A New Look at Cultivar Preference in Hoplocampa testudinea (Hymenoptera: Tenthredinidae) on Apple in the Annapolis Valley of Nova Scotia, Canada

SIMPLE SUMMARY: Many insect species show a preference for specific varieties or cultivars within a host plant type, e.g., apple. The European apple sawfly, Hoplocampa testudinea Klug was found to show preference for apple cultivars in Nova Scotia in 2013 and 2014. We hypothesized that this preference could result from either the female selecting specific cultivars for egg deposition or differential survival of the larvae on these cultivars. We studied 15 cultivars over a four-year period (2016–2019) to determine the distribution of egg deposition within the orchard, we bagged fruitlets to closely monitor the damage and impact of H. testudinea during the growing season and evaluated the fruitlets for soluble solids (sugars), acidity and firmness. We determined that female choice in combination with fruitlet chemistry is likely responsible for the cultivar preferences observed. ABSTRACT: (1) Background: The European apple sawfly, Hoplocampa testudinea Klug (Hymenoptera: Tenthredinidae), can be an economically important pest in eastern Canada and shows preference for apple cultivars in Nova Scotia, Canada. We hypothesized that this preference could be due to oviposition by female H. testudinea (preference-performance hypothesis) during the bloom period or differential larval survival during development due to fruitlet physicochemical properties. (2) Methods: Fifteen commercial and experimental apple (Malus domestica Borkh.; Rosaceae) cultivars located at the Kentville Research and Development Centre (Kentville, Nova Scotia) were chosen and examined for H. testudinea oviposition, larval performance during fruitlet development, fruitlet physicochemical properties and damage assessment at harvest from 2016–2019, inclusive. (3) Results: H. testudinea showed significant cultivar preference during oviposition, during development and at harvest, but the ranking of these cultivars was not the same throughout the season. Total impact by H. testudinea was consistent for most cultivars over multiple years of the study. (4) Conclusion: Correlation of oviposition with damage provided weak evidence for the preference-performance hypothesis. We propose that this relationship is weak due to differential survival of larvae during development.