Comparison of the severity of selection among beech leaves prior to egg-laying between a leaf-mining and two gall-inducing insects

Abstract

Abstract. Quantitative behavioural traits associated with egg-laying, such as the level of selectivity for host-supports and the size of egg clutches, are generally thought to be of great importance for the subsequent survival and development of offspring. These quantitative traits, however, are often difficult to assess reliably by direct observation in the field. This is particularly the case when the insects are very tiny, which is the case for most galling and leaf mining insects. However, a new approach, the "Melba" procedure, allows the indirect inference of these quantitative traits, using easily recorded field-data only. Application of this diagnostic procedure to a large series of samples of beech leaves (Fagus silvatica), harbouring either a leaf miner, Phyllonorycter maestingella (Lepidoptera: Gracillariidae) or one or the other of two galling insects, Mikiola fagi or Hartigiola annulipes (Diptera: Cecidomyiidae) indicates that the leaf miner differs significantly from the two species of galling insect in term of combined values of hostacceptance ratio and average clutch-size, while the two gall-inducing species remain substantially undistinguishable from each other according to these traits. Thus, the galling insects (i) show stronger selectivity for a host than does the miner at any given average clutch-size and (ii) show larger average clutch-size at any given level of selectivity. That is, for at least these three species, the galling insects show a greater level of selectivity when choosing leaves to oviposit on but, then, tend to lay larger egg-clutches. These differences may be due (i) to the gall-inducing process requiring far more of leaf tissues than being simply palatable, which makes it likely that galling species will be more selective in their choice of leaves than leaf miners and (ii) to the capacity of galls to become nutrient sinks, which may help explain why the galling insects laid larger egg clutches. However, whether these trends can be regarded as general rather specific to this particular case, depends on the outcome of future studies on other groups of insects with similar life histories.

Quantifying the gap between the actual incidence of leaf-mining or leaf-galling insects and the proportion of host-leaves potentially acceptable by them

“…It should be noticed that this diff erence in the average level of exploitation ratio of the acceptable resource adds a new point of distinction between gall-inducers and mineformers operating on a same host-plant species: another strong diff erence between them was already reported, regarding the average degree of mothers' selectivity prior to oviposition, i.e. the higher requirements of gall-inducing species regarding the minimum quality of leaves they consider acceptable to be egg-laid (Béguinot 2010(Béguinot , 2011). …”

“…In short (see Appendix for a brief but more detailed description of the method), this procedure provides an estimation of the leaf-acceptance ratio α (and incidentally of the average number n c of mines/galls issued from the deposit of anyone egg-clutch) making only use, as the data input, of the easily recorded distribution Π(ŋ) of the number ŋ of either mines or galls upon each of the leaves of the sample under study. Th en, seeking for the best adjustment between this recorded distribution and the corresponding theoretical distribution Π(ŋ), explicitly parameterised in α and n c as freely adaptable parameters, will eventually deliver the estimations of α and n c (examples of practical applications are given in Béguinot 2005Béguinot , 2009aBéguinot ,b, 2010Béguinot , 2011.…”

Quantitative analysis of the causes of outbreaks in leaf-mining insects: application to the common leaf-miner Leucoptera malifoliella (Lepidoptera: Lyonetiidae) on apple tree

“…Several types of causal factors govern together the pattern of mines distribution among host-leaves, the resulting level of insect incidence and its variations in space and time. In particular, quantitative aspects of ovipositing mothers' behaviour prior to egg-laying and their local density within the host canopy are explicitly involved in determining the level of incidence and impact of leaf-mining, as detailed below [2][3][4][5].…”

“…In leaf-mining insects, ovipositing mothers actively search, at first, for individuals belonging to the proper host species and then, having reached the foliage of the host, they seek for the most convenient host-leaves to be egg-laid. To this end, mothers actively visit and probe host-leaves, prior to egg-laying, in order to assess their respective level of quality and ultimately select those leaves only that meet their own minimum requirements (the latter intended to mirror, more or less tightly, the future needs of larvae -beyond the prime necessity of developing upon the appropriate host-species [3][4]6]). Schematically, the impact of mines upon host-leaves depends upon:  The average number of mothers probing visits (µ)received per host leaf, directly related to the density of mothers within the host canopy ([*]: see Appendix 1)  The proportion (α)of leaves of the selected host which would be considered acceptable for egg-laying by mothers, i.e.…”

“…To overcome this obstacle, an efficient way is to trace back from the resulting, as-recorded mining-impact pattern to the causal factors 'µ', 'α', 'nc'. This way has now become practicable, using a recently derived procedure [4][5][7][8] which allows accurate estimates of parameters 'µ', 'α', 'nc', retrospectively. Thus, the respective contributions of (i) the mothers' density of visits µ, (ii) the leaf-acceptability ratio α and (iii) the mine clutch-size nc may be adequately disentangled, thereby casting new light upon what actually makes the mining impact of a given insect either high or low, according to sites or years.…”