Use of systemic fipronil and imidacloprid to control regeneration pests of loblolly pine

Regeneration pests of loblolly pine (Pinus taeda L.) threaten growth and survival in intensively managed loblolly pine plantations throughout the southeastern United States. The Nantucket pine tip moth, Rhyacionia frustrana (Comstock), in particular, often reduces growth of loblolly pine but has been difficult to control with traditional insecticides due to multiple annual generations and multi-year infestations which are difficult to predict in timing and location. Relatively new systemic insecticide products offer a solution in that their efficacy persists through multiple generations and years after a single application. Efficacy of systemic imidacloprid and fipronil were evaluated side by side across multiple sites in Virginia. Significant reductions in Nantucket pine tip moth damage were noted in trees treated with either the imidacloprid or fipronil product compared with check trees. After 2 yr, growth improvement of treated trees relative to controls was modest and not significant at all sites, but per acre volume indices were significantly greater in treated blocks as a result of higher tree survival. Reduced seedling mortality was attributed primarily to prevention of damage by pales weevil, Hylobius pales Herbst (Coleoptera: Curculionidae), by both insecticide treatments. Control of pales weevil in addition to pine tip moth suggests that systemic insecticide products with a long window of efficacy might control additional nontargeted pests.     

Effects of intensive forest management practices on insect infestation levels and loblolly pine growth

Intensive forest management practices have been shown to increase tree growth and shorten rotation time. However, they may also lead to an increased need for insect pest management because of higher infestation levels and lower action thresholds. To investigate the relationship between intensive management practices and insect infestation, maximum growth potential studies of loblolly pine, Pinus taeda L., were conducted over 4 yr using a hierarchy of cultural treatments. The treatments were herbaceous weed control (H), H + irrigation (I), H + I + fertilizer (F), and H + I + F + pest control (P). These treatments were monitored for differences in growth and insect infestation levels related to the increasing management intensities. The Nantucket pine tip moth, Rhyacionia frustrana (Comstock), was consistently found infesting study trees. In the third field season, the H + I + F + P treatment had significantly more southern pine coneworm, Dioryctria amatella (Hulst), attacks than the H and H + I treatments. There were significant differences in volume index (D2H) among all treatments after each of the four growing seasons. This study indicated that tree fertilization can increase coneworm infestation and demonstrated that tip moth management can improve tree growth initially. Future measurements will determine if the growth gains from tip moth management are transitory or sustainable.     

Effects of fertilization and herbicides on growth of young loblolly pine and infestations of Nantucket pine tip moth（Lepidoptera： Tortricidae）

A 2-year-old pine plantation was selected to receive treatments of fertilizers and herbicides to evaluate effects on Nantucket pine tip moth infestations and the tree growth parameters of height, diameter and volume increment. Nitrogen and phosphorus fertilizers, and hexazinone and sulfometuron methyl herbicides were used in creating six treatments： （i） control; （ii） phosphorus; （iii） nitrogen and phosphorus; （iv） phosphorus and herbicide; （v） nitrogen, phosphorus and herbicide; and （vi） herbicide. Treatments were applied in 1987 and 1988. In 1987, trees treated with nitrogen, phosphorus and herbicide had significantly greater height, diameter and volume growth than trees not receiving fertilizer treatments, but did not have significantly higher tip moth infestations than control trees. Treatments receiving phosphorus only had much lower tip moth infestation rates than other treatments except nitrogen and phosphorus. In 1988, tip moth infestations were uniformly low, with no differences in treatment effects observed.     

