A sampling unit for estimating gall densities of Paradiplosis tumifex (Diptera: Cecidomyiidae) in Christmas tree stands

Abstract

We used field surveys in central New Brunswick, Canada to establish efficient sampling procedures for evaluating densities of balsam gall midge, Paradiplosis tumifex Gagné (Diptera: Cecidomyiidae), and its associated damage in balsam fir, Abies balsamea (Linnaeus) Miller, Christmas trees. Infestation was greater in larger trees than smaller trees and in mid-crown and upper-crown branches than in the lower crown. However, the relationship between gallmaker infestation and site, height class, and crown level was highly complex and may involve covariation of shoot length with height class and crown level. As a result, patterns in infestation did not lend themselves to simple interpretation. This complexity highlights the need to find sampling units that provide simpler but reasonably accurate predictors of gallmaker impact at the whole-tree scale. We identified such a sampling unit: gallmaker density in first-order current-year shoots of a mid-crown branch explained 81% of the variance in total infestation among trees.

A Simulation Approach to Assessing Sampling Strategies for Insect Pests: An Example with the Balsam Gall Midge

“…Our sampling data indicate that both within- and between-site distributions of P. tumifex are complex. Within sites, we were not able to fit raw gall counts to any simple distribution, most likely because the shoots we sampled varied in size, and P. tumifex attack is influenced by shoot length [2] . Rather than exploring distributions with additional parameters to allow direct modeling of this dependence, we calculated infestation as the percentage of galled needles, which is a measure of direct importance to host trees and to farmers.…”

“…Among major pests of Christmas tree crops is Paradiplosis tumifex (balsam gall midge), a univoltine, needle-galling cecidomyiid that attacks balsam fir across the tree's natural range. Most needles galled by P. tumifex turn yellow and fall from the tree in the year of attack, with defoliation most severe high in the crown [2] , [24] . The natural history of P. tumifex is further described by [25] .…”

“…Giese & Benjamin's [24] recommended sampling schemes were labour-intensive and impractical for application by growers. Only recently has a functional sampling unit been determined at the tree level [2] , and no formal analysis has been available to guide site-level density estimation or decision making with respect to pesticide-application thresholds. Second, the development of comprehensive pest management strategies for P. tumifex (and for other Christmas tree pests) is further hampered by a high diversity of agricultural practices in the industry.…”

“…We converted GPS coordinates from degrees latitude and longitude to north and east distances in metres from a point near the centre of our study area (46°N, 66°W). We used sampling methods prescribed by [2] to assess P. tumifex infestation. Briefly, for each sampled tree, we collected terminal shoot clusters from one south-facing, dominant mid-crown branch.…”

“…For simplicity, in this paper, we use vocabulary associated with insect pests, although our discussion is equally applicable to other types of pest. Estimating insect densities in the field is far from a simple task, and it involves decisions about when to sample during host or insect phenology (e.g., [1] ), what to sample (quadrat, whole plant, appropriate organ, or representative module; e.g., [2] ), and which and how many plants, or other sampling units, to sample from the large number available at a site. This last decision in particular has spawned an enormous literature [3] , with thousands of idiosyncratic recommendations for different systems but with a simple underlying truth: in general, more accurate estimation is achieved by including more samples and selecting them in more sophisticated ways; but doing so requires more time, money and labour.…”