Psyllid sprays have long been a point of contention in Florida citrus groves. How many sprays are enough and how many are too much? Psyllid sprays were the focus of a session at the Florida Citrus Show in January presented by UF/IFAS Professor of Entomology and Nematology Dr. Lukasz Stelinski. The session looked at the role sprays and Asian citrus psyllid control methods play in combating citrus greening. There were some important takeaways.
Psyllid Spray Takeaways
The first takeaway of Stelinski’s session is that the concentration of Candidatus Liberibacter asiaticus, or CLas, the bacterium that causes citrus greening, is not related to frequency of inoculation. Essentially, the amount of CLas found in a citrus tree is not affected by how often an infected psyllid feeds on the tree. Research shows that CLas concentrations fluctuate, correlating to vegetative plant growth, or when citrus trees flush. He maintained that research suggests the CLas bacterium is transported through the phloem of a citrus tree—the vascular tissue of a plant that transports sugars and more from the leaves to the rest of the plant—during annual vegetative plant growth cycles.
The research also found that plants respond to pulses of ACP feeding—bursts of psyllid feeding activity rather than a single feeding or continuous feeding—by boosting their natural defenses. However, long-term feeding by psyllids suppresses plant immunity and inhibits growth, which points to another takeaway: the importance of suppressing psyllids as part of an overall citrus greening management strategy. Essentially, psyllids must be managed on a large, continuous scale, such as using initial sprays during the dormant period and then continuing to spray when populations begin to rise; sprays don’t need to occur when there are no psyllids, but intermittent spraying does not adequately suppress psyllid populations.
Another piece of the puzzle is that conventional management—or 10 to 12 sprays per year—had the highest rate of suppressing psyllids, with intermittent sprays—around three sprays—had the least effect on suppressing psyllids, even when compared with organic-managed groves and abandoned groves.
While a high number of sprays are best at controlling psyllid populations, Asian citrus psyllids can quickly develop a resistance to sprays. According to Stelinski, research has observed a 200-500 fold resistance with just 2 back-to-back sprays of the same modes of action (MOA), and an approximately 2,000-fold resistance after three consecutive uses of the same MOA.
However, another takeaway is that psyllid resistance can be undone. Stelinski said rotating five different modes of action in sequence over the course of about 20 weeks—or about five or six generations of psyllids—can cause a reversal to spray resistance for an overused MOA. All in all, Stelinski recommended using a combination of cultural, chemical and biological control tactics to suppress psyllid populations.
