Journal of Economic Entomology – Arjun Khadka, Sandra A Allan, Daniel Cho, Emma N I Weeks
Abstract
Asian citrus psyllid, Diaphorina citri Kuwayama, negatively impacts the citrus industry as it transmits Candidatus Liberibacter spp., the causal agent of citrus greening. Monitoring D. citri population levels is critical for management of vectors and citrus greening and is usually through use of yellow sticky traps. In our study, use of odors, odor blends, and visual targets were evaluated to determine whether attraction to yellow sticky traps could be improved. Methyl salicylate consistently increased D. citri attraction to decoy yellow but not to other yellow or yellow/green traps. Addition of a visual target did not enhance attraction to sticky traps. While several chemical blends were evaluated, they did not increase psyllid attraction to decoy yellow traps. The increased attraction to traps with methyl salicylate is promising and may contribute to trapping efficiency under field conditions.
Color, Odor Tests or Both
The first experiment examined whether odor increased attraction to a trap (n = 12). To isolate attraction due to visual cues from odor cues, assays were conducted using a one-choice assay. Colors used were Olson yellow, lime green, decoy yellow, neon green, and clear controls. Tests were conducted with a trap with no odor or in conjunction with methyl salicylate or p-cymene, previously reported attractants for D. citri (Mann et al. 2012, Patt et al. 2018). To avoid unintended contamination with odors, traps with no odor, those with methyl salicylate or those with p-cymene were conducted on different randomized days.
A second experiment was conducted to determine whether a contrasting visual target (black, UV-reflecting white, neon green, decoy yellow, or clear) would enhance attraction to decoy yellow, Olson yellow, lime green, or neon green traps (n = 8). Neon green traps were tested with a decoy yellow visual target, instead of a neon green target. Tests were conducted as four-choice assays with comparisons made between trap/target color combinations.
Based on the first two experiments, a third experiment evaluated combination of odor and visual targets (n = 18). Attraction was assessed in one-choice assays to attractive traps (decoy yellow, lime green), traps with the most attractive odor (methyl salicylate) or traps with odor and visual target (neon green).
In a fourth experiment, one-choice assays were used to evaluate attraction to decoy yellow traps combined with individual volatiles previously reported as D. citri attractants (n = 12). These included methyl salicylate, β-ocimene, p-cymene, formic acid, caryophyllene, limonene, β-myrcene, ethyl butyrate, citral, acetic acid (Mann et al. 2012, Aksenov et al. 2014, Coutinho-Abreu et al. 2014b, Patt et al. 2014, George et al. 2016), and a methanol control.
In a fifth experiment, three odor blends previously formulated for D. citri and a methanol control with decoy yellow traps were evaluated in one-choice assays (n = 12). Aksenov blend consisted of β-caryophyllene: citral: limonene in a 9: 1: 5 ratio and tested at 1 and 5% (Aksenov et al. 2014). Coutinho-Abreu blend consisted of p-cymene: ethyl butyrate: myrcene (1: 1: 1) (Coutinho-Abreu et al. 2014b). Lapointe blend consisted of p-cymene: formic acid: acetic acid (1.0: 3.5: 1.6) (Lapointe et al. 2016).
Spectral Analysis
Reflectance from traps and visual targets was measured from 300 to 700 nm using a concave grating spectrometer with a tungsten–deuterium light source (Stellarnet, Tampa, FL) and quartz light guides. Spectral data are presented in Fig. 1. Reflectance from the surface of traps and targets used in the study.
Results
All yellow or green traps in the absence of odor were equally attractive and collected more psyllids than clear traps. However, with odor, attraction varied between different color traps. With methyl salicylate, decoy yellow traps were the most attractive traps and neon green and Olson yellow traps moderately attractive. Least attractive were clear and lime green traps. Methyl salicylate addition increased attraction to decoy yellow traps only. With p-cymene addition, decoy yellow traps were more attractive than other colored traps; however, attraction was reduced to neon green and Olson yellow traps. Visual target addition (black, white, neon green) did not enhance collections on decoy yellow (F3,31 = 1.17; P = 0.336), Olson yellow (F3,31 = 0.68; P = 0.58), lime green (F3,31 = 2.59; P = 0.07), or neon green (F3,31 = 1.15; P = 0.37) traps compared with those without targets (data not presented).
