A comparison of three sampling techniques to estimate the population size of caterpillars in trees

Abstract

Three sampling techniques commonly used to estimate the population size of caterpillars and sawfly larvae in trees (branch samples, frass production, water basins), were compared with respect to sampling error and economic costs. At the level of tree populations (e.g. forests), on an arbitrary date, the mean caterpillar intensity per tree (expressed in numbers of larvae or their biomass per 100 shoots) was predicted from the mean frass production per tree (expressed in mg frass per m forest floor per day). At the level of the single tree, the frass production on an arbitrary date was related to the population intensity, but, due to the large sampling error, did not provide an accurate prediction. Summing the frass produced over the whole season reduced this error and predicted the seasonal abundance of larvae in single trees, estimated as their maximum intensity or their density (numbers of larvae or their biomass per m forest floor). The maximum population intensity was not related to the population density. The sampling techniques suffer from large errors unrelated to larval abundance. The main sources of error (i.e. weather or predation of the larvae) usually cause an underestimation of population size. Labour, the main cause of high costs, was low in the basin technique and high in the frass production technique. Possible ways of reducing errors and applications of the three techniques are discussed.

Effects of structural and functional habitat gaps on breeding woodland birds: working harder for less

“…Thus great tits in the park worked harder for less return. We did not measure food abundance directly (the high tree species diversity in parkland makes frass trapping ineffective [Zandt 1994]), but the increase above average DEE in birds whose territories had more than about 35% gap indicated that the availability of trees per se had a direct effect on the costs of rearing young. Work on great tits in the Netherlands (Verhulst and Tinbergen 1997;Tinbergen and Verhulst 2000) has suggested that adults feeding young are constrained by time, rather than by intrinsic (e.g.…”

Spatial variation in haemoglobin concentration of nestling Blue Tits (Cyanistes caeruleus): a long-term perspective

“…The amount of frassfall is directly associated with the ecological production of caterpillars and, therefore, is a reliable indicator of caterpillar numbers (Zandt 1994). The caterpillar frassfall was collected into tissue collectors, 1 m 2 on a metal framework, hanged below tree canopies, five in the parkland and nine in the woodland.…”

Large-scale variation in the temporal patterns of the frass fall of defoliating caterpillars in oak woodlands in Britain: implications for nesting woodland birds

“…Many designs of frass trap have been published (Liebhold & Elkington 1988, Seki & Takano 1998, Tinbergen 1960, Tinbergen & Dietz 1994, Zandt 1994, but in this study we deployed simple, inexpensive traps consisting of a standard plastic seed tray (approximately 35cm × 25cm) lined with permeable horticultural fleece folded and stapled to be a good fit. Each trap was covered in plastic horticultural netting (black 1.5-cm mesh) and pegged to the ground below an oak tree (1-2 m from the trunk).…”