Tracking insect outbreaks: a case study of community-assisted moth monitoring using sex pheromone traps

Spruce budworm in eastern Canada undergoes one generation per year. In early spring, young larvae emerge from diapause to mine year-old needles on spruce (Picea sp.) or balsam fir (Abies balsamea (L.) Mill). As the young shoots begin to shed their bud cap, larvae enter the shoots and spin a protective tunnel in the midst of the expanding needles to protect them as they feed. Larvae continue to feed on young foliage until pupation, which occurs usually around mid-summer (late June to mid-July in eastern Canada). About 10–12 d after pupation, adult moths emerge and proceed to mate, lay eggs, and in some instances disperse to new stands. As soon as moths emerge from cocoons, the females begin to emit a multi-component sex pheromone to attract males in search of mates (Silk et al. 1980). Eggs hatch after 10–12 d and the newly emerged larvae seek out crevices on the tree branches or trunk in which to spin a hibernaculum for protection during overwintering.

We provided BTP start-up kits free-of-charge to volunteers. Kits consisted of the following components: supply checklist and set-up instructions, pheromone trap (Unitrap, Contech Enterprises Inc., pre-assembled with a unique identity number; Fig. 2a), a budworm pheromone lure, an insecticidal strip, prelabeled paper bags (40), a freezer bag, one pair of gloves, a wooden stick for sweeping moths into bags, a pencil or pen, a data collection sheet, a contact information sheet, and a prepaid and addressed return envelope (Fig. S1). In 2018, the insecticidal strip was switched for a solution of soapy water, which provided a simpler and cleaner way to kill moths in traps. Each year, volunteers who wished to continue with the program were encouraged to keep and store their trap, and they were shipped a small refill kit with the same components except the pheromone trap, which was sent at a reduced shipping cost. Kits were sent in a box large enough to enclose all its components (for our full kit (L × W × H): 20 cm × 20 cm × 25 cm box; for refill kits (no trap): 30 cm × 22 cm × 5 cm bubble envelope). We purchased, packaged, and stored all kit components between February and April in preparation for the upcoming season, starting in mid-June. To preserve their quality, sex pheromone lures were kept frozen and were not placed in the delivery box until just before their distribution. BTP protocols were provided in English and French (Fig. S1). We branded all kits and outreach material with our easily recognizable logo (e.g., on trap in Fig. 2b). In 2015, we shipped or hand-delivered BTP kits to local organizations to distribute and collect kits, data, and samples at the end of the season. In all subsequent years, we used mail service (prepaid) almost exclusively to deliver traps and have the data returned.

Following delivery of kits, we guided assembly using a combination of the component checklist, and photographic and video assembly guides (Figs. S1 and S2). Once traps were assembled, we asked that volunteers choose trap locations that were convenient to visit every few days and that had an appropriate host tree (i.e., spruce or balsam fir) with an accessible lower branch from which to hang the trap at about eye level (Fig. 2). Traps were often placed near homes, offices, field stations, parks, or in nearby forests. Following trap set up, volunteers were encouraged to begin checking the trap immediately. We encouraged volunteers to check traps ideally 2–3 d per week; however, trap check frequency varied among volunteers depending on their schedule. If multiple checks per week were not possible, we asked for a minimum of one weekly trap check, ideally on the same day of the week. Spruce budworm moth flight occurs after dusk (Greenbank et al. 1980), so we recommended checking traps during daylight hours. When checking traps, volunteers placed captured moths into one of the provided paper bags and filled in the label with trap number and collection date. Bags with moths were to be placed in the provided freezer bag and then into a freezer to prevent mold. We encouraged volunteers to count collected moths, though there was no requirement to do so because we always recounted moths once they were returned. We asked that volunteers use the provided paper data sheet to record each time the trap was checked and the number of moths that were caught. We told volunteers that even when there was nothing captured they should still report their observations—from our perspective noting the absence of moths on a given day was just as important as noting their presence. To assist BTP volunteers to report data on their moth counts we prepared a webpage and mobile phone applications (Android or Apple iOS). Each trap was given a unique QR code applied as a sticker on each trap (Fig. 2b). This QR code directed users of the mobile phone application to their personal profile on the BTP webpage where they could report the number of moths in their trap. These tools were to be used in addition to the paper records written on the provided data sheets.

We asked that volunteers cease collecting and return moths no sooner than the end of August, which was weeks after most moth flight would have ended in the region. After an initial reminder to return collected moth samples and data, we sent occasional follow-up reminders through social media, post cards, and e-mail. If samples were not received by mid-October, we followed up with another e-mail or phone call.

To recruit volunteers we used many of the same outreach tools that have been used in other community science programs (Mulder et al. 2010; Graham et al. 2011; Council and Horvath 2016). Our first step was to ask local organizations to solicit their members or employees to sign up as volunteers (e.g., forestry companies, government agencies and departments, woodlot associations, Co-ops, park groups, outfitters). Next, we engaged in several periods of concentrated outreach and recruitment of the public via social media and traditional media (i.e., newspaper ads, radio and TV interviews).

When contacted by groups or individuals asking to join the program, we requested a contact name, mailing address, and contact information (phone and e-mail). We also requested the likely location of the trap (i.e., latitude and longitude, determined by GPS or site information provided by volunteer). As the volunteer network developed and the spatial pattern of trap locations began to emerge, we sought to fill in any gaps on the map by targeting communities in the nonsampled area via outreach to local media outlets and community groups.

