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In the Spotlight ...
In this section of the Vermont Apple Newsletter, a different
arthropod or disease will be featured. This information is part of "Key
Arthropods, Diseases, and Vertebrates affecting Apples: A Synopsis" by
Jessica Reardon and Lorraine Berkett and which appeared in the Back to Basics Resource
Notebook, March 22, 2000.
Leafminers (LM)
Apple Blotch Leafminer, Phyllonorycter crataegella (Clemens)
Spotted Tentiform Leafminer, Phyllonorycter blancardella (Fabricius)
The biology, phenology, and management of both leaf miner species are similar and are
often collectively referred to as leafminers (LM).
Description: Moths are very small, light brown in color, appear shiny in
flight, and have white spots that look like transverse bands when the wings are folded.
The first three instars (sap feeders) are clear or pale yellow, flattened, and possess no
legs. The last two instars, (tissue feeders) are darker yellow, cylindrical in shape, with
thoracic legs and abdominal prolegs. Pupae are brown, elongated and cylindrical, with the
future eyes, antennae, and wings of the adult visible.
Lifecycle: Leafminers overwinter as pupae within leaf mines from the previous
fall. Adults emerge in April to late May, mate, and commence egg laying. In the sap
feeding stages the larvae feed just above the lower leaf surface, and in the tissue
feeding stage, begin feeding just below the upper leaf surface, producing densely spotted
mines. Pupation occurs in June, followed shortly by adult moth emergence. Second
generation mines occur in July, with adult emergence in August. Third generation larvae,
present by mid-August, pupate in the mines and overwinter, emerging the following spring.
Generations may overlap due to the extended period of egg laying and long larval
development.
Damage: Densely spotted mined areas on the leaves. Heavy foliar damage can
affect fruit quality and quantity indirectly, resulting in a decrease in fruit size, early
ripening, premature fruit drop, and reduced fruit set the following season.
Monitoring
- Key times for monitoring: Silver tip - mid August.
- Monitoring method: Sticky red visual traps should be stapled to tree trunks at
silver tip. Place a minimum of 4 traps per 8-acre block. Beginning at petal fall,
check 10 fruit cluster leaves per tree in at least 10 trees throughout the orchard for
signs of mine development. In July, monitor for second generation population levels.
Observe later folding fruit cluster leaves as well as leaves on the basal portion of the
present seasons vegetative shoots. Again, observe 10 leaves per tree in at least 10 trees
though out the orchard. Wing traps baited with pheromone may also be used for monitoring
of LM.
Action Threshold: Action threshold for red visual trap captures are a
cumulative average of 4 LM/trap on McIntosh trees, 8 LM/trap on non-McIntosh trees during
tight cluster. During late pink the action threshold is 9 LM/ trap on McIntosh trees and
21 LM/trap on non-McIntosh trees. The action threshold for leaf mines at petal fall is
7 mines/100 leaves for McIntosh trees and 14 mines/100 leaves for non-McIntosh trees. The
action threshold for second generation monitoring in July is 50 mines/100 leaves for
McIntosh and 100 mines/100 leaves for non-McIntosh.
Cultural: Flail mowing of leaf litter in autumn may reduce
overwintering leaf miner populations.
Biological control: There is a complex of parasitoids which
decrease populations of leafminer. The two most common are Sympiesis marylandensis
and Pholetesor ornigis. Both are wasps whose larvae feed on LM larvae in the
tissue feeding stage. Selecting insecticides which are least toxic to these natural
enemies will enhance biological control.
Chemical: See New England Apple Pest Management Guide
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