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Developing Additive Action Thresholds for Spotted Tentiform Leafminer and European Red Mite 2000

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Project Leaders: J. Nyrop and A. Lakso, NYEAES, Cornell University, Geneva, NY

Abstract:

Apple trees grown in the eastern United States are subject to attack by several insects and mites that feed on foliage. While injury caused by foliage feeding pests may vary due to the time and type of feeding, the principal mechanism of damage appears to be the reduction of photosynthesis by leaves. If individual pests affect the apple tree via reductions in leaf photosynthesis, it is reasonable to hypothesize that the influence of multiple pests is an additive reduction in leaf photosynthesis. Affirming the additive affect of multiple sources of pest injury would bolster current pest management practices. Spotted Tentiform Leafminer (STLM) (Phyllonorychter blancardella) is an important apple insect pest. Larvae burrow beneath the layers of apple leaves creating mines where most of the green tissue has been removed. We conducted experiments to better understand the influence of damage by this insect on apple tree photosynthesis. We found that photosynthesis by apple leaves declined linearly as the leaf area removed by STLM larvae increased. On shoot leaves of 20-25 cm2 leaf area, each mine damaged about 2% of the leaf area. There was no clear effect by the mines on the photosynthetic capacity of the surrounding, healthy leaf tissue, except an apparent reduction in photosynthesis above 15% leaf area damaged.

We observed no effect of up to 3 mines per leaf on whole-canopy photosynthesis. STLM damage on photosynthesis of individual leaves on the same trees showed similar responses to other leaf studies. The lack of effect on the whole tree was likely due to inherent variations (variability in tree size and vigor, a low number of replicates, and spatial variation in pest injury), and the relatively low level of injury ( 3 mines/leaf represented about 8-10% leaf area damage with those leaves) that had no significant effect on the whole tree. The undamaged leaf area in that study showed no photosynthetic compensation in response to the miners.

Comparison of results from the single leaf photosynthesis measurements with comparable measurements taken for leaves damaged by European red mite indicates that each STLM mine causes about a 2.5% reduction in photosynthesis that is equivalent to approximately 125 mite days. Current recommendations are that no than about 500 mite days (or about a 10% reduction in leaf photosynthesis) be allowed on apples trees with a moderate crop load in order to prevent yield or quality reductions in the fruit. This suggests that apple trees can tolerate at least 4 STLM mines per leaf before adverse affects accrue to the fruit. This threshold may be even greater given the results from our whole tree photosynthesis measurements. A threshold of 4 mines per leaf is considerably higher than the current recommended threshold of 2 mines per leaf. Further work is required to verify and extend the results we obtained in this study.

STLM mine injury was simulated with similar areas treated with oil paint, latex paint, punched holes, or finger nail polish. The nail polish treatment was promising with similar effects to STLM's on photosynthetic reduction, although the reduction was somewhat stronger for the same leaf area affected.