Why spray if you don’t need to? Putting the IPM back into cItrus IPM by ground truthing spray thresholds

Why spray if you don't need to? Putting the IPM back into cItrus IPM by ground truthing spray thresholds

Report Date: 09/04/2020
Project: 19-002   Year: 2020
Category: ACP Vector
Author: Lukasz Stelinski
Sponsor: Citrus Research and Development Foundation

Objective: Develop threshold-based models for current use in Florida citrus. Background: In areas where huanglongbing is widespread, the occurrence of disease can reach a 90-100% within citrus groves. The use of insecticides addresses none of the symptoms of disease but instead reduces ACP populations. The objective of this experiment has been to optimize management of ACP using an economic threshold to time insecticide applications that also involves insecticide resistance management. We are investigating various insecticide rotation programs that we have developed based on previous research on managing resistance for ACP. These rotations are comprised of insecticides of various modes of action that are currently registered and used for ACP control; these are: acetylcholinesterase inhibitor, acetylcholine receptor agonist, an inhibitor of chitin biosynthesis, chloride channel allosteric modulator, sodium channel modulator and a ryanodine receptor modulator. By rotating these modes of action, we should be able to prevent development of resistance in ACP entirely. Our purpose is to determine the economic threshold level that controls ACP populations effectively and whether this population level can be effectively maintained with an insecticide rotation that will prevent resistance development. Ongoing work and preliminary data In our first report, we described two areas where experiments are being carried out (Frostproof and Lake Alfred). In this report, we focus on the partial results obtained from the Frostproof location.  This is a commercial site with an estimated size of 80 hectares. Eight 30-tree blocks were established for monitoring. These consisted of either ‘Hamlin’ or ‘Valencia’ trees. There are 8 blocks in total with 4 blocks per variety. Each block is being monitored weekly by counting psyllid densities and feather flush intensity per block. Within each tree variety, two insecticide rotations are being compared and a treatment threshold of 0.5 ACP per tap sample is in place for triggering sprays in each rotation. Insecticides are applied when ACP populations reach approximately 0.5 ACP per tap. We are investigating the possible differences in ACP densities and feather flush intensity between these citrus varieties (‘Valencias’ and ‘Hamlins’) when using this threshold and two insecticide rotations. There were no differences in ACP densities between the four blocks of Valencias during the month of January 2020 prior to when treatments were made with ACP populations in all blocks averageing approximately 0.1 ACP per tap. The first insecticide application was made during the first week of March 2020 after ACP reached the 0.5/tap threshold population in the four blocks of Valencias. Two of them were sprayed with Movento (spirotetramat, inhibitor of acetyl CoA carboxylase) as part of ‘rotation A’ and other two blocks were sprayed using Exirel (anthranilic diamide, ryanodine receptor modulator) as part of ‘rotation B’. After insecticide applications, ACP populations were not reduced in blocks sprayed with Movento and weekly counts reached 0.4 ACP per tap. A second insecticide (Minecto Pro) application was triggered in these blocks on June 2 2020. In contrast, blocks sprayed with Exirel nearly completely eliminated the ACP population until May 21th, 2020.  The results observed in Hamlin have been different. The first insecticide (Exirel) application in these blocks was triggered by the 0.5 ACP/tap threshold in March 2020. Interestingly, in these blocks, the ACP population has remained below the thredhold until August, 2020.  Thus far, we have observed differences in ACP populations due to choice of insecticide that were not entirely unexpected. Exirel is a much more effective ACP adulticide than Movento, and the results have confirmed that Exirel reduces populations more effectively than Movento after application. Even though Movento can be a useful early season choice for application during bloom, the results suggest that ACP populations must be effectively eliminated after budbreak and prior to bloom instead of relying on the bee-safe Movento during bloom, if a threshold is going to be used for subsequent timing of sprays. The difference in ACP population densities between Valencia and Hamlin blocks treated with the same insecticide (Exirel) is more unexpected. These results were not related to differences in flush intensity between the two varieties. Flush intensity was similar in Valencia and Hamlin blocks during this period. Numerically, flush intensity was actually higher in the Hamlins than Valencias during some of the sampling dates, depite higher psyllid numbers in the Valencias than Hamlins. We will continue to monitor this pattern as the experiment progresses. To date, the data appear to indicate that the 0.5 ACP/ tap threshold is equivalently effective in Hamlins and Valencia, if a potent adulticide, such as Exirel is applied during the dormant period to knock down psyllids before trees begin flushing heavily. In constrast, if Movento is used an an early season treatment, as a means to comply with bloom period restrictions for bee safelty, psyllid populations are not effectively reduced and remain high throughout the spring and summer, triggering more sprays and sooner. These results appear to confirm that a broad spectrum and potent insecticide needs to be scheduled and fit in soon after flushing and before bloom and harvest in order to maintain ACP low during bloom and the subsequent early spring period. Without effectively suppressing ACP populations in the spring, a subsequent threshold-based psyllid management program is unlikely to be successful.        

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