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Native generalist natural enemies and an introduced specialist parasitoid together control an invasive forest insect

Citation

Broadley, Hannah; Boettner, George; Schneider, Brenda; Elkinton, Joseph (2022), Native generalist natural enemies and an introduced specialist parasitoid together control an invasive forest insect, Dryad, Dataset, https://doi.org/10.5061/dryad.1g1jwstrt

Abstract

Specialized natural enemies have long been considered a major force driving the population dynamics of outbreaking forest insects. While research has traditionally focused on the role of specialist parasitoids, recent studies and reviews reflect an appreciation of complex interactions among many regulatory factors. The sources suggest that specialist parasitoids and generalist predators can each inflict strong top‐down effects and that specialists and generalists can interact to regulate insect herbivore populations. Here we use the model study organism winter moth (Operophtera brumata) in its invasive range in the northeast United States to investigate interactions between the introduced, host-specific tachinid parasitoid Cyzenis albicans and native, generalist pupal predators. Prior research in Canada showed that predation of winter moth pupae increased after C. albicans establishment. To explain this phenomenon, the following hypotheses have been suggested: (1) parasitoids suppress the winter moth population to a density that can be maintained by generalist predators, (2) unparasitized pupae are preferred by predators and thus experience higher mortality rates, or (3) C. albicans sustain higher predator populations throughout the year more effectively than winter moth alone. We tested these hypotheses by deploying winter moth pupae over six years spanning 2005 to 2017 and by modeling pupal predation rates as a function of winter moth density and C. albicans establishment. We also compared predation rates of unparasitized and parasitized pupae and considered additional mortality by a native pupal parasitoid. We found support for the first hypothesis; we detected both temporal and spatial density dependence, but only in the latter years of the study when winter moth densities were lower. We found no evidence for the latter two hypotheses. Our findings suggest that pupal predators have a regulatory effect on winter moth populations only after populations have been reduced, presumably by the introduction of the host-specific parasitoid C. albicans.

Methods

Study organisms

Winter moth, Operophtera brumata L., is a geometrid moth native to Europe and northern Asia and can be an important defoliator, particularly in its invasive ranges in coastal North America (Nova Scotia, British Columbia, and New England) (Roland and Embree 1995, Elkinton et al. 2015) and in sub‐arctic birch forest in northern Fennoscandia (Jepsen et al. 2008). Winter moth caterpillars hatch in synchrony with bud-break of their host plants, a broad range of deciduous trees, particularly oak and maple. The caterpillars feed on the foliage in early spring before dropping to the soil in mid-late May to pupate. Winter moth has a long pupal period (6–7 months during the summer, representing most of its life) and it pupates in the top layer of soil or leaf litter. They emerge as adults in early winter, from early November through early January at which point they mate and lay their eggs in bark cervices. These eggs overwinter and the cycle repeats with one generation per year. Cyzenis is a tachinid parasitoid that co-evolved with winter moth. It deposits its eggs on leaves, and any subsequent caterpillars that feed on those leaves ingest the eggs. The fly maggot develops inside the caterpillar, eventually killing it, and pupates inside its host’s pupa. Like winter moth, Cyzenis has one generation per year, but the adult flies emerge the following spring rather than in the winter when winter moth adults emerge.

 

Pupa Deployment

Pupae were deployed in 2005 and yearly from 2013 through 2017 to estimate mortality from predation over the winter moth pupal period. We deployed unparasitized winter moth pupae and Cyzenis puparia (winter moth pupae parasitized by Cyzenis), both reared from spring larval collections. Methods for pupal collection, rearing, and storage are described in Appendix 1. Pupae were deployed in eastern Massachusetts at eight study sites used for long-term assessment of winter moth population dynamics (Elkinton et al. 2015) in either three or five consecutive rounds per year, from mid-June until end of October. Five deployments (one every three weeks) were completed in 2005, 2013, and 2014; three deployments (one every six weeks) were completed in 2015 - 2017. In 2005, sets of 40 winter moth pupae were deployed, while in all the subsequent years (2013 – 2017) sets of 100 winter moth pupae and 50 Cyzenis puparia were deployed. To evaluate season-long mortality as compared to the cumulative mortality calculated from the consecutive deployments, we deployed an additional 200 winter moth pupae at two sites (Table 1, sites B and D) from 12 Jun. - 20 Oct. 2014. To evaluate mortality of Cyzenis puparia over the winter, we deployed a set of 100 puparia at sites B and D from 26 Oct. 2013 - 3 Apr. 2014. 

Deployed pupae were attached to small burlap squares (one per square) using beeswax, as per Broadley et al. (2018). Pupae on burlap were placed haphazardly 2.5 cm below the soil surface under the drip line of a red oak (Quercus rubra) at the study sites (Table 1, Appendix S1: Figure S2). The deployment depth was chosen to mimic natural pupa placement (Holliday 1977). Study sites coincided with winter moth long-term study sites and reflected a range of winter moth and Cyzenis establishment histories (Elkinton et al. 2014). All sites were in mixed hardwood forests dominated by red oak (Q. rubra) and red maple (Acer rubrum). Due to limited supply, only select sites received deployments of Cyzenis puparia (Table 1). To test for an effect of our deployment method as compared to previously published mortality estimates using a wire tag method (Buckner 1969, East 1974, Raymond et al. 2002, Horgan and Myers 2004), in 2005, we compared mortality of pupae attached to wire tags as compared to pupae attached to burlap squares. We buried 30 wired pupae and 30 burlap-attached pupae at two sites for two weeks.