Efficacy tests and determination of optimal spray timing values to control nantucket pine tip moth (Lepidoptera: Tortricidae) infestations

The Nantucket pine tip moth, Rhyaciona frustrana (Comstock), a common regeneration pest of loblolly pine, Pinus taeda L., has been shown to reduce tree volume yields through larval feeding. Chemical applications can be effective in protecting trees from the growth losses associated with this feeding, and optimum spray timing values are commonly used to reduce the number of necessary applications and to increase insecticide efficacy. Optimal spray timing values for the Georgia Piedmont were obtained for the following four insecticides available for use in loblolly pine plantations: permethrin (Pounce), lambda-cyhalothrin (Warrior T), spinosad (SpinTor 2 SC), and Bacillus thuringiensis variety kurstaki Berliner (Foray 48B). Optimal timing values were similar between the first and second generations for each of these compounds. All of the insecticides used in this study significantly reduced tip moth damage below the control treatment levels. Lambda-cyhalothrin was the most efficacious and had the longest spray timing window. B. thuringiensis was the least effective and had the shortest timing window. Spinosad and permethrin were similar in efficacy and spray timing values. This information is applicable to regions where there are three tip moth generations per year, as found in the southern Piedmont region and the coastal plain of Virginia and most of North Carolina.     

Importance of resin ducts in reducing ponderosa pine mortality from bark beetle attack

The relative importance of growth and defense to tree mortality during drought and bark beetle attacks is poorly understood. We addressed this issue by comparing growth and defense characteristics between 25 pairs of ponderosa pine (Pinus ponderosa) trees that survived and trees that died from drought-associated bark beetle attacks in forests of northern Arizona, USA. The three major findings of our research were: (1) xylem resin ducts in live trees were >10% larger (diameter), >25% denser (no. of resin ducts mm⁻²), and composed >50% more area per unit ring growth than dead trees; (2) measures of defense, such as resin duct production (no. of resin ducts year⁻¹) and the proportion of xylem ring area to resin ducts, not growth, were the best model parameters of ponderosa pine mortality; and (3) most correlations between annual variation in growth and resin duct characteristics were positive suggesting that conditions conducive to growth also increase resin duct production. Our results suggest that trees that survive drought and subsequent bark beetle attacks invest more carbon in resin defense than trees that die, and that carbon allocation to resin ducts is a more important determinant of tree mortality than allocation to radial growth.     

Evaluation of biological and biorational control tactics for suppression of Nantucket pine tip moth damage in Virginia pine Christmas trees

A study was performed to evaluate the potential of the egg parasitoid Trichogramma exiguum Pinto and Platner (Hymenoptera: Trichogrammatidae), the newly registered insect growth regulator (IGR) tebufenozide (Confirm), and a modified spray technique (top whorl only pesticide application) for suppression of Nantucket pine tip moth, Rhyacionia frustrana (Comstock) (Lepidoptera: Tortricidae), damage in Virginia pine, Pinus virginiana Mill., Christmas trees. Augmentative releases of T. exiguum failed to increase parasitism levels in release plots compared with controls, and significant reduction in tip moth damage did not occur. High predation levels on released T. exiguum may have contributed to the failure of parasitoid augmentations. Whole-tree and top whorl tebufenozide treatments provided significantly greater damage control than corresponding applications of acephate (Orthene), a commonly used pesticide in Christmas trees. Damage to trees receiving whole-tree chemical applications did not differ significantly from trees receiving top whorl treatments for most measurements of damage. Top whorl chemical treatments resulted in a 67% reduction in time required for application and a 70% reduction in pesticide used.     

Radial and stand‐level thinning treatments: 15‐year growth response of legacy ponderosa and Jeffrey pine trees

