Research Aids Fight Against HLB

Monday, September 19, 2016
By: Steve Olson, Product Manager - Horticulture
Dani Ruais studies the ectoparasitic wasp Tamarixia radiata

We connected with one of the graduate students at Cal Poly Pomona who is working to find a solution to the current Asian citrus psyllid populations. Dani Ruais studies the ectoparasitic wasp Tamarixia radiata. This parasitic wasp is currently being released in California as a biological control agent that feeds on Asian citrus psyllid consequently killing the psyllid.

As you may know, the Asian citrus psyllid is a tiny insect that feeds on citrus trees, and spreads the devastating disease known as Huanglongbing (HLB). In 2015, Bayer donated $25,000 to Cal Poly Pomona to back HLB research through its partnership with California Citrus Mutual. Read along to learn more about Dani Ruais and her research, and visit to learn how you too can help save California citrus.

Q: Can you briefly explain the research you’ve been working on regarding the Asian Citrus Psyllid and Huanglongbing in citrus?

A: Absolutely. I have been working with the ectoparasitic wasp (Tamarixia radiata) that is currently used and released in Southern California to reduce Asian Citrus Psyllid insect pressures and slow the spread of Huanglongbing. Tamarixia, in lab situations, has had great success and really high rates of parasitism of ACP nymphal stages. I am currently working on how diet can affect the egg load of these wasps. We are encouraging increases in egg load in the wasps so that when they are released, they are full of eggs and will start laying. Of course, they will also host feed, but in order to bring the ACP populations down to a minimal level, we need to encourage egg laying to provide the next generations of wasps that will continue to feed and reproduce specifically on ACP.

Wasps are fed various diets and kept in a light controlled chamber
Wasps are fed various diets and kept in a light controlled chamber until they are dissected to record egg load.

Q: Can you explain more specifically the relationship of the Tamarixia radiata wasp and the Asian Citrus Psyllid?

A: I can explain this all day - I never get bored of describing her physiological development. So far as we understand it, the female Tamarixia will hone in on a colony of ACP nymphs (which usually hang out in the new flush, or new growth, of the branches of plants in the citrus family). She will penetrate one of the ACP nymphs with her ovipositor, or tube she uses to deposit her eggs, and then suck out the haemolymph, or blood and body fluids, of the ACP with her mouthparts. She will also, and this is the cool part, stab the ACP nymph, usually a third, fourth, or fifth instar, and will release a paralytic into the nymph so that the nymph stays in place while she deposits one very tiny, translucent egg on the underside of the nymph, usually behind its sixth leg. The egg will hatch and the larvae crawls inside the ACP and consumes it from the inside as it is living.

Female (left) and male (right) Tamarixia radiata through microscope
Female (left) and male (right) Tamarixia radiata through microscope.

Q: Is there an example you can share with us?

A: There’s this really great YouTube video of it that I like to show students (who may or may not be interested). It’s great - she does her thing, flutters her wings and then it’s over and she moves on to kill the next nymph either by ingesting its body contents, or by oviposition. I just think the whole system is fantastic.

Q: Is the Tamarixia radiata a cure-all, or are there complementary steps that need to be taken to help stop the spread?

A: A major misconception is that Tamarixia radiata will be a panacea for the ACP problem at hand. The realities of dealing with insect vectored diseases are very harsh. Not only are we dealing with misunderstandings about how the vector is spread, but also about exactly what “control” means. It’s crucial that the public know that this is a deadly disease that is transmitted by these insects. Of course biological controls are an option in some cases (they are not a valid option in all cases, if we are intent on stopping the spread of the insect and thus the disease). But along with biological controls (carefully cycled into pest management programs), are pesticides that will provide adequate knock-down for the ACP.

Q: Is the Tamarixia radiata widely used throughout California right now?

A: It has only been released in Southern CA right now. It is my understanding that many counties are under heavy quarantine and are on a mandatory spray schedule that does not include the application of the parasitic wasp. As I previously mentioned, Tamarixia are typically a help in urban environments and in situations that are difficult for pesticide access. Tamarixia do not eliminate entire pest populations therefore HLB can still spread. It really just takes one psyllid carrying the bacterial disease HLB in its mouthparts to begin the devastation of a 100+ acre orchard.

Also, have you heard of the Nu Psyllid - a genetically engineered psyllid that cannot transmit HLB and will replace the wild type ACP? Like I said before, the face of agriculture is changing because we need to be more innovative and creative in the way we solve our problems of now and the future. We need to make assaults on ACP and HLB on all fronts and that means combining the powers of our plant pathologists, vector specialists, biological control experts, entomologists as well as growers and community members. We need to make a concerted effort because there is no “silver bullet”; at least not one that we have found yet.

ACP nymphs on citrus tree
ACP nymphs on citrus. They are easily identified by the white waxy secretions also called frass, or insect waste, that they excrete as they feed. Ants frequently tend to the ACP nymphs and harvest this frass which is really honeydew, or sugary water wastes produced by ACP feeding.

Q: Do you think GE technology will be successful?

A: We need to not only think of what we can do to halt HLB in its tracks, but what will also be an economically sustainable solution long term . I’m not sure anyone can say exactly what the face of the citrus industry in California will look like in ten years. With combined help from the industry, research and educational institutions such as Cal Poly Pomona, hopefully we’ll be looking at an HLB-free world.

Tagging ACP populations on citrus trees at Cal Poly Pomona
Dani and her team of lab assistants regularly tag ACP populations on citrus trees at Cal Poly Pomona. The second phase of her project includes releasing wasps that have been fed different diets and collecting them to record parasitisation rates, host feeding rates, and fecundity.

Q: How will this research positively impact the citrus industry in California?

A: This research will greatly impact the citrus industry, especially beginning with those orchards that are near urban areas in which community members may neglect citrus trees and personal orchards. It is in these hubs that ACP populations build and can easily migrate to open commercial citrus orchards. By upping the wasps’ fecundity and parasitism rate, we are creating a better, more efficient tool to challenge the ACP and HLB spread.

Citrus at Cal Poly Pomona
Citrus at Cal Poly Pomona

Q: How long have you been at Cal Poly Pomona?

A: I started at Cal Poly Pomona in the Fall of 2015. I had previously finished my undergraduate degree at Cal Poly San Luis Obispo, in Environmental Horticultural Sciences with a focus on Plant Protection Sciences. I have worked in the field as both a scout and pest control advisor.  

Citrus Orchard in California

“Cal Poly Pomona offers so much: not only do I have a lab, but a new state of the art greenhouse, and an entire orchard to carry out my experiments.”

Q: Tell us about the new 5,040 square-foot research greenhouse that was recently unveiled. What are you looking forward to the most? What opportunities does this give you?

A: I’m very excited about the greenhouse. Although before it was built I had great opportunities to get contacts in the California Department of Food and Agriculture at Mt. Rubidoux; entomologists such as Dr. David Morgan, Grace Radabaugh, and Alex Muniz have been so helpful in guiding my research and giving me the tools necessary to continue my research on Tamarixia and ACP. Now they are even closer and I can hang out in the greenhouse to observe Tamarixia behavior- so cool!

Grand Opening of the research and insect production greenhouse at Cal Poly Pomona
Grand Opening of the 5,040 square-foot research and insect production greenhouse at Cal Poly Pomona on July 25th, 2016

Dani Ruais is a Cal Poly Pomona graduate studentDani Ruais is a Cal Poly Pomona graduate student studying the link between diet and fecundity in the parasitic wasp Tamarixia radiata. Her research will lead to being accepted into a doctorate degree program in agricultural entomology and/or biological control.


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