THE EFFECT OF THE FUNGI CERATOCYSTIS MINOR (HEDGECOCK) HUNT, CERATOCYSTIS MINOR (HEDGECOCK) HUNT VAR. BARRASII TAYLOR, AND SJB 122 ON REPRODUCTION OF THE SOUTHERN PINE BEETLE, DENDROCTONUS FRONTALIS ZIMMERMANN (COLEOPTERA: SCOLYTIDAE)

The southern pine beetle, Dendroctonus frontalis Zimmermann, was studied in the laboratory to determine the influence of associated fungi on its reproduction. First-generation (P) surface-sterilized beetles associated with mycangial fungi (Ceratocystis minor [Hedgecock] Hunt var. barrasii Taylor or SJB 122) constructed more galleries and laid more eggs, at faster rates, than P beetles not associated with these mycangial fungi. No significant differences occurred among non-surface-sterilized P beetles associated with the phoretic blue staining fungus Ceratocystis minor (Hedgecock) Hunt and mycangial fungi or among progeny of P beetles (F1 generation) carrying mycangial fungi. P and F1 surface-sterilized beetles produced more eggs at a greater density than non-surface-sterilized beetles associated with blue stain, but gallery length and the rate of construction were not different. P and F1 surface-sterilized beetles laid more eggs and constructed galleries faster than surface-sterilized beetles that carried no mycangial fungi. The re-emergence rate of beetles was fastest for P beetles associated with C. minor and significantly slower for fungus-free P beetles, P beetles carrying only mycangial fungi, and F1 beetles, respectively. The F1 generation emerged fastest when associated with both mycangial fungi and slowest when associated with SJB 122, and C. minor var. barrasii or no fungus, respectively. This study employed a successful new rearing technique for isolating specific southern pine beetle/fungal associations.

Average radial growth rate and chlamydospore production of Ceratocystis minor, Ceratocystis minor var. barrasii, and SJB 122 in culture

Two symbiotic fungi (SJB 122, an unidentified basidiomycete, and Ceratocystis minor (Hedgecock) Hunt variety barrasii Taylor) and one pathogenic phoretic fungus (C. minor (Hedgecock) Hunt variety minor) of the southern pine beetle, Dendroctonus frontalis Zimmermann, were inoculated onto six different concentrations of D. frontalis frass, loblolly pine (Pinus taeda L.) phloem, and uric acid media to observe radial growth rates and chlamydospore production. The average radial growth rate per day of C. minor var. barrasii increased significantly from the control on all three media, but growth was faster at increased concentrations of added phloem compared with the other supplemented media. Significant increases in chlamydospores produced by C. minor var. barrasii from the control occurred only on frass media, with more chlamydospores being produced at higher concentrations. The average radial growth rate per day of SJB 122 fungus increased significantly from the control on only one concentration of phloem and two concentrations of uric acid, but decreased significantly on low concentrations of frass media. SJB 122 chlamydospore production increased with increasing concentration on frass, was not different from the control on phloem, and increased significantly at intermediate concentrations on uric acid. Ceratocystis minor var. minor average radial growth rate per day increased with increasing concentration on both frass and phloem media but on uric acid decreased significantly at higher concentrations, following an initial signficant increase as compared with the controls.

Response of loblolly pine to different inoculum doses of Ceratocystis minor, a blue-stain fungus associated with Dendroctonus frontalis

Infection and invasion by the southern pine beetle, Dendroctonus frontalis Zimm., and its associated fungi stimulate the inducible defense system of loblolly pine and result in production of a hypersensitive-like lesion around the affected tissue. The length of the lesion stimulated by inoculation is not related to the amount of inoculum introduced into the tree, even with an eightfold difference. The extent of the response is greater with large inoculum doses, but that is likely to be a function of initial inoculation wound size.

Phenolic metabolites of Ceratocystis minor from laboratory cultures and their effects on transpiration in loblolly pine seedlings

Several low molecular weight phenolic metabolites of Ceratocystis minor (Hedgcock) Hunt, a blue-stain fungus of loblolly pine, were isolated from cultures grown on 2.5% malt-extract liquid medium. The most abundant compounds were 6,8-dihydroxy-3-hydroxymethyl isocoumarin and 6,8-dihydroxy-3-methyl isocoumarin. Two unknown phenolic compounds were also isolated. Solutions of these fungal metabolites stimulated transpiration in loblolly pine seedlings when administered through the cut stem. Rates of water loss were doubled within 24 h after treatment with isocoumarin solutions. Administration of raw or sterile fungal culture (in liquid medium) on the other hand produced a small but significant inhibition of water loss shortly after treatment. Reduction in transpiration after treatment with fungal culture probably is the result of partial tracheid blockage by as yet unidentified fungal products. Effects of metabolites of C. minor on the water relations of loblolly pine are discussed in terms of a possible mutualistic association between C. minor and the southern pine beetle, Dendroctonus frontalis Zimmermann.

EFFECTS OF MYCANGIAL FUNGI ON GALLERY CONSTRUCTION AND DISTRIBUTION OF BLUESTAIN IN SOUTHERN PINE BEETLE-INFESTED PINE BOLTS

Southern pine beetles, Dentroctonus frontalis Zimmermann, treated to remove their mycangial fungi, were infected with Ceratocystis minor (Hedgcock) Hunt spores and placed in freshly-cut pine bolts. Galleries constructed by beetles without mycangial fungi were significantly shorter and contained significantly more bluestain than galleries constructed by beetles with mycangial fungi. It was concluded that southern pine beetle mycangial fungi limit the distribution of bluestain in southern pine beetle-infested trees. Inhibition of the spread of the bluestain fungus by mycangial fungi may be necessary for optimal beetle development.

Seasonal response of ponderosa pine to inoculation of the mycangial fungi from the western pine beetle

The hypersensitive response obtained by inoculating the two mycangial fungi of the western pine beetle, Dendroctonus brevicomis LeC, and the blue-stain fungus, Ceratocystis minor Hedge, into ponderosa pine was measured during three different seasons in the same 12-month period. The lesions were produced faster and were significantly longer in the fall than in the summer. Similarly, lesions were longer and were produced faster when inoculations made in the summer were compared with spring inoculations. However, the response to a nonpathogenic fungus, a Penicillium sp., was not different from the response to mycangial fungi, suggesting that lesion production is a generalized response to infection that isolates fungus-colonized tissue from the rest of the tree. Ethanol extracts of hypersensitive-response lesions produced during the spring did not reduce growth of the mycangial fungi or C. minor when bioassayed at concentrations similar to those found in host tissue. This is different from other bark beetle–host tree systems that have been investigated.