Scolytid bark beetles that colonize living conifers are frequently associated with specific fungi that are carried in specialized structures or on the body surface. These fungi are introduced into the tree during the attack process. The continuing association suggests that there is mutual benefit to the fitness of both beetles and fungi. The fungal species may benefit from the association with the beetles by transport to new host trees. Beetle species may benefit from the association with fungi by feeding on the fungi, or by the fungi contributing to the death of the host trees through mycelial penetration of host tissue, toxin release, interactions with preformed and induced conifer defenses, or the combined action of both beetles and fungi during colonization. Extensive research has been directed towards characterizing the interactions of beetle-fungal complexes with live host conifers and determining the ecological advantages for maintaining the associations. However, differences among systems and how species interact under different population and environmental conditions make it difficult to generalize about the importance of the separate biological components in successful host colonization.

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Interactions among scolytid bark beetles, their associated fungi, and live host conifers

The present review is devoted to the exocrine glands derived from epidermis; glands associated with the preoral cavity (mandibular, salivary, etc) and genital apparatus are not dealt with except for the sake of comparison. As defined, the epidermal glands manifest an exceptional diversity as far as location, morphology, and func­ tion are concerned. In the past, the studies were mainly anatomical, very rarely cytological, but the interest in insect glands was recently renewed by the chemical approach and the recognized importance of the secretions as far as behavior and physiology are concerned. Concurrently, the high resolution of the transmission electron microscope permitted a far more precise elucidation of the structures, with the hope of establishing some correlations between these structures and the func­ tions. Additionally, the scanning electron microscope appeared well suited for the examination of some cuticular differentiations.

Fine structure of insect epidermal glands

Insect-fungus relationships in the bark and ambrosia beetles.

The nitrogen content of plant tissue is low relative to that of herbivores; as a consequence, dietary N can limit the growth and reproduction of herbivores and select for attributes that increase N acquisition. Bark beetles face a particularly severe challenge because the phloem that they consume is very low in nitrogen and phosphorus relative to their requirements. We quantified variation in the phloem concentrations of N and P in the host tree, Pinus taeda, and evaluated the following hypotheses regarding the role of sym- biotic fungi in nutrient budgets of the herbivore Dendroctonus frontalis: D. frontalis ex- perience variation in phloem nutrient concentrations across several spatial scales (HI); mycangial fungi enhance the diet of D. frontalis larvae by contributing to the acquisition of N and P (H2); Ophiostoma minus, an apparently antagonistic fungal symbiont, hinders D. frontalis larvae because it does not enhance nutrient concentrations of the phloem as much as mycangial fungi do (H3); and larvae of bark beetle species that lack mycangial fungi must consume more phloem to accomplish the same growth as larvae of D. frontalis (H4). In addition, we developed a general model for the N budgets of herbivorous insects that identifies the possible combinations of dietary and physiological parameters that can allow developmental success on low-nutrient diets. Spatial variation in phloem N was mostly at the level of trees within sites (a scale of meters) while P mostly varied among sites (a scale of kilometers). Trees with higher N content produced larger D. frontalis adults. Prior to infestation by beetles, phloem nutrient concentrations were very uniform within trees and very low relative to that of the bark beetles (N and P concentrations of D. frontalis adults were 28 and 8 times greater, re- spectively). During infestation, phloem nutrient concentrations increased overall and be- came highly variable within trees. Nitrogen concentrations increased from 0.40 + 0.01% (mean + 1 SE) in uninfested phloem to 0.86 + 0.03% in the phloem surrounding successfully developing D. frontalis larvae, which are typically associated with one or two species of mutualistic mycangial fungi. Nitrogen concentrations were intermediate in other micro- habitats within infested trees, including regions with no adult colonization, with failed larval development, or colonized by the antagonistic bluestain fungus 0. minus. We pa- rameterized a general nutrient-budget model for D. frontalis and a sympatric non-mycangial bark beetle, Ips grandicollis, which indicated that (1) mycangial fungi provide their benefits by concentrating dietary N for larvae; (2) 0. minus may exert its antagonistic effects on D. frontalis larvae by failing to concentrate dietary N as much as mycangial fungi do; (3) non-mycangial bark beetles meet their N budgets through high consumption of unaltered, low-N phloem; and (4) larvae should easily meet their P requirements with any combination of consumption rate and development time that allows them to meet their N requirements. Alternative strategies for N acquisition may have general consequences for the population dynamics and community interactions of bark beetles.

