Competition Between Exotic Species: Scale Insects on Hemlock

Abstract

Two exotic armored scales, Fiorinia externa Ferris and Tsugaspidiotus tsugae (Marlatt), native to Japan occur sympatrically in the northeastern United States where they often attain high population densities on eastern hemlock, Tsuga canadensis Carriere. Both scales preferentially colonize the young needles of the lower hemlock crown, which intensities competition between them when food and space become limiting at high scale densities. Studies conducted for 2 yr in the greenhouse and in a hemlock forest located in Darien, Connecticut, demonstrated that summer generations of F. externa and T. tsugae compete intraspecifically and interspecifically at densities occurring in the field. F. externa had a greater advderse affect (reduced survival and fecundity) on itself and on T. tsugae than did T. tsugae have either on itself or on F. externa. The superior competitive ability of F. externa over T. tsugae was likely due to its earlier colonization time. Hatching in early June some 2—4 wk prior to its competitor, F. externa is afforded a distinct spatial and nutritional advantage (1) by colonizing young, nitrogen—rich needles at a time during the year when concentrations of foliar nitrogen and water are high, (2) by reducing the amount of foliar nitrogen available to T. tsugae nymphs, and (3) by forcing T. tsugae crawlers to colonize older, nitrogen—poor foilage where they subsequently suffer significantly greater mortality. Aspidiotihagus citrinus Crawford (Hymenoptera: Aphelinidae), a primary internal parasitoid of F. externa and T. tsugae, exhibited a density dependent response to both scales in single—species infestations in the Darien forest. In mixed infestations, however, the parasitoid did not selectively attack the more abundant species but parasitized each scale in proportion to its frequency. Because it did not discriminate between hosts, A. citrinus did not alter the outcome of the competition between summer generations of the two scales. The geographical distributions of the two scales in Faifield County, Connecticut, and their population trends for 3 yr at 20 sites where they occur sympatrically indicate that competition has excluded T. tsugae. At 18 to 20 sites coinhabited at different densities during 1976, T. tsugae was excluded or nearly excluded within 3 yr. Only at two sites, where T. tsugae was abundant and F. externa was sparse, was T. tsugae able to maintain its initial relative density.

Exotic herbivores on a shared native host: tissue quality after individual, simultaneous, and sequential attack

“…After 3 years, EHS-infested foliage had a 24.2% lower total amino acid content than foliage from uninfested trees. Although this trend was only marginally significant, it was consistent with previous studies documenting that increases in EHS density were accompanied by decreases in nitrogen (McClure 1980b). In contrast, HWA-infested trees had much higher free amino acid concentrations than did uninfested trees.…”

Exploitative Competition Between Invasive Herbivores Benefits a Native Host Plant

“…The fact that our results are qualitatively identical to those produced by similar unbagged branch manipulations (McClure 1991) suggests that the 'bag effects' on herbivore densities were not a major source of experimental error. In addition, the observed densities of both species fall well within the range of naturally occurring densities (McClure 1980a(McClure , 1991. Finally, there is the concern that hemlock branches are not functionally ''autonomous,'' i.e., that trees re-allocate resources to stressed branches.…”

“…. sometimes trees die following sustained and heavy attacks'' (McClure 1980a(McClure :1392; however, we are unaware of any research documenting tree death as a direct result of EHS infestation.…”

“…Although HWA overwintering mortality undoubtedly plays a role in reducing hemlock losses in the north (Parker et al 2002), another potential (but nonexclusive) explanation involves the recent rapid range expansion of a second introduced hemipteran, the elongate hemlock scale Fiorinia externa (EHS). Although EHS can reduce hemlock growth and may be capable of killing already-stressed trees (McClure 1980a), this insect is generally considered far less dangerous than HWA. The gloomiest predictions of hemlock mortality were made in the mid-1990s; since then, however, research assessing changes in insect density and forest health in 142 southern New England hemlock stands from 1997 through 2005 found that HWA densities have substantially declined and many hemlock stands still appear relatively healthy (Preisser et al 2008).…”

Two Shoot-Miners,Ceutorhynchus alliariaeandCeutorhynchus roberti, Sharing the Same Fundamental Niche on Garlic Mustard

“…All parasitoid species we reared have been recorded previously from other curculionid species (U. Kuhlmann, personal communication), which makes it unlikely that these parasitoids will distinguish between larvae of C. alliariae and C. roberti. Low discriminative behavior of parasitoids has little potential to inßuence species coexistence (McClure 1980).…”