Restoration efforts to improve vigor of large, old trees and decrease risk to high‐intensity wildland fire and drought‐mediated insect mortality often include reductions in stand density. We examined 15‐year growth response of old ponderosa pine (Pinus ponderosa) and Jeffrey pine (Pinus jeffreyi) trees in northeastern California, U.S.A. to two levels of thinning treatments compared to an untreated (control) area. Density reductions involved radial thinning (thinning 9.1 m around individual trees) and stand thinning. Annual tree growth in the stand thinning increased immediately following treatment and was sustained over the 15 years. In contrast, radial thinning did not increase growth, but slowed decline compared to control trees. Available soil moisture was higher in the stand thinning than the control for 5 years post‐treatment and likely extended seasonal tree growth. Our results show that large, old trees can respond to restoration thinning treatments, but that the level of thinning impacts this response. Stand thinning must be sufficiently intensive to improve old tree growth and health, in part due to increasing available soil moisture. Importantly, focusing stand density reductions around the immediate neighborhood of legacy trees was insufficient to elicit a growth response, calling into question treatments attempting to increase vigor of legacy trees while still maintaining closed canopies in dry, coniferous forest types. Although radial thinning did not affect tree growth rates, this treatment may still achieve other resource objectives not studied here, such as protecting wildlife habitat, reducing the risk of severe fire injury, and decreasing susceptibility to bark beetle attacks.     

Drought stress limits the geographic ranges of two tree species via different physiological mechanisms

Range shifts are among the most ubiquitous ecological responses to anthropogenic climate change and have large consequences for ecosystems. Unfortunately, the ecophysiological forces that constrain range boundaries are poorly understood, making it difficult to mechanistically project range shifts. To explore the physiological mechanisms by which drought stress controls dry range boundaries in trees, we quantified elevational variation in drought tolerance and in drought avoidance‐related functional traits of a widespread gymnosperm (ponderosa pine – Pinus ponderosa) and angiosperm (trembling aspen – Populus tremuloides) tree species in the southwestern USA. Specifically, we quantified tree‐to‐tree variation in growth, water stress (predawn and midday xylem tension), drought avoidance traits (branch conductivity, leaf/needle size, tree height, leaf area‐to‐sapwood area ratio), and drought tolerance traits (xylem resistance to embolism, hydraulic safety margin, wood density) at the range margins and range center of each species. Although water stress increased and growth declined strongly at lower range margins of both species, ponderosa pine and aspen showed contrasting patterns of clinal trait variation. Trembling aspen increased its drought tolerance at its dry range edge by growing stronger but more carbon dense branch and leaf tissues, implying an increased cost of growth at its range boundary. By contrast, ponderosa pine showed little elevational variation in drought‐related traits but avoided drought stress at low elevations by limiting transpiration through stomatal closure, such that its dry range boundary is associated with limited carbon assimilation even in average climatic conditions. Thus, the same climatic factor (drought) may drive range boundaries through different physiological mechanisms – a result that has important implications for process‐based modeling approaches to tree biogeography. Further, we show that comparing intraspecific patterns of trait variation across ranges, something rarely done in a range‐limit context, helps elucidate a mechanistic understanding of range constraints.     

Neighboring trees affect ectomycorrhizal fungal community composition in a woodland-forest ecotone

Ectomycorrhizal fungi (EMF) are frequently species rich and functionally diverse; yet, our knowledge of the environmental factors that influence local EMF diversity and species composition remains poor. In particular, little is known about the influence of neighboring plants on EMF community structure. We tested the hypothesis that the EMF of plants with heterospecific neighbors would differ in species richness and community composition from the EMF of plants with conspecific neighbors. We conducted our study at the ecotone between pinyon (Pinus edulis)-juniper (Juniperus monosperma) woodland and ponderosa pine (Pinus ponderosa) forest in northern Arizona, USA where the dominant trees formed associations with either EMF (P. edulis and P. ponderosa) or arbuscular mycorrhizal fungi (AMF; J. monosperma). We also compared the EMF communities of pinyon and ponderosa pines where their rhizospheres overlapped. The EMF community composition, but not species richness of pinyon pines was significantly influenced by neighboring AM juniper, but not by neighboring EM ponderosa pine. Ponderosa pine EMF communities were different in species composition when growing in association with pinyon pine than when growing in association with a conspecific. The EMF communities of pinyon and ponderosa pines were similar where their rhizospheres overlapped consisting of primarily the same species in similar relative abundance. Our findings suggest that neighboring tree species identity shaped EMF community structure, but that these effects were specific to host-neighbor combinations. The overlap in community composition between pinyon pine and ponderosa pine suggests that these tree species may serve as reservoirs of EMF inoculum for one another.     

