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: 12/01/2021
Project: 19-002   Year: 2021
Category: ACP Vector
Author: Lukasz Stelinski
Sponsor: Citrus Research and Development Foundation

We aimed to investigate the potential of a usable economic threshold to increase sustainability of Asian citrus psyllid (ACP) management in citrus under conditions of high huanglongbing (HLB) incidence. Specifically, we measured efficacy of three nominal thresholds by relating ACP densities to cost of application and yield. Moreover, two spray programs of insecticide MoAs were compared in a region of Florida where insecticide resistance in ACP was previously identified in an effort to evaluate the need for an effective dormant season spray and to combine integrated pest management (IPM) with integrated resistance management (IRM) practices.  The highest yield was observed with the 0.2 ACP / tap threshold that required 7 annual sprays, while reducing the number of sprays to 5 and below with higher thresholds caused a significant decline in yield. The estimated profit obtained with using the 0.2 ACP / tap threshold was higher than with the two higher thresholds tested in this study, indicating that we were unable to reduce the number of sprays below 7 per year under this set of circumstances without compromising yield. Fewest adults were observed in plots with the lowest threshold evaluated, which required seven annual sprays. ACP populations were lowest overall in plots where treatments were triggered by the lowest threshold evauated (0.2 ACP/tap). The input cost of spraying at the 0.2 adults per tap threshold were estimated at $451.93/hectare. The costs associated with the 0.5 adults per tap economic threshold were estimated at $284.38 per hectare. Finally, at the 1.0 adult per tap economic threshold treatment, costs were $35.62 per hectare. There were no significant differences in fruit drop between the three threshold treatments compared. Susceptibility of ACP to thiamethoxam in treated field plots after each insecticide application was monitored with an insecticide bioassay. In treatments that were managed with the 0.2 ACP/ adults / tap threshold, resistance ratios ranged between 1.63-5.25. For the 0.5 ACP adults / tap threshold, the resistance ratios varied from 1.75-5.25. For the 1 ACP adult / tap threshold, the resistance ratios ranged between 2.89-3.25. Overall, there were no statistically significant changes in susceptibility of ACP following the three treatment thresholds tested. Significant differences were observed between two insecticide rotation programs depending on whether or not an effective dormant season insecticide spray was applied near budbreak of the first seasonal flush. 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. Our current results also indicate that the 0.2 psyllid per tap threshold shown to effectively predict need for ACP treatment application in mature citrus may also be useful in young trees, but going above this threshold may not be possible without reducing yield. This outcome may have been affected by the frequent flushing that occurred in young trees combined with the small size of our treatment plots, which likely promoted re-colonizaiton of treated areas due to psyllid movement. Our results also suggest that combining an action threshold with an appropriate rotation program can effectively prevent development of insecticide resisitance among psyllid populatons. Furthermore, our results indicate that a highly effective dormant season insecticide spray targeting both adult and immature psyllids near budbreak of the first seasonal flush will be required in order to implement a low (0.2 psyllds/tap) treatment threshold during the remainder of the season. Implemention of thresholds to predict need for ACP sprays could allow for transitioning away from calander-based spray programs and development of more sustainable citrus management programs. Further research on the consequences of using economic thresholds on populations of secondary pests and beneficial arthropod species in Florida is warranted.                                  

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