With the high volume of globally traded agricultural products as well as the impacts of climate change on pest patterns, growers are facing the arrival of exotic pests which can quickly spread and threaten yields. This is especially threatening when a newly introduced pest has no natural enemies in its new habitat. One of the current threats citrus growers around the world are facing is the Asian citrus psyllid, which is a vector of the lethal citrus greening disease.
Dr. Mark Hoddle, an entomologist and biological control specialist at the University of California Riverside, has led several research projects evaluating methods to control these invasive pests in citrus, avocados and grapes.
During our conversation we discussed how the state of California has managed to hold the Asian citrus psyllid (ACP) at very low levels thus far by using a classical biocontrol approach. Mark elaborated on the importance of augmented and conservation biocontrol as well as ant management in the fight against ACP and other sap-sucking pests, and he also told me more about implementing proactive biocontrol programs as insurance against potential invasive exotic pests.
Classical biocontrol of the asian citrus psyllid
According to the USDA, the citrus greening disease (also known as Huanglongbing or HLB) has wiped-out 74 percent of the Florida citrus industry since 2005, creating a high level of nervousness among growers of California and other citrus producing areas who fear that the same situation could repeat itself in their orchards.
Asian citrus psyllid adult feeding on citrus
(Credit: Mike Lewis, Center for Invasive Species Research, UC Riverside)
But over 12 years after the Asian citrus psyllid was first detected in California, the state still hasn't witnessed any massive epidemic level outbreaks of Asian citrus psyllid in commercial citrus orchards. How did it manage to hold this pest in check?
Mark Hoddle is one of the best people to answer this question. When the spread of the Asian citrus psyllid became an obvious threat in Southern California he, along with members of the team he is leading at UC Riverside, started to monitor field sites from Los Angeles all the way to Riverside County for about three years. They noticed the insect densities were very high on unmanaged trees in urban areas and in parks.
In parallel, they began to study the potential biocontrol options and identified Tamarixia radiata, a parasitoid of the Asian citrus psyllid that was imported from Pakistan. After doing safety testing and quarantine in the UC Riverside facility to demonstrate that it wouldn't be an environmental threat, they finally got the permission to start releasing it.
Mark Hoddle in Pakistan collecting Tamarixia radiata, a natural enemy of Asian citrus psyllid, Diaphorina citri.
(Credit: Unknown, University of Agriculture Faisalabad, Pakistan)
The releases were done primarily in urban areas instead of commercial orchards for two main reasons. The first one is that the industry was very worried about the spread of HLB, so citrus growers were almost always spraying their orchards if they found Asian citrus psyllids, giving the beneficials no chance to survive. “We'd be putting our parasitoids, which are difficult and expensive to rear, into a toxic environment where they'd probably die almost immediately. So it makes no sense to do that. On the other hand, trees in urban areas are generally not sprayed with insecticide,” points out Mark.
The second reason is the extent of the agricultural-urban interface in California, which are the zones pressing up very closely to agricultural areas. These urban trees could act as incubators for Asian citrus Psyllid which would fly out of people's backyards and into citrus orchards. Mark adds that, “we thought that if we could establish our natural enemies in people's backyards, not only would we control Asian citrus psyllid, but we would reduce that invasion pressure coming out of these urban areas into commercial citrus groves.”
After the initial releases, Tamarixia established very readily and spread thanks to the abundance of food for it in the citrus trees. At the time, no predators were using Asian citrus psyllid populations for food, so Tamarixia were finding hundreds of psyllid nymphs on the trees that it could parasitize for its larvae to feed on.
Tamarixia radiata parasitizing an Asian citrus psyllid nymph
(Credit: Mike Lewis, Center for Invasive Species Research, UC Riverside)
They performed various monitoring research: life table studies where they followed the fates of individual Asian citrus psyllids on citrus trees, population counts and also videography studies. Mark details the process, “we set up little micro video cameras in citrus trees that were solar-powered and we filmed little clusters or colonies of Asian citrus Psyllid 24 hours a day, seven days a week until they had either emerged as adults or they had been killed and eaten.”
Based on the results of their three years' study, the UC Riverside team concluded “that natural enemies were the driving factor that had caused Asian citrus psyllid to decline by about 70% in people's backyards.”
