Scientists release wasps to control citrus pest
Curry leaf may capture the imagination of gardeners who love Indian cuisine, but the tree is also of interest to Mark Hoddle, a biological control specialist at UC Riverside. Curry leaf can be a host for the Asian citrus psyllid, above, an aphid-sized insect that spreads disease, can kill citrus crops and has become a major concern of California growers.
On Friday in Pico Rivera, Bell Gardens and Southgate, Hoddle's team released 300 tiny parasitic wasps from Pakistan that feed only on the Asian citrus psyllid. The wasps, Tamarixia radiata, right, are a potential alternative to insecticides. The wasps had been used as beneficial insects in Florida and abroad but never in California.
“If someone has curry leaf and are in the L.A. area, we’d be interested in looking at their plants and maybe using them for our parasite release,” said Hoddle, whom we interviewed for this edited Q&A about the Asian citrus phyllid and the wasps his team released.
Why are you doing this project?
California has a new pest, the Asian citrus psyllid. It's become very widespread in L.A. County.
What prompted you to go to Pakistan to look for a solution?
Two reasons: The Punjab area of Pakistan and India is thought to be the center of where the insect evolved and subsequently spread out. Also, the climate of the Punjab is a very good match for commercial citrus areas in California. We thought, if we go to the area where this insect evolved, we're more likely to find different species of natural enemies attacking it and using it for food. And if we want natural enemies to do well back in California, it would make sense if they would have evolved in the climate similar to California's. They should be held to tolerate hot dry summers and cold damp winter conditions.
The wasp’s effectiveness is variable. In certain parts of the world -- Mauritius, Réunion [an island in the Indian Ocean, near Madagascar], the Caribbean -- the parasite has provided very good control of this pest. The climate on these tropical and semitropical islands is very stable, no cold winters or super-hot summers. In California, our temperatures fluctuate a lot. That's what we see in the Punjab as well. There, certain times of the year the wasp might kill less than 25% of the psyllids, and then once the parasites come out of the cold phase, they might attack and kill more than 95% of the psyllids. There are these big swings.
Was there concern about introducing a new insect here?
Yes, we were very concerned about that. It's something that we don't take lightly. We've been working in Pakistan for the last 18 months, and for about 14 to 16 of those months, we've been doing safety testing at the bio-security Level 3 quarantine facility we have at UC Riverside. We have run tests to see what other insects our parasite would attack -- native insects, beneficial insects. Our data demonstrated that the parasite that we are releasing only attacks Asian citrus psyllid, to the best of our knowledge. It's not going to attack nontargeted insects, such as native California psyllids. The wasp has been used in Florida, Central America and the Caribbean, and scientists have no findings to suggest that this parasite has ended up attacking a lot of things that it shouldn't be attacking. There's no reason for us to suspect it is going to go crazy here and cause a lot of problems.
Has the insecticide been effective against psyllids?
In Florida, it's been quite effective, and now they are seeing the development of resistance to some of their most efficacious products. So that's a big concern. If you use the stuff too much, the bugs develop a resistance to it. We need to come up with an integrated pest management program. One component of that is the natural enemies we have brought in from Pakistan. If we get rid of 20% to 30% of the population, that's 20% to 30% fewer psyllids that growers need to worry about. Maybe they won't spray as often.
The adults are about the size of an aphid. They have a mottled brown body, but what catches your eye is that when they are feeding on the plant, their bodies are at an angle of about 45 degrees to the stem or leaf. In silhouette, they're very easy to see because of this 45-degree angle. The eggs and the nymphs are bright yellow. They will be very densely clustered into the new flush growth that is just pushing out of the end of the twigs. When they get bigger, you can see them with the naked eye; when they are small you can see them readily with a hand lens.
Will there be symptoms such as leaf curl or yellowing?
When you have high infestation of the nymphs and the adults feeding on the young growth, it'll become twisted and distorted as they inject their saliva and feed. The Huanglongbing disease that the psyllids can spread has not been detected yet in California. HLB could well be here, but we have not found it yet. Should that show up, the leaves will become asymmetrical chlorotic -- part yellow, part green.
Will the psyllids kill a tree?
Without the HLB bacterium, the insects can't kill a tree. But once psyllids are infected, they can fly off and carry the bacteria with it and settle on a clean tree and infect it. And then some uninfected psyllids come along, and they get infected and carry the disease to other trees, and the disease starts hop-scotching along with the psyllids.
So homeowners should be looking more for psyllids than for HLB disease?
Exactly. And if you have no flush on your citrus, that does not mean that you do not have psyllids. Adults can feed quite well on leaves and stems, but they really prefer that tiny flush growth to lay their eggs on.
Because the eggs and nymphs are bright yellow on a green background, are they easy to see?
Yes, they can be, if you have the sun on the right angle and if there are enough eggs packed into that flush growth. I can see them quite easily with my 44-year-old eyeballs. I know the color, and I know where to look.
Do the parasitic wasps sting?
They cannot sting people or pets. They pose no hazard to humans or animals. Most people will never know they're in the backyard. They are so tiny -- about half the size of a chocolate sprinkle.
If people believe they have the psyllids in their plants, whom should they call?
The best bet would be to go to the California Department of Food and Agriculture. The department has a website about the Asian citrus psyllid.
-- Jeff Spurrier
Photos of Asian citrus psyllid and Tamarixia radiata wasp on leaf: Mike Lewis / Center for Invasive Species Research
Photo of Tamarixia radiata wasp on gray background: Jason Mottern