Exploring Navel Orangeworm Monitoring Tools for Almond Orchards Under Mating Disruption
Jhalendra Rijal, UCCE IPM Advisor, Stanislaus, San Joaquin and Merced Counties Charles Burks, Research Entomologist, USDA-ARS, Parlier, CA
Background
Navel orangeworm (NOW), Amyelois transitella is the primary pest of almonds and pistachios, and a significant pest of walnuts in California. NOW females lay eggs on hull-split nuts in which young larvae bore into and cause direct damage to the nutmeat. Additionally, damaged nuts are highly susceptible to mold fungus, Aspergillus spp. that can produce carcinogenic aflatoxin. A comprehensive IPM approach that combines various monitoring and con- trol measures is essential for navel orangeworm management. Recent studies have demonstrated that synthetic pheromone-based mating disruption could effectively be integrated into navel orangeworm management programs to reduce damage by this pest in nut crops. For monitoring purposes, the oviposition bait-based egg trap has been used to monitor egg-laying activity in nut orchards since the 1980’s. After the discovery of the NOW pheromone and its commercial production, the use of pheromone lure has become a regular practice in nut orchards across the Central Valley. However, the pheromone trap is not effective in tracking NOW adult activities in mating disrupted orchards as synthetic pheromones from mating disruption products impair the male moth’s ability to find females or the pheromone lure. In the last couple of years, new types and formulations of attractants such as oviposition baits, a naturally occurring phenolic compound — phenyl propionate (PPO), and host-derived ovipositional attractants are being used to monitor NOW adults in nut orchards, including almonds. In this article, we present the results of multiple studies conducted to examine the efficacy of commercially available attractants in capturing NOW adults in almond orchards, with or without a mating disruption program, and discuss the potential implications of these newer attractants in monitoring navel orangeworm as a part of the NOW IPM program.
Study Design
We conducted trapping studies in seven almond or- chard sites in the Modesto and Fresno area. This represented the upper and lower SJV in the 2020 field season. In upper SJV, attractants were tested in three sites (60-90 acres) located in Stanislaus County. In each site, we had two plots (with commercial mating disruption, and without mating disruption - grower standard) of the same commercial planting, with a separation distance of ~400 ft. between the plots. In the mating disruption plot, navel orangeworm mating disruption dispensers (CIDETRAK NOW MESO; Trece, Inc.) were used at the rate of 20 dispensers/acre. Commercially available attractant types used in the study consisted of pheromone lure (Pheromone), two sources of phenyl propionate lure (PPO1; PPO2), the combination of PPO lures with pheromone lures (PPO1+pheromone; PPO1+pheromone), and ground pistachio-based Peterson bait (ovipositional bait Ovibait). The orange delta trap was used for pheromone lure, while the white wing trap with sticky liners was used for the rest of the attractants. The attractants were deployed inside the traps following the manufacturer’s directions, and four traps of each attractant were installed in two tree rows separated by five rows (approx. 100 ft) around the center 5-10 acres of individual plots. Traps were checked and serviced weekly for 20 weeks from May through mid-September.
In the lower SJV study conducted in Fresno County, three attractants (Pheromone, PPO1, Ovibait) were used, and all white wing traps were deployed in 160 acre almond orchard sites. The 40-acre quarters of these were used as experimental plots, with mating dis- ruption or alternative treatments applied to the central 30 acres of these plots. All traps were at least 400 ft. from the edge of the orchard. The pheromone and ovibait traps were one tree apart because they attract different sexes at different times of night, and previous data indicate that there is no interaction between these trap types. PPO1 was used by itself and was 120 ft.
away from the pheromone and ovibait traps. The wing traps were prepared by bending the wire to clip on and off the bottom half containing the trap liner. The ovibait traps were prepared with both the top and bot- tom about two and a half inches apart instead of the one-inch separation used for wing traps for phero- mones. The wing traps with PPO1, also used the wider separation. Traps were checked weekly from April to September.