Providing feedback to volunteers regarding their contribution to the program is a key part of retaining volunteers and ensuring that the outreach and educational benefits of volunteer science are achieved. Thus, at the end of each year, we prepared an annual report for volunteers summarizing the monitoring season, any interesting insights, and adjustments to the protocol for the upcoming year (e.g., Fig. S2). We provided periodic updates and additional information on budworm during the off-season in the form of written and video blogs to keep volunteers engaged in the program and to prepare them for the upcoming season.

All moths returned to the program were verified as spruce budworm based on morphological features and counted. Moth counts were compared to those provided by the volunteers and, where there was a discrepancy, we used our own counts for subsequent analyses. All samples were placed in a freezer (−18 °C) for later analyses in related ecological studies (e.g., morphological, isotopic, and genomic). All other information provided by the volunteers on the data sheets was digitized (e.g., when traps were set up and taken down, weather, sampling frequency, etc.). We used these to assess compliance rates of volunteers with the program’s sampling protocol (e.g., were set up and take down of traps around the recommended dates, were samples collected at least weekly, etc.).

Data from the BTP provided two types of data for use in managing and studying spruce budworm outbreaks. First, budworm moth monitoring is an important part of gauging year to year variations in the distribution and abundance of budworm throughout the region. To this end, the overall moth density data collected by BTP complements that collected by provincial and state jurisdictions. In New Brunswick for example, data from BTP essentially doubled the total number of trapping sites where pheromone traps were set up. These data can help to map regional trends of budworm moth abundance and are could be used to identify areas of potential interest for more intensive sampling of overwintering larvae to inform decisions to manage populations (Johns et al. 2019).

Second, the more frequent estimates of budworm moth density provided by volunteers (compared with conventional trapping) can offer additional insight on the potential origin of captured moths. Because of the large geographic range of the spruce budworm, different populations experience different temperature regimes. This results in correlated, spatial variations in the pacing of larval development across the range of the insect and associated regional variations in the timing of moth flight. Fortunately, the timing (i.e., phenology) of moth flight can be reliably forecast using a temperature-based model (i.e., BIOSIM: Régnière et al. 2013). However, since typical survey programs only sample traps once during a season these patterns of flight can be obscured since discrete flight “events” are all combined in a single trap. By using a combination of information on the date and location of moth capture from the BTP and projected phenologies from temperature-based models, we can distinguish between flights of local, phenologically synchronous, spruce budworm populations and influxes of migrant, phenologically asynchronous, moths. By collecting moths weekly, the BTP volunteers provide high-frequency temporal data on moth flight phenology that can be used to determine the extent of moth flight, for instance during local and mass dispersal events (e.g., Dobesberger et al. 1983; James et al. 2015; Pureswaran et al. 2017). We illustrate this approach in the Results section using a subset of our trapping sites from the 2016 season. The 2016 trapping season was punctuated by a mass-exodus flight event that occurred roughly between 20 and 22 July (i.e., day of year 202–204). This event was first detected by weather radar imagery and indicated that moths were swept southward from the main outbreak defoliation range on the north and south shores of Quebec through Atlantic Canada.

Volunteer participation varied between 216 and 375 people during the first three years of the program (Table 1). The return rate of data and moth samples from volunteers ranged from 65% to 89% of volunteers annually. In total, we received 3006–5324 samples (i.e., sites × sample dates) each year for a total of 16 311–54 525 moths. Approximately 71%–89% of volunteers opted to remain in the program from year to year. We maintained a waiting list of additional people interested in volunteering. With an increasing number of volunteers keeping traps each year, we were able to spend less on traps and shipping, leading to a total drop of ∼45% in shipping costs even as the program expanded. Most volunteers submitted data on the paper data sheets when they returned samples at the end of the year, rather than through the various internet portals provided (e.g., mobile apps and the BTP website; <10% per year).

In general, spruce budworm moth misidentification in traps by volunteers was rare (<1%) as the provided pheromone lure attracts spruce budworm almost exclusively. Some traps occasionally captured a late-season relative (i.e., the blackheaded budworm, Alceris variana Fern.) if traps were left out beyond the normal spruce budworm flight period, though these are easily differentiated from spruce budworm as the two species are morphologically distinct.

Volunteers that did not return any data had a variety of reasons ranging from personal health or family issues to simple forgetfulness. One fortunate volunteer won the lottery and thereafter abandoned her trap-checking efforts. Several others lost traps to mischievous bears.

Regional patterns of spruce budworm moth distribution and abundance, based on the sum of moths captured by each volunteer, illustrated the high concentration of moth activity in and around the main outbreak area of Quebec (Fig. 3). Regional trends provided by the BTP were relatively consistent with those obtained through the traditional centralized trapping programs of provincial and state jurisdictions.

Data from many volunteers who sampled at least once per week showed strong evidence of the large scale of moth dispersal during the 2016 mass-exodus flight event (Fig. 4). For example, in locations south of the mass dispersal event, warmer temperatures facilitate more rapid development and local budworm had ceased flying prior to the mass-exodus flight (Figs. 4a, 4e, 4f). Similarly, in locations where cooler temperatures delayed budworm development (i.e., northern and some coastal areas), local flight occurred days or weeks after the mass-exodus flight (Figs. 4b–4d). Therefore, based on the phenological asynchrony of moths collected at these sites, we can infer that such moths are not likely residents.