https://escholarship.org/content/qt7zs5v0st/qt7zs5v0st.pdf?t=nu227f

A review and synthesis of monoterpene speciation from forests in the United States.

The economic impact of insect damage to trees is so great that research efforts aimed at controlling insect pests are easily justified and research accomplishments are readily measured. As a result of past research on many insect-tree combinations, progress has been made towards the discovery and development of control methods including insecticides, silvicultural techniques, attractants, and the use of parasites, predators, and diseases. Remarkably little use has been made of tree resistance for control of pest populations despite the fact that this is generally regarded as the ideal method ( lO, 66). Although tree losses are being reduced by nongenetic means and will continue to be reduced as methods are refined, insect damage to our forest and ornamental trees remains enormous. Genetic methods have proved successful in combating many crop insects and offer as much or more potential for alleviating damage to trees (77). Development of host resistance as an effective tool for long-term control of insect pests requires a multidisciplinary approach by �eneticists, physiologists, and en­ tomologists. Unfortunately, one or more of the disciplines is usually missing and this has partially contributed to the general lack of progress in this area. This has recently been discussed by Philogene (79) with regard to the need for plant physiolo­ gists to work on host-insect relationships, but it is equ.ally true with respect to forest geneticists. Stark (105) covered several facets of tree resistance to insects in his 1965 review of forest entomology. The book edited by van Emden (25) also contains much information pertinent to the topic. The objective of the following discussion is to critically examine the concept of tree resistance to insects and to review our knowl­ edge of the physiological mechanisms involved in resistance. Hopefully, this may at least stimulate more discussion of the problem and perhaps productive research among the three disciplines.

Physiology of Tree Resistance to Insects

Plant resins pose interesting ecological, taxonomic, physiological, and biochemical problems. This chapter briefly describes the resins in chemical terms and presents their contrast with certain other plant products. Resins are nonvolatile products of plants, from which they exude naturally (surface resins) or can be obtained by incision or infection (internal resins). They are insoluble in water but soluble in organic solvents. Stable, inert, and amorphous, they become sticky when heated and are fusible with no sharp melting points. They are mixtures of compounds, including flavonoids, terpenoids, and fatty substances. Resins are usually produced in specialized surface glands (glandular hairs) or internal ducts. Such ducts are widespread in certain families and occur in both woody and nonwoody plants. They are more common in gymnosperms and dicotyledons than in monocotyledons. The chapter focuses on the external resins that are secreted onto leaf surfaces, but it also provides information on resins, which remain within the plant, and other related plant products.

Plant resins—their formation, secretion and possible functions

Phytophagous insects are natural bioassays that may be used to segregate closely related plant taxa. To examine the value of this method, we conducted a comprehensive survey of insects and arachnids in a natural zone of overlap and hybridization between Fremont (Populus fremontii) and narrowleaf cottonwood (Populus angustifolia). Three categories of trees were distinguished by leaf morphology and identified by previous genetic analyses as Fremont, F1 and BC1 hybrids, and complex backcrosses (BC2–BC4). These trees were subsequently reclassified by a discriminant function analysis of their associated herbivores. The level of concordance between the two methods was 98% (n = 43 trees). Because insect distributions may reflect differences in plant morphology, chemistry, phenology, and other characters, insect bioassays may be a more rigorous method of distinguishing closely related plant taxa than reliance solely on morphological or chemical analyses. In contrast with chemical and genetic analyses, insect surv...

Insects as traits in plant systematics: their use in discriminating between hybrid cottonwoods

Mechanisms of Resistance in Conifers Against Shoot Infesting Insects

Results of experiments in which white-pine weevils, Pissodes strobi (Peck), were caged on cut leaders show that weevils are capable of distinguishing and expressing feeding preferences among three of their host species—eastern white pine, jack pine, and red pine—very similar to patterns observed in the field. There was no difference in amount of feeding on eastern white pine or western white pine, or between two classes based on past weeviling, despite a large difference in susceptibility. In none of the 4 species was amount of feeding correlated with any of the morphological features which were measured at 10 in. from the base of the terminal bud.

Gordon H. Plank

Papers

Evaluating Host Resistance to the White-Pine Weevil, Pissodes strobi, (Coleoptera: Curculionidae) Using Feeding Preference Tests

Monoterpene variations in vapors from white pines and hybrids.