllid, it is our closest ally in the fight to save California’s billion dollar citrus industry.
The Vine spoke with Dr. David Morgan, biological control program manager with the California Department of Food and Agriculture (CDFA), about the insidious nature of HLB, what the state is doing to fight it, and what will happen if this disease spreads unchecked.
“There are some very interesting parallels to COVID,” says Morgan. “But there's also some big differences. So the disease we deal with, Haunglongbing, is actually more like malaria because it's vectored by an insect. So that's how it gets around. And so we have the opportunity of trying to kill the disease or trying to kill the insect that carries it. Humans are not very good at killing diseases in plants, unfortunately. We’re much more capable of killing insects. So that's where a lot of our work is targeted to, at least in the beginning.”
HLB infections in Orange County.
The introduction of the Tamarixia wasp as a biological control against HLB required the cooperation of many researchers as well as local and foreign governments.
“We went through several stages,” says Morgan. “When a new disease arrives in a country, what we do is we go back to the country where the pest or disease or the insect comes from, and we find out what eats it. Then we run lots of tests to make sure that thing that we want to release doesn't do any other damage. And then we request approval from the government, both from the federal government but also, if the insect comes from outside the continent, we also get approval from Mexico and Canada because they’re right next door to us. And then we do the releases. Basically, we look for where the pest is present, and we release the good insect on the pest. Then we follow through to make sure that the good insect is killing the bad insect, and also surviving over the winter. And if that works, then we move to the next strategy, which is actually to try and spread the insect out as much as possible. So my responsibility is to release these good insects, the Tamarixia, in all the urban areas in California that has Asian citrus psyllid present.”
Part of the challenge in spreading these wasps is due to the urban environment in which the Asian citrus psyllid chose to inhabit. It’s a tricky place to for researchers to root it out.
These Asian citrus psyllids were stopped in their tracks by the Tamarixia radiata wasp.
Photo credit UC Riverside
“What happened was this pest, Asian citrus psyllid, got a ride in California and it really got established in urban areas,” says Morgan. “It's very hard to manage a pest there and it's very expensive. So the responsibility was given to bio control to try and control the pest in urban areas. In Southern California, we have about 4,000 square miles of urban areas. And we did releases every three miles for about four years in these areas. So we just try to release as many as possible and as frequently as possible everywhere from Imperial all the way up to Santa Barbara. And we carried on doing those releases for all those years. Then we moved to a different strategy. We needed to concentrate our releases on the highest risk areas. So that's where we are at the moment.”
Morgan defines high risk areas as those where the Asian citrus psyllid is likely to show up as well as those where it has already been found. In Irvine, although no citrus trees have been discovered with HLB, it’s likely already here.
A vial of Tamarixia radiata wasps.
Photo credit Citrus Pest and Disease Prevention Program
“We do seem to be having a really good impact on managing the disease, but it's only a matter of time before it spreads to Irvine,” says Morgan. “We have a large team of people who go around searching for disease plants, and then those are tested. So we remove those trees as soon as we find them. And that's the best way. Just like with COVID. Removing the source of infection for spread is one of the best way of preventing the disease from becoming a real problem. Early detection is really essential. And so that's what we do. But having said that, the disease takes about two years for it to become symptomatic so that people can see that the plant is diseased. But during all that time, it can still be there spreading the disease in the local area. So there is a good chance that HLB is present.”
Similar to COVID’s asymptomatic spreaders, trees diseased with HLB may be infecting other trees undetected. In order to catch these spreaders before they show symptoms, the CDFA has turned to an entirely different type of biological control: highly trained dogs.
“There's actually a number of canine teams; one is being trained to smell the insect,” says Morgan. “They go around trying to smell the insect in new areas so we know where to schedule a treatment to eradicate. Florida really started that idea up and we've made use of it as well. We also have dogs that are trained to sniff the diseased plant, and they do seem to be pretty capable of finding diseased plants. It's a pretty rapid way of diagnosing it. At the moment a dog finding a potentially disease plant isn't enough for us to say that that plant has the disease. It then has to be tested again to make sure that it has a disease before we can actually remove that plant.”
In addition to the use of disease and pest-smelling dogs, scientists are exploring a myriad of possible solutions to save citrus trees from this threat.
“There's a huge number of potential solutions that people are looking at, including the same sort of things that are happening with COVID.,” says Morgan. “We have RNAi technology that’s being used to try and find ways of blocking the development of the disease. There's a whole lot of research. There are millions and millions of dollars spent every year trying to tackle this problem. The citrus industry is worth billions of dollars in California and the federal government is very supportive. It helps bring in millions of dollars for research to try and find ways of blocking the disease. You can kill the insect that carries the disease or you can make the plant resistant. And that's a very strong topic of research. You can either do that by breeding classically to find resistant plants and then breed them into our existing citrus stock, or you can actually use transgenics, which is more questionable because people are less accepting of that. But you can find a way so the disease doesn't exist in the citrus plant, for instance, and develop plants that are resistant to the disease. And then also there are chemical routes. There are all sorts of insecticides but also there are injections you can give to plants, like antibiotics. Florida has some antibiotics which have been approved to be used to treat the disease in the plant. Then there are people looking at other things such as metals to try and block and the development of the disease inside the plant.
This enormous range of treatment options is yet another connection between HLB and COVID-19.
A map of Tamarixia radiata released in and around Orange County.
Photo credit California Department of Agriculture
“It's a huge range,” says Morgan. “And it's really fascinating looking into it because, you know, you've got sort of basic people like me, who just look at insects. And then you've got people who are very high tech geneticists who are looking at really the cutting edge of technology. The parallels in dealing with a human disease like COVID and dealing with the plant disease are amazing.”
All of this effort and all of these resources are being directed at HLB for good reason: if the disease is not stopped, the economic and ecological implications are disastrous.
“First of all, we estimate that in Southern California that we have an average of two citrus trees on every property,” says Morgan. “So everyone could potentially lose their own citrus trees. They wouldn't be able to go out and pick their oranges to keep themselves healthy. It would reduce the number of trees present, so that would be really sad. But also, we have a very huge industry in California. We have the nation’s largest whole fruit industry for citrus. And Florida has more citrus, but they juice it because basically, their citrus plant fruit are not as good looking, whereas our fruit are really healthy looking. We have a lot of exports, and so that would be a problem, and also we produce a huge amount of citrus for the USA. So that would all be under threat. We're talking about lemons and limes and grapefruits and oranges and kumquats. All of the produced citrus could potentially be affected.”
In addition to the other ramifications of losing our citrus trees, Morgan points out that our region has deep historical ties to our citrus trees.
“The county you’re in is Orange County,” says Morgan, “because it was one of the first big orange producing areas in California. Riverside and Ventura, they really started up because of the citrus industry. And so we have a huge history of citrus in California. And it would be devastating to the agriculture and the whole history of California to lose citrus.”
If you suspect HLB has infected a tree on your property, please reach out to The Citrus Pest & Disease Prevention Program. And find out what you can do to fight HLB in your own backyard.