Early-season defoliation of Sorbus aucuparia (L.) and Acer platanoides (L.) can induce defense mechanisms against the spider mite Tetranychus urticae (Koch)

Artificial defoliation is often used in studies of induced defense reactions of plants to damage by arthropods. However, little is known about the role of various external factors that may determine the nature of inducible defenses. Here we tested whether light level under which plants grow could affect the natural invasion of two-spotted spider mite (Tetranychus urticae Koch) with a broad range of host plants. For this study we used two host species that differ widely in shade tolerance: shade-adapted Norway maple (Acer platanoides L.) and sun-adapted European mountain ash (Sorbus aucuparia L.). The purpose of this study was to explore mite invasion and to test whether prior simulated defoliation, light conditions and host species differences in secondary defense metabolites (soluble phenolics) play a role in the pattern of invasion. One-year-old maple and ash seedlings growing in pots were placed into two shade houses that produced a treatment with 5% light transmittance, and the second group of seedlings was placed in full sunlight. The experiment was carried out in a location chronically affected by spider mite presence. The defoliation was performed in mid-May by manual removal of 0 (control), 25, 50, 75 and 100% of leaves. Natural two-spotted spider mite invasion took place in mid-July and was observed only on seedlings growing in the shade. Mites were found on leaves of almost all seedlings. However, the intensity of feeding damage to leaves was related to the level of earlier defoliation. Control and slightly to moderately (25-50%) defoliated seedlings of both species were most affected by spider mites. Mite feeding behavior was directly linked to changes in concentration of leaf carbon and phenolic compounds. Carbon content was positively correlated with the percent of seedlings damaged by spider mites in both tree species (R²> 0.80; P <0.05). The highest concentration of phenolic compounds was observed in leaves of seedlings subjected to 50 and 75% defoliation. Our results indicated that two-spotted spider mites preferred plants grown in a shade environment and within these plants favored leaves which are high in carbon and low in phenolic compounds. Both plant species were able to survive early season leaf damage inflicted by spider mites. Inducible phenolic compounds were among defensive secondary metabolites contributing to low spider mite success in plants recovering from artificial spring defoliation.