TITLE:
Excised Leaf Bioassay: Phenotypic Expression of Natural Resistance in Apple against the Obliquebanded Leafroller
AUTHORS:
Joseph J. Schwarz, Jay F. Brunner
KEYWORDS:
Choristoneura rosaceana, Malus, Genotype, Lethal Effects, Sub-Lethal Effects
JOURNAL NAME:
Advances in Entomology,
Vol.5 No.1,
December
22,
2016
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.