 

Estimating site pupal densities and parasitism by Cyzenis

To estimate winter moth pupal density and percent Cyzenis parasitism at each site, 16 plastic buckets (16 cm wide x 28 cm long x 10 cm high) were filled 3 cm deep with sifted peat moss and placed under each study tree in late May, before pre-pupal winter moth caterpillars began to spin down from the tree canopies. Each bucket was placed at a randomly selected distance between the tree stem and the edge of the tree canopy, along one of eight evenly-spaced directions radiating from the tree stem. Parasitism rates on winter moth by Cyzenis were estimated both from the proportion of Cyzenis-parasitized pupae from these pupal bucket collections and from collections of 100 to 500 late-instar larvae collected at each site.  The collections were made one week prior to the onset of pupation. The larvae were reared in groups of up to 500 in 20 liter buckets with screen tops. In July, after Cyzenis had completed development, we dissected all the cocoons and scored them for the presence of fly puparia.

 

Pupal mortality estimates

After each deployment, pupae were retrieved and stored in a growth chamber (Percival Scientific, Inc.) at 12°C in dark until analysis. We characterized the fate of the pupae as intact, predated, parasitized, or diseased. Predation was assumed for pupae that had been removed from the burlap square and for pupae with only the crushed cuticle remaining, holes chewed in them, and evidence of teeth or claw marks in the wax, as shown in Broadley et al. (2018). Parasitism by native Pimpla wasps was assumed for pupae with characteristic wasp emergence holes and pupae that yielded wasps as shown in Broadley et al. (2019). No data on Pimpla were recorded in 2005 and 2013 because we had not yet identified the characteristic emergence holes these wasps leave in the pupal cuticle, and we did not hold the pupae to allow for wasp development. We excluded diseased pupae in our mortality estimates since the desiccation or mold likely occurred as a result of rearing conditions; these pupae accounted for a small proportion of mortality (< 6%). To allow for the development of pupal parasitoids, seemingly intact pupae were stored in an growth chamber over the winter, as outlined in Broadley et al. (2019), and were re-examined in the spring.

Predation rate and parasitism were calculated for each pupal deployment. The proportion parasitized was calculated as the number of pupae parasitized by Pimpla wasps divided by the number of pupae that remained after predation. This method of calculating percent parasitism estimates the true, underlying mortality rate of contemporaneous mortality factors and incorporates the fact that parasitism rates can be obscured by predation rates because predation typically occurs on the pupae whether or not they were parasitized (Elkinton et al. 1992). Because previous research found no relationship between the proportion of pupae parasitized by Pimpla and the proportion lost to predation (Broadley et al. 2019), we assumed the estimate of Pimpla parasitism determined after predation acted on the deployment will be the same as that prior to predation.

Annual cumulative (life stage long) survivorship values were calculated as the product of successive survivorships of each pupal deployment (e.g., Sc = S1a x S1b x S2 x S3) and the cumulative mortality values (Pc) were calculated as (Pc = 1−Sc). To analyze mortality rates of deployments across years and months, even when the number of days deployed varied (from 19 to 45), we standardized mortality proportions to the mean deployment duration of 31 days. We estimated the daily survival rate (S) = Si1/n where n is the variable number of days the pupae were deployed (ranged from 19 to 45 days, mean of 31 days). The daily survivorship (S) was raised to an exponent of 31 for the expected survivorship over a standardized 31 days (S31 = [(Si)1/n]31). To estimate pre-experimental mortality of the pupae (S1a), we used the estimated daily survival of the first deployment (S1a = S1b1/n) where n is the number of days the first deployment was out and raised this to the estimated number of days of the winter moth pupal period that elapsed before to the onset of the study. Based on prior research (Elkinton et al. 2015), we estimated the start of the pupal period to be 1 June.

 

Predator exclusion and community experiments

In 2013 and 2014, we used a combination of predator exclusion studies and pitfall traps to identify predators in the community and to evaluate their relative contribution to pupal mortality. We ran six deployments of a predator exclusion experiment. Using two sites for each deployment (Table 1, sites A and D), we ran one deployment in 2013 (12 Aug. - 20 Oct.) and two in 2014 (26 Jun. - 11 Aug. and 18 Aug. - 30 Oct.). For each deployment, 100 pupae were deployed in a 100 m by 100 m grid with one pupa placed every 1 m2 in the array. The pupae were attached to burlap squares and these squares were secured to the bottom of one of three cage treatments—cages with 3.2 mm (1/8”), 6.4 mm (1/4”), or 12.7 mm (1/2”) square openings or a control (just the wire mesh bottom of one of the other three cages). The cages were placed 2 to 3 cm into the ground and covered with soil and leaf litter. At the end of each deployment, the pupae were characterized as intact, predated, parasitized, or diseased. In 2013, two pitfall traps were placed at each study site and checked weekly. The pitfall traps were each made of a 0.91 L (24 oz) plastic cup, buried flush to the surface, covered with a lid elevated five centimeters above with a wire stand, and partially filled with 70% ethyl alcohol solution. All beetle larvae, adult staphylinid and carabid beetles, and any small mammal by-catch were counted.           

Funding

Animal and Plant Health Inspection Service, Award: 12 13 14-8225-0464-CA

USDA Forest Service, Award: 13-CA-1140004-236-CA

Dissertation Research Grant and a Graduate Fieldwork Grant from the UMass Amherst Graduate School, Award: NA

Natural History Collections at UMass Amherst, Award: NA