Results
Cumulative trap counts from mating disruption or grower standard plots of all three sites in the upper SJV or four sites in the lower SJV were combined for statistical analyses. In the upper SJV, traps that con- sisted of pheromone alone or the combination of pher- omone and PPO1 or PPO2 caught significantly higher numbers of moth than Ovibait, PPO1, or PPO2 used traps in grower standard plots (Table 1). In mating dis- ruption, the highest moth catch was recorded in PPO1+ pheromone traps. PPO2+ pheromone or Ovibait caught fewer moths than PPO1+ pheromone traps but higher than the rest of the treatments (Table 1). Pheromone alone traps resulted in significantly lower catch than the traps mentioned earlier under mating disruption (Table 1).
Table 1. Cumulative numbers of navel orangeworm per trap for various at- tractants in almond orchards in upper San Joaquin Valley.
*Numbers following the same letters within the column are not statistically different.
In the southern SJV, in grower standard plots, though treatments were not significantly different regarding total moth catch, a higher number of moth were caught in PPO1 and pheromone traps, respectively (Table 2). In the plots under mating disruption; however, PPO1 and Ovibait caught significantly higher numbers of moths than the pheromone traps (Table 2).
Table 2. Cumulative numbers of navel orangeworm per trap for various attractants in almond orchards in lower San Joaquin Valley.
*Numbers following the same letters within the column are not statistically different
Although we present combined numbers in tables and used cumulative counts for statistical analysis, NOW adult activities across attractant types in all 14 plots across seven sites were highly variable (Figs. 1 & 2). It is important to understand the orchard and population factors that can play an essential role in trap efficiency.
Fig. 1. Average weekly navel orangeworm adults/trap captured in various attractant types in almond orchards in upper San Joaquin Valley
Fig. 2. Average weekly navel orangeworm adults/trap captured in various attractant types in almond orchards in upper San Joaquin Valley
Summary
Our study indicated that the trap with PPO combined with pheromone lures shows excellent promise to use in orchards with or without mating disruption to monitor the navel orangeworm population. Also, the efficacy of PPO seems to be different among orchards under vary- ing levels of infestation. For example, PPO alone may provide a reasonable estimation of adult activity if the navel orangeworm population in the area is consistently moderate or high (e.g., lower SJV sites; Fig. 2). Under relatively low NOW pressure (e.g., sites 2 & 3 of upper SJV; Fig. 1), the combined use of PPO with pheromone seems to be critical. The selection of attractant type can depend on the purpose of the monitoring. If the moni- toring is aimed at tracking the flights, PPO combined with pheromone lures seems to be the best choice. In contrast, if the monitoring is focused on navel orange- worm female moth activity, which is a better predictor of nut damage, the use of ovibait trap counts can be a valuable decision support tool for NOW pest manage- ment. (Rosenheim et al. 2017; Ref: J. Econ. Entomol. 110: 2692–2698; doi: 10.1093/jee/tox226). Additional- ly, although delta traps are easier to use and preferred traps among pest control professionals, the rate of cap- ture in delta traps, even the modified delta trap in which a rectangular cut covering approximately half of the surface area of the delta trap ‘roof’ was made for better air flow, showed poor performance compared to the wing traps for both PPO (with/without pheromone) and Ovibait attractants (Burks et al., 2020; Ref: J. Econ. En- tomol. 113: 1270-1278; doi: 10.1093/jee/toz363; Fig. 3).
Fig. 3. Performance of different types of traps and lure combinations tested for cap- turing orangeworm adults in lower San Joaquin Valley. (Source: Burks et. Al 2020)
The type of attractants that can effectively be used to track navel oranageworm flights in various field conditions and population levels is still a moving target. However, for information we generated so far, the use of PPO + pheromone lure in wing traps seems to be the best option to track navel orangeworm flights In conclusion, the type of attractants that can effectively be used to track navel oranageworm flights in various field conditions and population levels is still a moving target. However, for information we generated so far, the use of PPO + pheromone lure in wing traps seems to be the best option to track navel orangeworm flights in almond orchards under mating disruption. We will continue to explore and refine various monitoring tools that can help growers and pest control advisors implement IPM practices in nut crops.
Acknowledgments
The Almond Board of California partially funded these studies. We thank field technicians F. Hengst, D. Rivers, F. Hengst, L. Salinas, J. Salinas, and S. Stephens for their help in conducting trapping studies