María J. Lombardero

Assessing the consequences of global change for forest disturbance from herbivores and pathogens.

Herbivores and pathogens impact the species composition, ecosystem function, and socioeconomic value of forests. Herbivores and pathogens are an integral part of forests, but sometimes produce undesirable effects and a degradation of forest resources. In the United States, a few species of forest pests routinely have significant impacts on up to 20 million ha of forest with economic costs that probably exceed

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

1 billion/year. Climatic change could alter patterns of disturbance from herbivores and pathogens through: (1) direct effects on the development and survival of herbivores and pathogens; (2) physiological changes in tree defenses; and (3) indirect effects from changes in the abundance of natural enemies (e.g. parasitoids of insect herbivores), mutualists (e.g. insect vectors of tree pathogens), and competitors. Because of their short life cycles, mobility, reproductive potential, and physiological sensitivity to temperature, even modest climate change will have rapid impacts on the distribution and abundance of many forest insects and pathogens. We identify 32 syndromes of biotic disturbance in North American forests that should be carefully evaluated for their responses to climate change: 15 insect herbivores, browsing mammals; 12 pathogens; 1 plant parasite; and 3 undiagnosed patterns of forest decline. It is probable that climatic effects on some herbivores and pathogens will impact on biodiversity, recreation, property value, forest industry, and even water quality. Some scenarios are beneficial (e.g. decreased snow cover may increase winter mortality of some insect pests), but many are detrimental (e.g. warming tends to accelerate insect development rate and facilitate range expansions of pests and climate change tends to produce a mismatch between mature trees and their environment, which can increase vulnerability to herbivores and pathogens). Changes in forest disturbance can produce feedback to climate through affects on water and carbon flux in forest ecosystems; one alarming scenario is that climate warming may increase insect outbreaks in boreal forests, which would tend to increase forest fires and exacerbate further climate warming by releasing carbon stores from boreal ecosystems. We suggest a list of research priorities that will allow us to refine these risk assessments and adopt forest management strategies that anticipate changes in biotic disturbance regimes and mitigate the ecological, social, and economic risks.

Interactions between fire and bark beetles in an old growth pine forest

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.

Cold Tolerance of Four Species of Bark Beetle (Coleoptera: Scolytidae) in North America

Abstract Management strategies for old growth pine forests have recently begun to include prescribed burns. Fire could influence interactions between bark beetles and mature pine trees, but we cannot predict the effects because we know too little about the numeric and functional responses of bark beetle populations to fire, and because we do not know how fire affects the oleoresin defense system of pine trees. We estimated population abundance of Ips spp. (Coleoptera: Scolytidae), and the resin flow of mature red pines (Pinus resinosa), before and after a prescribed burn, inside and outside the burn, in an old growth forest at Itasca State Park, Minnesota. Following a prescribed burn in April, the local abundance of Ips pini increased by two-fold during May, decreased by a comparable amount during 6 weeks starting in mid-July, and was otherwise unchanged. The abundance of I. grandicollis and I. perroti were unaffected, while that of a specialist predator, Thanasimus dubius (Coleoptera: Cleridae) increased by 30–90% during May. Many mature trees that sustained no visible crown damage from the fire were attacked by Ips within the scorched region of the lower bole. Oleoresin flow increased substantially in trees with scorched boles, which may limit the probability that trees will be killed by bark beetles following a ground fire. We tested whether fire increases the probability that a healthy tree will sustain bark beetle attacks by locating beetle-infested trees inside and outside the burned area, and comparing their growth history (from growth rings) with paired, unattacked trees. Surprisingly, there was no indication of recently declining growth, or chronically slow growth, in beetle-infested trees, either inside or outside the prescribed burn. Half of the trees attacked by Ips in 1998 were dead in 1999 and the remainder were partly girdled by the attacks, which increases their subsequent vulnerability to fires, insects, and pathogens. Ips bark beetles can exert meaningful effects on the survivorship of red pine populations, and their demographic impact is probably increased by ground fires.

Biology, demography and community interactions of Tarsonemus (Acarina: tarsonemidae) mites phoretic on Dendroctonus frontalis (Coleoptera: Scolytidae)

Abstract We investigated the overwintering biology of four temperate-latitude bark beetles: Dendroctonus frontalis Zimmermann, Ips pini (Say), I. grandicollis (Eichhoff), and I. perroti Swaine. All four species were freeze-susceptible. However, there was variation within and among species in overwintering biology that related to their geographic distribution. D. frontalis and southern populations of I. grandicollis continued to reproduce and develop under the bark of their host plants throughout the winter and did not show any seasonal adjustments in their lower lethal temperatures: mean supercooling point ± SD = −12.15 ± 4.02 and −12.25 ± 2.50°C. In contrast, northern populations of I. grandicollis and I. pini employ a behavioral strategy in which adults migrate to the forest soil, where they are insulated from temperature extremes by litter and snow. Furthermore, adult supercooling points of both northern populations declined from about −13°C in summer to about −17°C in early winter. A concomitant decline in lipid content suggests that lipid metabolism may be involved in seasonal adjustments of cold tolerance in I. pini. An assortment of temperature manipulations failed to provide any evidence of cold tolerance acclimation. Immatures, which remain in the inner bark of their host trees, have lower lethal temperatures of −5 to −12°C, and are especially vulnerable to mortality from freezing. I. perroti, a northerly distributed species, had similar cold tolerance and overwinter behavior as northern populations of the other two Ips species. Winter mortality from freezing could be an important determinant of population dynamics in all four species. Understanding variations in cold tolerance and overwinter behavior among insects species may help predict population dynamics and distribution of potential pests.

