Host-Plant Selection by the Hemlock Woolly Adelgid, Adelges tsugae Annand: Sensory Systems and Feeding Behavior in Relation to Physical and Chemical Host-Plant Characteristics

The hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), is an invasive insect causing extensive mortality to hemlocks in the eastern United States. It has become increasingly important to understand the insect-plant interactions of this system for management and eventual hemlock restoration. Insect-plant interactions were observed using light, scanning and transmission electron microscopy. HWA may exploit alternative stylet insertion sites during high infestation (typical insertion site is between the pulvinus and the stem, below the abscission layer). Five dentitions occur at the tip of the mandibular stylets. Stylet bundle cross-section reveal separate salivary (0.24--0.54 mum in diameter, n=11) and food canals (0.48--1.0 mum in diameter, n=11), typical of Hemiptera, and single stylet innervation. Sensilla of the labium appear mechanosensory and an antennal sensorium is present, indicating that morphology and chemical host characteristics may play roles in host-plant acceptance. Tarsal setae are likely used in adhesion to surfaces. Feeding biology was further investigated in an enzymatic survey. We detected the presence of trypsin-like protease, amylase, peroxidase, and polyphenol oxidase. HWA had four times lower protease activity (0.259 BAEE U mug protein-1 min-1) and eight times lower amylase-like activity (0.1088 mU protein-1 min-1) than L. lineolaris, the positive control. The presence of protease and amylase suggests that, if injected into the plant, it could be used for digestion of insoluble plant proteins and starches. HWA had twice as much peroxidase (0.2323 mU mug protein-1 min-1 ) and three times as much polyphenol oxidase activity (0.0569 abs mug protein-1 min-1) compared to L. lineolaris , signifying the possibility of plant defense detoxification.;Hemlocks were scrutinized for differences in morphology and chemical composition of the epicuticle. Comparisons were made between species using low temperature-scanning electron microscopy and related to known HWA resistance. Trichomes likely do not relate to host-plant acceptance by HWA, as species that lack and are covered with trichomes are both colonized. Cuticle thickness is significantly thinner at the stylet insertion point than at other locations of the pulvinus. A thinner cuticle may facilitate tissue penetration, potentially the reason for specificity in insertion site. In addition, cuticle thickness at this point is significantly different between species, with the hybrid, Carolina, southern Japanese, and northern Japanese hemlocks possessing the thickest cuticles; therefore, cuticle thickness may confer some resistance to HWA. Wax structures were generally smoother in eastern and Carolina hemlocks, hypothesized to facilitate sheath adhesion to the plant surface. Because there are many factors that contribute to wax morphology, this research should be pursued further. Extractable lipids of the hemlock surface were analyzed with gas chromatography-mass spectrometry. Results indicate inter- and intraspecific variation. In addition, an unidentified compound appears in the resistant species. This compound should be further scrutinized, both in its identification and its association to HWA behavior.;Interspecific variation in resistance among eastern, Carolina, and western hemlock was assessed in the field. Two of five blocks were artificially infested, two blocks were untreated, and one block was chemically treated. A binary assessment of HWA infestation revealed that eastern hemlock was 36 times more likely and Carolina hemlock was half as likely to become infested as western hemlock. Infestation rates declined significantly in subsequent years as a result of winter temperatures.;Cutting success was also studied at the species level and propagation methods are suggested for hardwood rooted cuttings for all species except western hemlock. Rooted cuttings offer a viable option for clonal propagation, which will be useful in breeding programs for resistance screening. In addition, the establishment of a common-garden is discussed and will be useful for ongoing and future research in the field.