The mass rearing and release program has since then been passed over to the California Department of Food and Agriculture (CFDA), allowing the operations to scale up and go from thousands of these parasitoids reared per year to millions of them.
The role of IPM and conservation biocontrol in the fight against the Asian citrus psyllid
Is classical biocontrol the only non-chemical method proven to prevent major Asian citrus psyllids outbreak? Not necessarily. Naturally occurring enemies or augmented release of beneficial insects in IPM orchards, while probably not enough by themselves, could also have an important part to play. Similar to what Chris Sayer, a citrus grower in Ventura County,California also suggested, Mark believes that one reason why Asian citrus psyllid has not blown up in areas with a long tradition of integrated pest management (IPM) is because orchards that are under minimal chemical management have very good biotic resistance, mainly from natural enemies. “I've completed a couple of studies which suggest that the most important predator attacking Asian citrus psyllid nymphs are the larvae of hover flies and syrphid flies. They recruited very strongly to Asian citrus psyllid colonies. The female hover flies laid their eggs on those colonies and the larvae just obliterated colonies of Asian citrus psyllids. They destroyed them all!”, Mark relates.
To attract and help these beneficial insects establish, Mark has looked at sweet alyssum as a cover crop in citrus orchards. He demonstrated that it is highly attractive to hoverflies, which, as a bonus to growers, is also a predator of mealybugs. This low growing, flowering creeping plant does very well in sunny areas. It requires minimal management and doesn't interfere with machining equipment.
Proactive biological control as an insurance against new invasive pests.
HLB, and its vector the Asian citrus psyllid, is also a threat to other citrus growing countries such as Chile, Perú, Spain or Italy. These countries are not impacted by the pest yet, so could they get ahead of the game? Mark is promoting a simple, but yet effective concept called “Proactive biological control” that should be seen as an insurance against potential invasive exotic pests.
He observes that “If you're looking out on the pest horizon from California, or maybe even Chile and you see some obvious pests that have high invasion potential, doesn't it make sense to get ahead of that problem before it actually arrives in your country or in your state? Typically, what we do in classical biological control is that we react to the invasion. We have a pretty good idea that the pest is coming, maybe it's even been intercepted a few times, but we don't do anything. We react when the pest is finally established, spreading, and causing economic damage. Only then we initiate the biological control program. In that scenario, we lose years of time. So when there is an obvious threat like the Asian citrus psyllids, why not start saving that time now by beginning a proactive program?”
He also explains the complexity of a classical biocontrol program, justifying this proactive approach. “We have to do the foreign exploration, look for the natural enemies, bring them back to quarantine, do the required safety testing, analyze all the data, begin the process of requesting permission to release those natural enemies from quarantine if the data indicates that they're safe... You're probably looking at about three to five years before you could get anything released into the environment. So why not start saving that time now by beginning a proactive program? You could have the tests done, safety demonstrated, have all the permits ready for release. And when Asian citrus psyllid shows up and establishes in Chile, for example, and has spread over such a big area that eradication and containment are no longer considered possible, you can begin the biological control program virtually the next day.”
Mark is currently working on two proactive biological control programs. The first one, in collaboration with Dr. Kent Daane at UC Berkeley, looks at the natural enemies of the spotted lantern fly, a pest of grapes and nuts with a high invasion potential into California. Mark and a UC Riverside team also started to develop pheromones and identify natural enemies of the avocado seed weevil, a pest native to Mexico which is now threatening California production.
The importance of ant control in IPM
In the fight against honeydew producing pests like the Asian citrus psyllid, most orchards managers know that ant control is key, as ants tend to those pests and protect them from natural enemies. It is also an extremely important thing to do in vineyards when, for example, trying to control Vine mealybugs which are another sap-sucking insect that can spread grapevine leafroll diseases. Mark explains that “the key to maximizing the natural, free pest control services that you get from biological control agents is to reduce the ant numbers to as low densities as possible.” Mark and his team have performed some promising research in this area, and he made a point of mentioning that people interested in learning more about this topic should also follow the work of another leading expert on this topic, Kent Daane of UC Berkeley.