Effects of fertilizer and herbicide application on Nantucket pine tip moth infestation (Lep., Tortricidae)

A study of fertilizer and herbicide effects on Nantucket pine tip moth (NPTM), Rhyacionia frustrana (Comstock) infestations was conducted in loblolly pine ( Pinus taeda L.) plantations in Nacogdoches Co., Texas, from 1988 to 1989. Both fertilizer and herbicide applications had effects on NPTM infestation level, pupal weight and host tree oleoresin production. Nitrogen fertilization increased infestation levels; whereas, phosphorus applications tends to decrease infestation rate. Herbicide treatment had a negative effect on NPTM infestations possibly because of decreased moisture stress and increased tree vigour. This is in contrast to general observations of increased NPTM infestations associated with reducing competing vegetation. Fertilizer application alone did not significantly improve pine growth due to competing vegetation. A combination of fertilizer and herbicide achieved the best growth.     

Height-related growth declines in ponderosa pine are not due to carbon limitation

Decreased gas exchange as trees grow tall has been proposed to explain age-related growth declines in trees. We examined changes of mobile carbon stores (starch, sugars and lipids) with tree height in ponderosa pine ( Pinus ponderosa ) at two sites differing in water availability, and tested the following hypotheses: (1) carbon supply does not become increasingly limited as trees grow tall; rather, the concentration of mobile carbon compounds increases with tree height reflecting greater reductions of carbon sink activities relative to carbon assimilation; and (2) increases of stored mobile carbon compounds with tree height are greater in drier sites. Height-related growth reductions were associated with significant increases of non-structural carbohydrates (NSC) and lipid concentrations in all tissues in the upper canopy and of NSC in the bole. Lipid concentrations in the bole decreased with tree height, but such decrease is not necessarily inconsistent with non-limiting carbon supply in tall trees. Furthermore, we found stronger increases of mobile carbon stores with tree height at the dry site relative to the moist site. Our results provide first direct evidence that carbon supply does not limit growth in tall trees and that decreases of water availability might negatively impact growth processes more than net-photosynthesis.     

Effects of the time of drought occurrence within the growing season on growth and survival of Pinus ponderosa seedlings

Key message

A drought event during spring produces a stronger and long lasting decrease in growth of ponderosa pine seedlings than a summer drought event. However, survival is not differentially affected.

Abstract

Although there is certainty about the increasing frequency of extreme climatic events, the consequences of changing patterns of drought events within the growing season on the growth and survival of different species are much less certain. In particular, little knowledge is available on the differential effect on tree seedlings of a drought event at different times within the growing season. The objective of this study was to quantify the effect of a drought event imposed at different times over the growing season on the growth, survival and some related morphological and physiological variables of Pinus ponderosa seedlings from two seed sources. Four treatments were applied: control conditions; spring drought; summer drought and spring plus summer drought (SpSuD). A drought event in spring reduced stem growth and biomass accumulation in ponderosa pine seedlings during the occurrence of the drought and afterwards, even when plant water status had recovered. The lack of growth recovery could not be associated with loss of stem hydraulic conductivity or reduction in stomatal conductance after drought. However, the spring drought did not differentially affect plant survival, as was the case with prolonged drought in the SpSuD treatment. The summer drought event had a significant but much smaller impact on plant growth. Our results suggest different consequences of a drought event in spring or in summer in ponderosa pine seedlings. This knowledge may be relevant to understand and predict tree seedlings responses to changing patterns of drought events within the growing season in the framework of climatic change.     

