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: 03/08/2021
Project: 19-002   Year: 2021
Category: ACP Vector
Author: Lukasz Stelinski
Sponsor: Citrus Research and Development Foundation

We investigated 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. A year-long study was initiated in the spring of 2020 in a young planting of “Hamlin” sweet orange grove under standard agricultural practices for citrus, including mowing and fertilization, located in Lake Alfred, Florida. ACP management strategies were tested in a randomized complete block design with four replicates. Insecticidal sprays for ACP were based on either of three nominal thresholds of 0.2, 0.5, and 1 adult per stem tap and ACP treatment sprays were comprised of two rotations of different MoAs designated as rotations A and B. There were seven applications for the 0.2 adults per tap threshold treatment; five applications for the 0.5 adults per tap threshold treatment; and two applications for the 1.0 adult per tap threshold treatment (Table 1). ACP were monitored by calculating the mean number of adults per tap across all four plots for each treatment. If the means reached or exceeded the target economic threshold, all replicate plots assigned to that treatment were sprayed. ACP sampling occurred every 7 to 15 d. If the mean ACP density for a threshold treatment was within ± 0.05 per tap below the target threshold, a decision was often made to spray that treatment rather than waiting until the following week. Sprays were made after plots were sampled, and insecticide susceptibility to thiamethoxam was determined. Thiamethoxam was chosen as bellwether to test for resistance development given that resistance in ACP is predominantly metabolic and this MoA typically predicts subsequent multiple resistance within populations of this pest. An insecticide program was designed to maintain ACP numbers as close to zero as possible in the treatments considering potential impacts on resistance risks. Broad-spectrum insecticides were restricted to the ‘dormant’ season, and when activity of natural enemies was expected to be reduced, whereas more selective insecticides were used during the primary growing season. Table 1. Description of insecticide rotation programs according to different economic thresholds to manage Asian citrus psyllid                                                              Rotation A                                                         Rotation B  App Date                         0.2*                         0.5                     1.0                             0.2                       0.5                     1.0 5-May-20                              dimethoate                   ————                  ————                      fenpropathrin             ————                 ———— Jun 9 10, 2020                     cyantraniliprole            dimethoate                    ————                      dimethoate              fenpropathrin             ———— Jul 7-10, 2020                      fenpropathrin              cyantraniliprole             dimethoate                       cyantraniliprole         dimethoate                 fenpropathrin      12-Aug-20                            thiamethoxam             fenpropathrin                 ————                       diflubenzuron          cyantraniliprole           ———— 24-Sep-20                            spinetoram                  thiamethoxam              cyantraniliprole                  thiamethoxam          diflubenzuron              dimethoate    28-Oct-20                             diflubenzuron               ————-                    ————                       spinetoram                ————                  ———— 18-Dec-20                             abamectin                   spinetopram                   ————                       abamectin                thiamethoxam              ———— *: 0.2, 0.5 and 1.0 was the designated action (economic) threshold that triggered treatment with insecticide  The average number ACP eggs, nymphs, and adults counted was higher in plots where the 1 adult/tap threshold was implemented than in plots where the 0.2 or 0.5 D. citri/tap economic thresholds were implemented for both rotation A and rotation B. There were no differences in treatment efficacy between the two rotations. Overall, there were no statistically significant changes in susceptibility of D. citri following the completion of the either rotation schedule triggered by either of the three treatment thresholds tested. Furthermore, GSTE1, GST1, EST6, CYP4D1, and CYP4C67 gene expression levels were not significantly different in ACP collected from different threshold treatment populations as compared with the susceptible control. These results allow us to conclude with confidence that resistance was effectively kept in check throughout the trial. The incidence of HLB was determined by the level of CLas pathogen in each of the economic threshold treatment plots. There was 100 % infection of trees before the first application in the early spring. Therefore, the scoring and decline index of HLB was observed after harvesting and nutritional applications. All 480 trees were scored for each economic threshold treatment after harvest on a 0-4 scale, where category 0 = no HLB symptoms, normal growth flushes; category 1 = some HLB symptoms, mostly normal growth of flush; category 2 = some HLB symptoms, some normal growth; category 3 = obvious symptoms with no flush growth. category 4 = obvious HLB symptoms, including small leaves and dead wood and no new growth. The decline index was calculated for each treatment.  Furthermore, for rotation A, greater input of insecticides (lower action threshold) was correlated with lower incidence of HLB. We determined insecticide application costs and fruit drop rate for each economic threshold treatment. Insecticide costs were compiled from the University of Florida extension reports and the 2020-2021 Florida Citrus Production Guide. All prices were based on the products used in our tests. For fruit drop counts, each treatment threshold, 20 trees were counted per replicate plot and 80 trees total were counted per economic threshold treatment to determine mature fruit drop. Dropped fruit numbers were counted weekly for four weeks before harvesting. To estimate the level of CLas pathogen infection among treatment plots, three trees were randomly chosen for sampling per plot for a total of 24 trees for each rotation x treatment threshold treatment combination. Every tree sampled from each treatment plot was CLas positive. Similarly, all trees exhibited  HLB symptoms, The results indicated that trees had similar symptoms of HLB in all treatment plots and ranged between categories 1-4. The input costs of spraying at the 0.2 adults per tap economic threshold were estimated at $451.50 and $ 451.93/hectare for rotations A and B, respectively. The costs associated with the 0.5 adults per tap economic threshold were estimated at $ 288.88 and $ 284.38 per hectare for rotations A and B, respectively. Finally, at the 1.0 adult per tap economic threshold treatment, costs were estimated at $ 101.12 and $ 35.62 per hectare for rotations A and B, respectively (Table 6 and Supplementary data Table 7 and 8). There were no significant differences in fruit drop between each economic threshold treatment compared. However, we have no yet finished analyzing the yield data that were collected. In summary, our results indicate that an economic threshold could be implemented as a decision tool for timing insecticide applications in Florida and is compatible with a range of possible rotations of available insecticides for ACP management. Such rotational programs can maintain psyllids below population levels that negatively impact yield but can reduce the number of insecticide sprays needed per season. In the current investigation, although psyllid populations were reduced more effectively with the lower threshold that necessitated the most insecticide sprays per year (7), there was no difference observed in fruit drop or tree health between plots treated 2 times / year using the 1.0 psyllid / tap threshold and those sprayed 7 times/ year using the 0.2 psyllids/ tap threshold. As we continue this research, we will continue to validate use of economic thresholds in mature trees and determine how use of economic thresholds impacts secondary pest and beneficial arthropod populations. Also, we plan on compating profits between the management programs evaluated by including the yield component into our calculations.                          

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