Relative Suitability of Virginia Pine and Loblolly Pine as Host Species for Dendroctonus frontalis (Coleoptera: Scolytidae)

1 Dendroctonus frontalis, the southern pine beetle, is associated with a diverse community of fungi and mites that are phoretic on the adult beetles. Tarsonemus ips, T. kranzti and T. fusarii (Acarina: Tarsonemidae) may interact within this community in ways that link the population dynamics of D. frontalis, the mites and three dominant species of fungi. We explored species associations by comparing the dietary suitability of different fungi for Tarsonemus spp.

Factors Influencing Bark Beetle Outbreaks After Forest Fires on the Iberian Peninsula

Abstract Dendroctonus frontalis Zimmerman is a major disturbance agent in American pine forests, but attack preferences for various host species, and their relative suitability for reproduction, are poorly known. We studied patterns of beetle attack and reproduction during an infestation of stands containing Virginia pine and loblolly pine. Nearly all Virginia pine were attacked and killed, whereas a third of the loblolly pine escaped attack. Among attacked trees, the density of landings and attacks on Virginia pine was 56–106% higher than on loblolly pine at one site, whereas it was similar between species at another site. Paradoxically, D. frontalis preferred the host that was least suitable for reproduction: mean ± SE = 0.89 ± 0.33 versus 4.65 ± 1.40 progeny/attack in Virginia pine versus loblolly pine. Poor reproduction in Virginia pine was attributable to increased adult mortality, decreased oviposition, and decreased larval survival. Phloem thickness and nitrogen content were similar between the two pine species. Loblolly pine was significantly more suitable for the growth of Ophiostoma minus, a fungal associate of D. frontalis. Resin flow was lower in Virginia pine than in loblolly pine, although oleoresin chemistry may partly explain poor reproduction in Virginia pine. A simulation model predicted that beetle infestations will tend to collapse within stands dominated by Virginia pine, and that increasing availability of loblolly pine will promote infestation growth. Because of beetle preferences, forests that contain even modest proportions of Virginia pine relative to loblolly pine may be less likely to sustain beetle infestations. Management of species composition may provide a means for mitigating the undesirable impacts of this herbivore in pine forests.

Host Use Patterns by the European Woodwasp, Sirex noctilio, in Its Native and Invaded Range

ABSTRACT Fires are among the most globally important disturbances in forest ecosystems. Forest fires can be followed by bark beetle outbreaks. Therefore, the dynamic interactions between bark beetle outbreaks and fire appear to be of general importance in coniferous forests throughout the world. We tested three hypotheses of how forest fires in pine ecosystems (Pinus pinaster Alton and P. radiata D. Don) in Spain could alter the population dynamics of bark beetles and influence the probability of further disturbance from beetle outbreaks: fire could affect the antiherbivore resin defenses of trees, change their nutritional suitability, or affect top-down controls on herbivore populations. P. radiata defenses decreased immediately after fire, but trees with little crown damage soon recovered with defenses higher than before. Fire either reduced or did not affect nutritional quality of phloem and either reduced or had no effect on the abundance, diversity, and relative biomass of natural enemies. After fire, bark beetle abundance increased via rapid aggregation of reproductive adults on scorched trees. However, our results indicate that for populations to increase to an outbreak situation, colonizing beetles must initiate attacks before tree resin defenses recover, host trees must retain enough undamaged phloem to facilitate larval development, and natural enemies should be sufficiently rare to permit high beetle recruitment into the next generation. Coincidence of these circumstances may promote the possibility of beetle populations escaping to outbreak levels.

Climate and the northern distribution limits of Dendroctonus frontalis Zimmermann (Coleoptera : Scolytidae)

Accelerating introductions of forest insects challenge decision-makers who might or might not respond with surveillance programs, quarantines, eradication efforts, or biological control programs. Comparing ecological controls on indigenous vs. introduced populations could inform responses to new introductions. We studied the European woodwasp, Sirex noctilio, which is not a pest in its native forests, is a serious invasive pest in the southern hemisphere, and now has an uncertain future in North America after its introduction there. Indigenous populations of S. noctilio (in Galicia, Spain) resembled those in New York in that S. noctilio were largely restricted to suppressed trees that were also dying for other reasons, and still only some dying trees showed evidence of S. noctilio: 20–40% and 35–51% in Galicia and New York, respectively. In both areas, P. sylvestris (native to Europe) was the species most likely to have attacks in non-suppressed trees. P. resinosa, native to North America, does not appear dangerously susceptible to S. noctilio. P. radiata, which sustains high damage in the southern hemisphere, is apparently not innately susceptible because in Galicia it was less often used by native S. noctilio than either native pine (P. pinaster and P. sylvestris). Silvicultural practices in Galicia that maintain basal area at 25–40 m2/ha limit S. noctilio abundance. More than 25 species of other xylophagous insects feed on pine in Galicia, but co-occurrences with S. noctilio were infrequent, so strong interspecific competition seemed unlikely. Evidently, S. noctilio in northeastern North America will be more similar to indigenous populations in Europe, where it is not a pest, than to introduced populations in the southern hemisphere, where it is. However, S. noctilio populations could behave differently when they reach forests of the southeastern U.S., where tree species, soils, climate, ecology, management, and landscape configurations of pine stands are different.