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1 Bunch-zone leaf removal reduced infestation by Lobesia botrana, although the mechanism responsible for this effect is unknown. 2 Based on the mortality of eggs and newly-hatched larvae exposed to high temperatures (≥37 ∘ C) in the laboratory, the present study aimed to assess the influence of (i) bunch-zone leaf removal and grapevine-row orientation on berry surface temperature and (ii) bunch exposure to sunlight on egg and larval mortality. 3 Berry temperatures were measured using a noncontact infrared thermometer in two vineyards and showed that, in direct sunlight, the temperatures of berry surfaces were 9 ∘ C or more above air temperature, and the highest mean temperatures occurred on southwest-side bunches followed by west-and south-side bunches. 4 The results of four two-choice field assays, carried out confining fertile females in cages with two bunches, one exposed and one non-exposed to sunlight, showed that: (i) the females did not avoid laying eggs on sun-exposed bunches and (ii) the lowest percentages of both egg-hatching and larval settlement occurred on sun-exposed bunches. 5 The hypothesis that the high temperatures reached by sun-exposed berries cause egg and especially larval mortality is confirmed. 6 Bunch-zone leaf removal combined with optimized grapevine-row orientations can improve L. botrana control.

The hypothesis that bunch-zone leaf removal reduces infestations of the European grapevine moth, Lobesia botrana (Denis & Schiffermüller) (Lepidoptera: Tortricidae), by increasing egg and larval mortality owing to sunlight exposure was evaluated in the laboratory by subjecting different egg stages (white, red-eyes, and black-head) and newly hatched larvae to high temperatures. Based on temperatures recorded in a northern Italian vineyard on sun-exposed berries belonging to south-west facing bunches, eggs were subjected to constant temperatures of 40 °C and 37 °C for one or two periods of 3 or 6 h, and to 24-h temperature cycle with peak of 40 °C. Larvae were exposed to 24-h high-temperature cycles with peaks of 35, 37, and 40 °C. The results showed partial egg mortality at 40 °C, increasing with exposure hours and periods, and as eggs matured. Egg mortality was not affected by exposure to 37 °C. Larval survival already decreased significantly at 37 °C and was even lower at 40 °C. These laboratory data are in agreement with the hypothesis that temperatures reached by berries exposed to sunlight cause egg and larval mortality. Data on egg and larval susceptibility to high temperatures have also implications for species distribution and effects of climate change.

Some studies have highlighted benefits for Lobesia botrana by adding Botrytis cinerea mycelium to an artificial larval diet and have suggested a mutualistic relationship between the two organisms on grapevine, hypothesizing that fungal sterols were the nutritional factor involved. Because the nutritional quality of an artificial diet should be similar to grapes to allow extrapolation of the results to the field conditions, in the current study L. botrana larval performance was compared when larvae were fed on grapes (berries) or two artificial diets either with or without enrichment with B. cinerea. Based on sterol analysis, the two artificial diets had high cholesterol content, but relative to berries showed comparable and low phytosterol contents, respectively (high‐ and low‐phytosterol, HPh, and LPh). While larval fitness on the HPh diet was similar to berries, the LPh diet led to higher mortality and worse larval performance. The addition of the fungus compensated for the shortage in the LPh diet but did not improve the HPh diet. Supplementing the LPh diet with linoleic acid, which is supplied also from B. cinerea, partially improved larval performance. In a field experiment, females did not show any egg‐laying preferences towards naturally botrytized bunches. The positive effect of B. cinerea on the moth's next generation that is reported in the literature could be a consequence of fungus developed inside berry tunnels bored by larvae. Therefore, based on our data and previous reports the existence of a mutualistic relationship between L. botrana and B. cinerea is not well‐founded.