Drought responses of conifers in ecotone forests of northern Arizona: tree ring growth and leaf δ<Superscript>13</Superscript>C

We sought to understand differences in tree response to meteorological drought among species and soil types at two ecotone forests in northern Arizona, the pinyon-juniper woodland/ponderosa pine ecotone, and the higher elevation, wetter, ponderosa pine/mixed conifer ecotone. We used two approaches that provide different information about drought response: the ratio of standardized radial growth in wet years to dry years (W:D) for the period between years 1950 and 2000 as a measure of growth response to drought, and ¹³C in leaves formed in non-drought (2001) and drought (2002) years as a measure of change in water use efficiency (WUE) in response to drought. W:D and leaf ¹³C response to drought for Pinus edulis and P. ponderosa did not differ for trees growing on coarse-texture soils derived from cinders compared with finer textured soils derived from flow basalts or sedimentary rocks. P. ponderosa growing near its low elevation range limit at the pinyon-juniper woodland/ponderosa pine ecotone had a greater growth response to drought (higher W:D) and a larger increase in WUE in response to drought than co-occurring P. edulis growing near its high elevation range limit. P. flexilis and Pseudotsuga menziesii growing near their low elevation range limit at the ponderosa pine/mixed conifer ecotone had a larger growth response to drought than co-occurring P. ponderosa growing near its high elevation range limit. Increases in WUE in response to drought were similar for all species at the ponderosa pine/mixed conifer ecotone. Low elevation populations of P. ponderosa had greater growth response to drought than high-elevation populations, whereas populations had a similar increase in WUE in response to drought. Our findings of different responses to drought among co-occurring tree species and between low- and high-elevation populations are interpreted in the context of drought impacts on montane coniferous forests of the southwestern USA.     

Cross-validation of Non-linear Growth Functions for Modelling Tree Height–Diameter Relationships

Six non-linear growth functions were fitted to tree height–diameter data of ten conifer species collected in the inland Northwest of the United States. The data sets represented a wide range of tree sizes, especially large-sized trees. According to the model statistics, the six growth functions fitted the data equally well, but resulted in different asymptote estimates. The model prediction performance was evaluated using Monte Carlo cross-validation or data splitting for 25-cm diameter classes. All six growth functions yielded similar mean prediction errors for small- and middle-sized trees. For large-sized trees [e.g. DBH (diameter at breast height)>100 cm], however, five of the six growth functions (except the Gompertz function) overestimated tree heights for western white pine, western larch, Douglas-fir, subalpine fir, and ponderosa pine, but underestimated tree heights for western hemlock and Engelmann spruce. Among these five functions, the Korf/Lundqvist and Exponential functions produced larger overestimations. The Schnute, Weibull, and Richards functions were superior in prediction performance to others. The Gompertz function seemed always to underestimate tree heights for large-sized trees.     

Effects of green tree retention, prescribed burning and soil treatment on pine weevil (Hylobius abietis and Hylobius pinastri) damage to planted Scots pine seedlings

1 Feeding damage and mortality caused to planted Scots pine seedlings by the pine weevils Hylobius abietis and Hylobius pinastri were studied on burned and unburned sites with 0, 10 and 50 m³ per hectare levels of green tree retention from the second to the fourth summer after logging and burning of the sites. 2 The rate of severe feeding damage to pine seedlings caused by pine weevils was higher on burned clearcut sites than on unburned ones, whereas burning did not increase the feeding damage rate on sites with groups of retention trees. The damage rate in the fourth summer was approximately the same on burned and unburned sites. 3 Pine weevil feeding was the major cause of mortality of freshly planted pine seedlings on unburned sites. On burned sites, mortality was higher than the rate of severe feeding damage, particularly in the second summer after burning, possibly owing to fungal attack and abiotic factors. 4 At a retention tree level of 50 m³, feeding damage to the seedlings was lower than on clearcuts and at a 10 m³ retention tree level. Furthermore, on sites with 50 m³ of retention trees, scarification of the soil was found to decrease feeding damage more effectively than on clearcuts and 10 m³ sites. If the seedlings were situated in the centre of scarified patches, scarification alone was as effective as insecticide treatment on unscarified soil for decreasing feeding damage and mortality. 5 The results suggest that when burning is applied as silvicultural treatment after clear‐cuts, retention of trees is recommended to reduce the damages caused by pine weevils on pine seedlings.     

