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

Abstract

Abstract. The impact of phyto-endophagous insects on their host-plants is restricted, at fi rst, by the plants own defences, which more or less limit the proportion of host-leaves potentially acceptable by egg-laying mothers in view of the future successful development of the brood. But, due to the supplementary contributions of top-down and horizontal regulations, the actual incidence of endophagous insects (especially leaf-miners and leaf-gallers) among leaves is likely being still (quite) lower than the proportion of potentially acceptable host-leaves. But, more precisely, how large and how variable is the gap between the proportion of potentially acceptable resource and the fraction actually exploited, i.e. the actual incidence of the insect? While the topic has been treated within a few particular couples herbivore-host, we still lack of a larger overview of the subject, at the scale of a reasonably representative set of members of the guilds of leaf-miners and leaf-gallers. We address the issue, taking advantage of a specifi cally designed procedure that allows the convenient and quick indirect estimate of the proportion of host-leaves potentially acceptable by mothers (a proportion that, otherwise, seems virtually impossible to record directly in the fi eld). Considering a large fi eld survey (115 samples, 22 couples insect/host-plant), the estimated gap between the potentially acceptable resource and the actual incidence reveals on average quite signifi cant, while strongly variable from case to case: indeed, the exploitation ratio of the potentially acceptable resource may, occasionally, be as low as 20% for some gall-inducers and even decrease to less than 10% for some mine-fomers. Accordingly, as expected, the recorded incidence of the insects may not at all serve as a reasonable predicator of the maximum possible incidence that would be authorised by the potential of acceptable host-resource.Résumé. Quantifi er l'écart entre l'incidence réelle des insectes mineurs de feuilles et inducteurs de galles de feuilles et la proportion de feuilles-hôtes potentiellement acceptable par ceuxci. L'impact des insectes endo-phytophages sur leurs plantes-hôtes est en premier lieu restreint par les propres défenses des plantes, lesquelles limitent la part des feuilles potentiellement acceptables par les mères pondeuses, afi n d'assurer le bon développement de leur descendance. En raison des contributions supplémentaires propres aux régulations «top-down» et «horizontale», l'incidence effective des insectes endo-phytophages (en particulier fonceurs de mines et inducteurs de galles foliaires) est ordinairement bien inférieure à la proportion des feuilles potentiellement acceptables. Mais quelle peut-être l'amplitude de l'écart entre incidence effective et ressource potentiellement acceptable? Bien que le sujet ait à l'occasion été traité pour quelques couples herbivore-hôte, on manque encore d'une vision plus globale de la question, à l'échelle d'un large échantillonnage raisonnablement représentatif des guildes d'ins...

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

“…In short [see the references 4-5, 7-8 for full details], the "Melba" procedure relates the hard-to-observe parameters α and nc to the resulting, easily field-recorded distribution Π(ŋ). The procedure is based on the fact that, thanks to commonly accepted assumptions [4][5], the distribution Π(ŋ) should theoretically answer a specific equation with the adaptable parameters such as α and nc. Conversely, the inferred values for α and nc are those that yield the best fit between the recorded distribution Π(ŋ) and the corresponding theoretical distributions Π(ŋ), parameterised in terms of α and nc.…”

“…Conversely, the inferred values for α and nc are those that yield the best fit between the recorded distribution Π(ŋ) and the corresponding theoretical distributions Π(ŋ), parameterised in terms of α and nc. After that, the inferred value of parameter µ is immediately derived from α and nc [4][5]. Note that, here, the recorded data implemented in the "Melba" procedure involves the distribution, Π(ŋ), of mines instead of eggs, as already mentioned above.…”

“…Accordingly, the resulting estimated clutch-size nc represents the "mines clutch-size", i.e. the number of mines resulting from the (more or less complete) hatching of the corresponding eggs-clutch [4][5]. Indeed, in term of mining impact, it is well the resulting "mines clutch-size" (rather than the eggs clutch size) which contributes to rule the mining impact level ([***]: see appendix 1).…”

“…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].…”

“…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.…”