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Nitrogen budgets of phloem-feeding bark beetles with and without symbiotic fungi

Bark Beetle?fungus Symbioses

The ultrastructure of the prothoracic mycangium of female Dendroctonus frontalis is examined. The mycangium consists of a cuticular invagination within which symbiotic fungi are cultured by the pine beetle and transported to new host trees. Secretions from two types of gland cells pass into the mycangial lumen. The plasma membrane of type-1 cells is invaginated to form an enclosed extracellular cavity. The secretory product passes into the cavity, then through fine cylindrical channels into an end apparatus and finally via an efferent cuticular ductule to the lumen of the mycangium. Secretion of the type-2 cells is released into a cavity just beneath the mycangial cuticle. The cuticle over this cavity is quite thin (1-2μ), consisting mostly of inner epicuticle riddled with irregular canals through which the secretion reaches the lumen. Beneath the patches of porous cuticle are ribs (up to 1Oμ in thickness) which flank the cavities and presumably provide structural support for the porous secretory zones.

Fine structure of the prothoracic mycangium, a chamber for the culture of symbiotic fungi, in the southern pine beetle, Dendroctonus frontalis.

The defensive glands of Anisomorpha buprestoides produce the terpene toxicant anisomorphal. Each gland consists of a cuticular secretion reservoir surrounded by the secretory epithelium and the musculature which serves to compress the gland and expel the secretion. Two types of cells make up the secretory epithelium: a squamous layer next to the cuticular reservoir and a layer of larger secretory cells responsible for production of the toxicant. The microvilli-laden plasma membrane of each secretory cell is invaginated to form a central cavity. It appears that secretory products pass into the central cavity and then flow out to the gland reservoir via an efferent cuticular ductule contained within the squamous epithelial cell.



Histochemical techniques demonstrate lipid reserves, carboxylic esterases, a variety of phosphatases, and an alcohol dehydrogenase, within the secretory cells. It is suggested that the lipid reserves are precursors of the terpenoid toxicant, that a mevalonic kinase has been histochemically demonstrated by the phosphatase test, and that an unusual alcohol dehydrogenase is active in the final steps of toxicant synthesis. The histochemical evidence is consistent with the hypothesis that anisomorphal is produced via the mevalonic acid pathway.

The terpene-producing glands of a Phasmid insect. Cell morphology and histochemistry.

The spermathecal accessory gland of female Tenebrio molitor is examined by histochemicai and electron microscopical techniques. Immediately after ecdysis of the female, neither Golgi regions nor the endoplasmic reticulum of the secretory cells are well developed. In two days' time, the cytoplasm is rich in rough endoplasmic reticulum and the Golgi areas are expanded. Membrane-bound droplets of secretion move from the Golgi zone to a central cavity, formed by the invaginated plasma membrane of this cell. As the secretion accumulates this cavity swells until the fourth day after ecdysis when the females first mate. An efferent cuticular ductule, ensheathed in a ductulecarrying cell, carries the product to the main axial duct of the tubular gland. By histochemical criteria, the product is a glycoprotein.

Fine structure and histochemistry of the spermathecal gland in the mealworm beetle, Tenebrio Molitor

A basidiomycetous yeast is a dimorphic fungal ectosymbiont associated with the southern pine beetle, Dendroctonus frontalis. Within the mycangium (a glandular integumental invagination) of the southern pine beetle, the fungus grows in a yeast-like manner, while on some media or in a plant host, it forms mycelial masses. The yeast stage has a lamellate cell wall and forms new cross walls in a manner rather similar to the basidiomycetous yeasts. Like certain ambrosial fungi (Ascoidea), the yeast stage is multinucleate. The mycelial stage, initially produced in culture and within the plant host, shows clamp connections and dolipore septa, characteristic of many Basidiomycetes, but its cell wall is not lamellate.The dolipore septa have septal pore caps and also mirror images of these caps. The septal pore caps (about 400 A in thickness) have five layers: the two outermost (40 A) are probably continuous with the 'unit membranes' of the endoplasmic reticulum; the middle layer is similar to the outermost; and th...

Bark beetle – fungal symbiosis. II. Fine structure of a basidiomycetous ectosymbiont of the southern pine beetle

SJB 133 is an isolate of a variety of Ceratocystis minor that is found in a Sporothrix imperfect state as an ectosymbiont of Dendroctonus frontalis Zimm. Within the mycangium (fungal transport pocket of the beetle), SJB 133 grows in yeast-like fashion. Cells contain prominent vacuoles which appear to bridge between dividing cells. Hypha-like transition cells are also present. In continuous culture on potato carrot agar, SJB 133 produces sympodial conidiophores. The resulting conidia are quite similar to the yeast-like cells in the mycangium and the conidiophores resemble some of the transition cells.

Christine M. Happ

Papers

Fine structure of the prothoracic mycangium, a chamber for the culture of symbiotic fungi, in the southern pine beetle, Dendroctonus frontalis.

The terpene-producing glands of a Phasmid insect. Cell morphology and histochemistry.

Fine structure and histochemistry of the spermathecal gland in the mealworm beetle, Tenebrio Molitor

Bark beetle – fungal symbiosis. II. Fine structure of a basidiomycetous ectosymbiont of the southern pine beetle

Bark beetle – fungal symbiosis. III. Ultrastructure of conidiogenesis in a Sporothrix ectosymbiont of the southern pine beetle