Robert Czokajlo & Elizabeth Beers, November 30, 2023
Washington State University, Tree Fruit Research & Extension Center, 1100 N. Western Ave., Wenatchee, WA
Based on the keen interest in the Asian figitid parasitoids of spotted-wing drosophila (SWD) and the variety of sampling methods employed by various research groups, we conducted a study on figitid sampling methods. We originally noted that our sentinel traps from the Areawide study were all zeros, while wine-vinegar baited jar traps in the same orchard were finding figitids. A second motivation was the very robust dataset presented by Chris Looney at the Pacific Branch ESA meeting, which found large numbers of figitids in the by-catch of northern giant hornet traps deployed across western Washington. This really highlighted the utility of by-catch examination in traps with a potentially unrelated target species.
Materials and Methods
The study was conducted summer/fall of 2023 in three counties in northwestern Washington state (Whatcom, Skagit, Snohomish) (Fig. 1). The six replicate sites were selected on the basis of high capture of the two Asian figitids in a 2022 survey. All sites contained a patch of Himalayan blackberry (Rubus armeniacus) >50 m long. Five positions 10 m apart were marked with a stake in each patch, and the treatments were randomly re-assigned to a position within a site at each of 10 sampling dates (June – October). Sites were visited every other week during that period, and the berries collected or trap contents collected and re-deployed. The experimental design was RCB with 5 treatments and 6 replicates, sampled repeatedly 10 times.
The treatments were representative of methods that had been used by various groups in North America (see below). The first was fruit collections of ripe or ripening blackberries, 10-20 per sample date. The low number was used to prevent stripping the fruit during repeated sampling; however, it should be noted that this sampling effort is less than that recommended by Abram et al. (2022). The second was the SWD-infested fruit sentinels containing 1st – 2nd instar SWD larvae, the correct stage for the figitid parasitoids; this method is used by the SWD Areawide and SWD OREI group. Treatments 3 and 4 were SWD jar traps; the trap body is the one developed by the Beers lab, but is similar to the commercial versions by Trécé and Scentry. All are designed to hold a certain volume of liquid bait or drowning fluid, with screened holes to diffuse the attractive odor of the bait or lure, and allow entry of flies and parasitoids. The screen size filtered out larger insects (e.g., wasps and hornets, moths) that might otherwise enter the trap, making by-catch more difficult to sort. The 5th treatment was a double-sided yellow sticky card (AlphaScents, Canby, OR) with a Scentry lure used as an attractant.
Treatment 1. Fruit samples
Treatment 2. Fruit sentinels
Treatment 3. SWD jar trap with wine vinegar
Treatment 4. SWD jar trap with Scentry lure
Treatment 5. Yellow sticky card with Scentry lure
At each sample visit, the traps were serviced and the contents returned to the lab for incubation (treatments 1 and 2) and identification of adults (all treatments). Fruit from the berry collections and fruit sentinels were incubated in a growth room at 23 °C for 6 weeks, at which time the adult parasitoids were placed in vials of alcohol. As time permitted, they were counted and identified to species using the key characters and photos provided by Matt Buffington and Paul Abram in the 2022 meeting (species identification is still in progress at this writing). SWD, other Drosophila, and other Diptera were recorded to verify target host presence, and the volume of by-catch present.
Results and Discussion
A total of 673 Asian figitids were collected during the course of the study. Of those, 0.74% were G. brasiliensis, and 99.26% were L. japonica. An additional 8 specimens of the native figitid, tentatively identified as L. heterotoma, were also collected.
A few figitids were collected in June-early July, but numbers were very low in late July through early September (Fig. 2). Capture began to increase in mid-September, and peaked in mid-October. Sampling ceased when an early season snow storm made travel difficult over the Cascades, although based on the final sampling date, detection may have continued into November.
The two jar traps (treatments 3 and 4) caught significantly more figitids than all other sampling types (Fig. 3). The fruit collections and yellow sticky card captured similar numbers, and although not statistically different from the former, the sentinel traps failed to capture any figitids during the 10 week period.
The advantages and disadvantages of fruit collection and fruit sentinels are discussed by Abram et al. (2022). The greatest advantage of fruit collections is that seasonal dynamics can be tracked, and host association and ecology can be assessed. The sentinels were considered more laborious than fruit collections, but allowed for monitoring of parasitoids when SWD density was low. The jar trap methods, despite being targeted for SWD capture, were surprisingly effective in capturing the parasitoids; it is possible that the same cues that attracts the host (SWD) also attracts the parasitoid attempting to locate SWD-infested fruit. Recording the number of SWD in the traps gives only a loose association with the host compared with separating SWD puparia and verifying this relationship. The yellow sticky card performed less well than anticipated, given that yellow is frequently attractive to parasitic Hymenoptera; in theory, the addition of a visual cue to an odor cue should have been more effective. However, alighting and retention on the sticky surface may have been less efficient than retention in the liquid in the jar traps.
A potential significant disadvantage of the jar traps is that they may be biased in favor of a species or sex; the Looney et al study indicated that the northern giant hornet traps (rice wine and orange juice) caught almost all female L. japonica. Too few G. brasiliensis were captured in this study (5 individual) to determine if sample method introduced bias; G. brasiliensis was captured in all sampling methods except the sentinels.
The potential utility of the jar traps may lie in delimiting surveys, where presence/absence or a relative density is the primary goal rather than % parasitism or host-parasitoid relationships. This may also be useful for release-recapture pre- and post-release sampling and give a more optimistic picture of the establishment of G. brasiliensis.
References Cited
Abram, P. K., X. Wang, T. Hueppelsheuser, M. T. Franklin, K. M. Daane, J. C. Lee, C.-H. Lue, P. Girod, J. Carrillo, W. H. L. Wong, R. R. Kula, M. W. Gates, B. N. Hogg, C. E. Moffat, K. A. Hoelmer, A. A. Sial, and M. L. Buffington. 2022. A coordinated sampling and identification methodology for larval parasitoids of spotted-wing drosophila. J. Econ. Entomol. 115(4): 922-942.