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

Suzanne Blatt,Kim Hiltz

doi:10.3390/insects12090769

Abstract

Simple SummaryMany 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.(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.

Highlights

Preference of insects for specific host genotypes has been broadly observed across numerous agricultural crops and insect families [1] and references therein
There are four main results from our study: 1. cultivar preference in H. testudinea is consistent for many cultivars across years, 2. evaluation of cultivar choice based upon observed damage at harvest does not reflect the full impact of Hoplocampa testudinea in apple, 3. choice of cultivar by female H. testudinea is weakly correlated with total damage and 4. fruitlet chemistry weakly correlates with larval success during development
The first observation of H. testudinea damage in these blocks occurred in 2009 and this study has followed the increase in population, and number of cultivars impacted, over the ensuing 10 years

Summary

Introduction

Preference of insects for specific host genotypes has been broadly observed across numerous agricultural crops and insect families [1] and references therein. For pests that occur at times throughout the production cycle when pesticide applications would be detrimental to beneficial predators and pollinators, use of resistant genotypes (cultivars or varieties) offers a control option that is effective and able to conserve naturally occurring ecosystem services. One such pest is the European apple sawfly, Hoplocampa testudinea (Klug) (Hymenoptera: Tenthredinidae), which emerges and oviposits during the bloom period of apple (Malus domestica, Borkh., Rosaceae). Larvae require 1–2 weeks to develop [11,16] and upon hatching burrow into the fruitlet. Adult female H. testudinea emerge from the soil in early to mid-May [14] and mature within 4–10 days [8,11,15,16,18]

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