He relates that “one of the most effective ways of doing this is through an approach that we refer to as liquid baiting. A 25% sucrose solution can be placed in reservoirs or in biodegradable hydrogel beds that are spread on the ground underneath the vines or the citrus trees, in which an ultra-low dose of insecticide is added. When the ants drink the sucrose solution, they take the very low concentration of insecticide back to the nest and they feed it to the queen. After a couple of days, those ant populations collapse.” This is backed by some studies Mark has worked on. “Our work in citrus has shown that when you cause the collapse of those ant populations in citrus orchards, within a month the natural enemies have literally cleaned out those sap-sucking pests. As a result, Asian citrus psyllid density has declined by about 75%. Soft scales like the brown soft scale, citricola scale, citrus mealybug have declined by over 95%!”
But Mark mentions something much less obvious. “Even the California red scale, a non-honeydew producing scale, is reduced by natural enemies after the ants are removed. Why would ant control affect the densities of California red scale, since it doesn't produce honeydew and has nothing that the Argentine ants want?” Mark explains, “what our work has suggested is that California red scale benefits from Argentine ants being in the citrus trees, because those ants, as they're moving around collecting honeydew from sap-sucking pests that may be close to California red scale colonies, disturb natural enemies and scare them away. Once the ants have gone, they don't benefit from the presence of the ants and the natural enemies come in and clean them up.”
Argentine ants in a citrus orchard drinking sucrose water infused with ultra-low concentrations of insecticide from biodegradable hydrogel beads.
(Credit: Mike Lewis, Center for Invasive Species Research, UC Riverside)
Besides baiting, some other techniques can be used to control ant populations. Mark has started to gather some promising preliminary data on an interesting technique which is more often associated with soil health and regenerative agriculture - placing organic composted mulch, such as branches and leaves that have been put through a chipper and composted, under the trees. His studies suggest that this technique also reduces Argentine ants activity in orchards. He comments that “lab studies have shown that if ants are walking over uneven terrain, their walking speed is reduced by about 42%. So we think that putting out these mulches to a depth of three to six inches under the trees and spread out to the margins of the canopy really slows down the walking rate of these ants. The other thing is that Argentine ants are very efficient at setting up their foraging routes. They like their foraging routes to be as linear as possible, with few bends or curves. Mulch really disrupts the ability for these ants to move very quickly in straight lines through these orchards.”
Mark believes that these tactics should be more widely used in vineyards and orchards “One of the things that I think maybe is under-appreciated within the IPM community is the intensity of the free services growers can get from natural enemies. When orchards are managed in a sustainable way, with a strategic use of cover crops like alyssum, ant control and of course, a reduction in insecticide use, it can potentially boost the free natural enemy services that you get and control. If you're able to do that, it could be a very effective way of significantly reducing your reliance even on small or infrequent applications of pesticides.”
The role of agriculture technology in sustainable pest management programs
During our conversation, Mark expressed how excited he is about the opportunities agriculture technology has brought to his field. “A lot of the pest control work has the potential to be automated. Human error could be greatly reduced, efficiency could be increased and these types of tasks would become more cost effective.”
He stresses that timing is very important in natural enemies' releases, as there needs to be enough pests to serve as food for the natural enemies if the goal is to have them established. But on another hand, if the release is performed too late, the pest population might be out of control. He believes that using drones to release those beneficials in orchards will greatly help because “you need an efficient way to distribute those natural enemies across hundreds of thousands of trees. When you do it by hand, it's just cost prohibitive.”
Ants sometimes increase their activity when the temperature goes down and some natural enemies such as the hoverfly larvae are nocturnal. Monitoring the orchards during the day when the temperature is high is not only an exhausting task, it might also lead to erroneous assumptions.
“We're in the process of developing infrared sensors that can count ants moving through the citrus orchards. We've already figured out the action thresholds for when ant control is needed. The data from the sensors are sent through cellular towers to your smartphone to be able to look at the counts in near real time. And then you can target your and control to sections of the orchard that are needed”, Mark comments.
The next step would be to automatize the ant bait dispersal. “You could have little motorized land drones that use GPS data to drive at nighttime when there's nobody in the orchard to accurately put out the ant bait in the sectors of the orchard that need control.”
What invasive exotic pests are threatening crops in your area? Do you have experience with classical, augmented, conservation biocontrol or ants management? Reach out to us, we’d love to hear your story and write about it!