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La Tour du Valat

Institut de recherche pour la conservation des zones humides méditerranéennes

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Titre :	Estimating caterpillar density on trees by collection of frass droppings
Auteurs :	M. Fischbacher, Auteur ; B. Naef-daenzer, Auteur ; L. Naef-daenzer, Auteur
Type de document :	Article
Dans :	Ardea (86(1) 1998)
Article en page(s) :	121-129
Résumé :	Caterpillar frass dropping collections are compared with caterpillar density and biomass as estimated by branch samples tal,en from the same trees. A multiple regression model reveals (1) a linear relationship between mean caterpillar mass and frass dropping rate and (2) a non-linear effect of caterpillar density on frass dropping rates. At low density the probability that frass dropping from the trees:is trapped in the collectors is reduced. The multiple approach explains 72% of the variance in the frass dropping rates. It therefore allows frass dropping rates to be converted into biomass estimates. Including air temperature and duration of sunshine per day does not significantly improve the model. Hence, we conclude that caterpillar size provides a good estimate to correct frass dropping rates for changing weather conditions. Analyses of within-tree and between-trees variances reveal that frass dropping collection allows comparison of individual trees with a resolution of about 1 mg m(-2) h(-1). Since data on caterpillar growth (not abundance) can be obtained from a relatively small sample of branches, the conversion of frass dropping rates provides a relatively cheap and valuable tool to estimate caterpillar abundance in individual trees over large areas, even without using heavy equipment to reach the canopy.

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