Different egg-laying strategies between a mine-former and two gall-inducers insects upon a same host Herbivores do not randomly eat the green world but can make sophisticated choices, avoiding many potential foods and selectively seeking others (...) In general, females herbivores seem more adept at making appropriate intraspecific choices among host plants than they do at choosing among host species ' (Karban & Agrawal, 2002). Indeed most herbivores, and especially insects, feature more or less highly host-specific. This holds true not only in term of restricted fraction of plant species that any plant-feeder may accept, but still further in term of a more or less restricted ratio of acceptance of host-parts belonging to a given host-plant species. Thus for example, according to their different 'quality', all the leaves on a same branch may attract differently the feeder or ovipositing mother, which, in turn, will have to make a selective choice among immediately available leaves. This would arguably stand further true for those insects for which the larval stage is concealed, as it is the case for mine-formers and gall-inducers. This choice may be that of the feeder itself (often the larval stage of the insect). It may be also the choice of mother on behalf of offspring, at the time of egg-laying, when as usual, mother care for its progeny. For gall-inducers specifically, the selection regarding host-part quality should involve not only palatability but even further the aptitude of the plant-support to undergo satisfying cecidogenous development answering the induction by mother and/or larva. It would then seems likely that gall-inducers might be more demanding upon host-part quality than would be free feeders or even mine-formers, although the latter have also their larval stage concealed. Now, testing this hypothesis would obviously make sense only when comparison between gallers and miners is applied to insects exploiting the same type of part of a same host-plant species. Here, we test for possible difference of acceptance ratio among leaves of beech (Fagus silvatica) by one mining species (Phyllonorycter maestingella) and two gall-inducing species (Hartigiola annulipes and Mikiola fagi). Sampling includes a few hundreds leaves issued from a same branch ; the number of samples were 21, 14 et 8 for each of three species respectively. The leaf acceptance ratio a (a way to measure the level of leaf-selectivity by ovipositing mothers), although practically out of direct observation, may be assessed indirectly, using the newly developed procedure 'melba' (Béguinot 2005, 2009a, 2009b, 2009c). Besides leaf-selectivity, an other quantitative parameter of oviposition strategy was assessed : the average density 8 of oviposition (density of mines/galls) per acceptable leaf. We report, here, very contrasted oviposition strategies between the miner and the two gallers, in terms of both parameters α and δ : see figure 1 . The mine-former tends towards high acceptance ratio a for leaves (low selectivity for leaf quality) and, besides, have lower density δ of mines per acceptable-leaf (at any given overall density) ; consequently the mine-former shows a more homogeneously scattered distribution of offspring locations. This strategy : -might be both detrimental and beneficial to offspring fitness (respectively owing to lower food quality linked to decreased leaf selectivity and possibly decreased parasitism efficiency resulting from the more scattered distribution) ; -might be beneficial to the total number of eggs laid by a same female, due to less time devoted to seeking for best leaves, if mothers are time-limited regarding oviposition. The gall-inducers are significantly more selective for leaf quality (lower acceptance ratio a) and, also, have higher gall density ô per acceptable-leaf (at any given overall density) ; consequently the gall-inducers have somewhat more aggregated distribution of offspring locations than the mining species. This strategy : -symmetrically reciprocal of that of the mine-former, should consequently have advantages and disadvantages in a figure opposite to that suggested above for the mine-former, -should be arguably beneficial to efficient gall-development, which likely requires more narrow and stringent requirements regarding leaf characteristics, as already suggested. Although both strategies may have advantages and disadvantages for both mine-formers and gall-inducers, in a balance not easy to decide, the choice of gallers for greater leaf-selectivity might well simply result from this last argument.
Le choix des sites de pontes est particulièrement important chez les insectes à développement larvaire sessile, comme le sont notamment les fonceurs de mines et les inducteurs de galles. Au-delà de la préférence voire de l'exclusivité au niveau de l'identité taxonomique de l'hôte végétal, la sévérité dans le choix des sites de ponte peut, chez ces catégories d'insectes, aller plus loin encore vers un choix sélectif au sein même de l'hôte végétal. Le degré sélectivité ultime peut alors s'exprimer quantitativement par le ratio d'acceptabilité 'a' parmi les unités-hôtes de même nature qui s'offrent aux mères pondeuses (par exemple les feuilles d'une même plante ou d'une même branche d'un même arbre). Cette possibilité de quantifier le degré de sélectivité ouvre la possibilité de comparaisons fines entre espèces d'insectes, en relation notamment avec le type d'habitat des larves, ici mines ou galles. On a en effet quelques raisons de penser, a priori, que la cécidogénèse pourrait requérir plus de qualités particulières de la part du support de ponte que la simple exploitation par minage des tissus végétaux. On a testé cette hypothèse en comparant les ratios d'acceptabilité des feuilles de Hêtre pour un insecte fonçeur de mine (Phyllonorycter maestingella) et deux espèces inductrices de galles (Hartigiola annulipes et Mikiola fagi) exploitant ce même type de support de ponte. L'hypothèse avancée s'est trouvée confirmée pour ce système particulier, sans qu'on soit encore en mesure, pour le moment, d'évaluer le degré de généralité de cet intéressant résultat. On manque en effet d'indications quantifiées dans ce domaine, en raison de la difficulté d'accéder à une estimation directe du ratio d'acceptabilité des supports de ponte par les mères, sauf à recourir à une approche indirecte comme celle nouvellement exploitée ici.
