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: 09/01/2021
Project: 19-002   Year: 2021
Category: ACP Vector
Author: Lukasz Stelinski
Sponsor: Citrus Research and Development Foundation

Importance of the dormant spray with regard to establishing threshold-based annual spray program.  Previous research has demonstrated the importance of reducing ACP populations during the dormant winter period as an effective method to initiate the citrus growing season with low psyllid populations. Our initial results indicated that it may be difficult to implement the use of treatment thresholds without effectively managing psyllids during the dormant period using appropriately timed insecticides that are effective against both psyllid adults and nymphs. Because of the differences in dormant season spray applications between commercial growers, we were able to investigate this question by monitoring psyllid populations and associated citrus flushing in groves that were managed differently, yet located in close proximity to one another. In doing so, we were able to compare psyllid populations in groves: 1) lacking an effective dormant season application (Grove 1 below) versus those 2) where an effective dormant season treatment had been applied (Grove 2 below). These applications were made because of various goals and constraints (harvest period, available budget) facing each particular grower involved.  Six and five insecticide sprays were performed at Grove 1 and Grove 2 groves, respectively, during the course of monitoring. Two insecticide rotations were compared; the first rotation (HB) consisted of Movento, Timectin, Minecto Pro, Timectin, and Micromite. The second rotation consisted of Exirel, Movento, Agri-Flex, Minecto Pro, and Apta.  Grove 1 did not receive an effective dormant season application, while Grove 2 did. In Grove 1, Movento was sprayed during the dormant season and this spray occurred more than 10 d after budbreak, because of the difficulty in timing the spray in between harvest and bloom. Grove 2 did receive an Exirel spray during the dormant period, which is highly effective against all stages of ACP. This proved to be a highly effective dormant spray even though it had not been timed perfectly and occurred a week after budbreak of the first flush. Grove 2 was then sprayed by keeping ACP near the 0.2 psyllids/tap threshold the remainder of the season. In Grove 1, it was not possible to maintain ACP near the 0.2 ACP/tap threshold despite application of several sprays. In Grove 1, there was no opportunity for an untreated area given that monitoring of treatment effects was conducted within commercial citrus and the main purpose was to compare Groves 1 and 2 with the main treatment difference being an effective dormant season application. Therefore, all of Grove 1 blocks were treated the same and had relatively similar ACP populations. The most intense periods of vegetative growth (feather flush structures) occurred from January to March during both years, 2020-2021. The presence of feather flush structures at the beginning of 2020 was associated to high counts of ACP adults. However; during the same period on 2021, this association between vegetative growth and the number of psyllids was disrupted, and a significant reduction in adults was observed. To analyze the interaction between ACP adults with vegetative growth, autocorrelation analyses were performed. We found that between 0 to 3 weeks after vegetative growth was detected on trees, there was an increase in ACP adults with a statistical association value of 0.5 between occurrence of flush and psyllids. This result means that at least 50% of the adult ACP population present on these Valencia trees emerged from the eggs oviposited during the previous vegetative growth. Also, the effectiveness of each pesticide used in Grove 1 rotation was analyzed, indicating that the Movento spray was the least effective insecticide in the rotation program. Following application of Movento, the number of ACP adults on trees did not change. However, after Timectin and Minecto Pro sprays, there was a significant reduction in ACP adults observed in citrus trees. Six blocks consisting of Valencia (2 blocks) and Hamlin (4 blocks) citrus trees were examined. Similar intense periods of vegetative growth (feather flush structures) were observed on Valencia and Hamlin trees from January to March during both years, 2020-2021. Similar to the results obtained on Valencia trees in Grove 1, high counts of ACP adults were observed at the beginning of 2020 associated with high intensity of feather flush structures. However, during the same period in 2021, this association was disrupted. Autocorrelation analyses showed that 1 to 2 weeks after vegetative growth, a high number of ACP adults were detected on flushing Valencia and Hamlin citrus trees. The data revealed a higher association value of 0.8 between occurrence of flush and psyllids, which indicates that 80% of the ACP population present in these trees was explained by the occurrence of flush.  The effectiveness of each pesticide used in the Grove 2 rotation was evaluated. We found that the Exirel spray to Valencia and Hamlin trees was the most effective insecticide used in the rotation program. The results indicated that Exirel eliminated ACP adults from treated trees for more than 30 days and ACP populations remained low (less than 0.1 ACP/tap) during 2020, except in one Valencia block. Significant differences were observed between the two insecticide rotation programs. High counts of ACP adults were associated with the presence of feather flush structures on Valencia and Hamlin citrus trees. Also, differences in vegetative growth intensity were observed between Valencia and Hamlin varieties, which could have an impact on ACP vector control programs if ACP adults are able to migrate to new groves where feather flush structures are present. Our results indicate that ACP management is most critical during the period between January to March, when citrus is characterized by flowering, fruit maturation (final stage), and the need for safe harvesting. When comparing the two rotation programs, significantly more ACP adults were observed in plots in Grove 1 that did not have an effective dormant season spray than plots in Groves 2, where an effective dormant season spray was applied. Our results highlight the importance of applying a highly effective insecticide, in this case Exirel, during the dormant winter period and soon after initial budbreak of the first seasonal flush.                             

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