Abstract
Different approaches were assessed in the development of a bioassay method that would provide data necessary to evaluate resistance of apple genotypes to the obliquebanded leafroller (OBLR) Choristoneura rosaceana (Harris). A leaf disc method used to evaluate the impact of pesticides on leafrollers resulted in high levels of leaf-abandonment of first and second instar larvae. Inserting leaf petioles into water soaked Oasis foam resulted in a significant reduction in overall leaf weight and diminished the appearance of leaf quality. When leaf petioles were inserted into water-filled Eppendorf vials, leaf weight increased and overall leaf quality appeared to be good. We opted to adopt this latter whole-leaf bioassay method, which though very time consuming, provided a robust assessment of OBLR life history parameters against different apple genotypes. Across all apple genotypes evaluated, male larvae developed faster than females to pupation as well as to adulthood, while females developed faster as pupae and were heavier than male pupae. OBLR larvae reared on leaves from different apple genotypes displayed significant variability in development time, pupal weight, larval survivorship, and number of offspring produced. We utilized three indices to characterize host acceptance, sub-lethal effects, and lethal effects of OBLR to apple genotypes. Our results indicated that ‘Lady’, ‘Viking’, and ‘Northern Spy’ show some of the highest levels of resistance against OBLR, while ‘Granny Smith’ was quite susceptible to OBLR and could be used in future studies identifying apple genotypes with relative resistance to OBLR.
Highlights
The obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris), is a tortricid moth native to and widely distributed in North America [1] [2]
A leaf-disc bioassay has been previously used to evaluate the impact of pesticides against OBLR [21]
In this bioassay leaf discs were removed from foliage treated with pesticides and used to establish concentration-response data to assess resistance of OBLR to pesticides or to follow larval mortality due to the residue decline of a pesticide
Summary
The obliquebanded leafroller (OBLR), Choristoneura rosaceana (Harris), is a tortricid moth native to and widely distributed in North America [1] [2]. OBLR larvae primarily feed on foliage; flower buds in spring and terminal shoot leaves in summer [7] [8] [9]. They can cause significant fruit injury [5] and is potentially one of the most destructive lepidopteran pests of apple in Washington, second only to codling moth, Cydia pomonella (L.) [10]. Very little research has focused on apple resistance against chewing insect pests. More recent studies have shown that there is natural variation in Malus that could be captured in breeding programs against chewing insect pests like leafroller and codling moth. The method maintains high-quality leaves while quantifying parameters of host acceptance (anitxenosis) and specific sub-lethal and lethal (antibiosis) effects through all OBLR life stages
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