云南切梢小蠹对云南松树的蛀干危害及致死机理

蛀干危害是云南切梢小蠹致死云南松树的关键环节。通过控制云南切梢小蠹蛀干密度,对云南切梢小蠹在自然条件下蛀干行为与危害进行了首次探讨。结果表明,云南切梢小蠹蛀干密度与云南松存活率呈负相关,蛀干密度直接决定云南松死亡或存活。研究发现,蛀干密度115坑/m2是云南松树的最低致死密度阈值,云南松树在蛀干密度低于26.4坑/m2情况下存活,在26.4-115坑/m2有部分存活,超过115坑/m2以后将被害致死。云南切梢小蠹对树干攻击形成有卵和无卵两类坑道。形成无卵坑道的蛀干攻击可导致树势衰弱,形成有卵坑道的蛀干危害严重破坏了韧皮组织,是导致云南松死亡的直接原因。     

Effectiveness of bifenthrin (Onyx) and carbaryl (Sevin SL) for protecting individual, high-value conifers from bark beetle attack (Coleoptera: Curculionidae: Scolytinae) in the Western United States

High-value trees, such as those located in residential, recreational, or administrative sites, are particularly susceptible to bark beetle (Coleoptera: Curculionidae: Scolytinae) attack as a result of increased amounts of stress associated with drought, soil compaction, mechanical injury, or vandalism. Tree losses in these unique environments generally have a substantial impact. The value of these individual trees, cost of removal, and loss of esthetics may justify protection until the main thrust of a bark beetle infestation subsides. This situation emphasizes the need for ensuring that effective insecticides are available for individual tree protection. In this study, we assess the efficacy of bifenthrin (Onyx) and carbaryl (Sevin SL) for protecting: ponderosa pine, Pinus ponderosa Dougl. ex. Laws., from western pine beetle, Dendroctonus brevicomis LeConte, in California; mountain pine beetle, Dendroctonus ponderosae Hopkins in South Dakota; and Ips spp. in Arizona; lodgepole pine, Pinus contorta Dougl. ex Loud., from D. ponderosae in Montana; pinyon, Pinus edulis Engelm. in Colorado and Pinus monophylla Torr. and Frem. in Nevada from pinyon ips, Ips confusus (LeConte); and Engelmann spruce, Picea engelmannii Parry ex. Engelm. from spruce beetle, Dendroctonus rufipennis (Kirby) in Utah. Few trees were attacked by Ips spp. in Arizona and that study was discontinued. Sevin SL (2.0%) was effective for protecting P. ponderosa, P. contorta, and P. monophylla for two field seasons. Estimates of efficacy could not be made during the second field season in P. edulis and P. engelmannii due to insufficient mortality in untreated, baited control trees. Two field seasons of efficacy was demonstrated in P. ponderosa/D. brevicomis and P. monophylla for 0.06% Onyx. We conclude that Onyx is an effective individual tree protection tool, but repeated annual applications may be required in some systems if multiyear control is desired.     

Historical Stem‐Mapped Permanent Plots Increase Precision of Reconstructed Reference Data in Ponderosa Pine Forests of Northern Arizona

Forest structural reference conditions are widely used to understand how ecosystems have been altered and guide restoration and management objectives. We used six stem‐mapped permanent plots established in the early twentieth century to provide precise structural reference conditions for ponderosa pine forests of northern Arizona prior to Euro‐American settlement. Reference conditions for these plots in 1873–1874 included the following historical attributes: tree densities of 45–127 trees/ha, mean tree diameter at breast height (dbh) of 43.8 cm with a corresponding quadratic mean diameter range of 41.5–51.3 cm, and a stand basal area of 9.2–18.0 m²/ha. The reconstructed diameter distributions (for live ponderosa pine trees with dbh ≥9.14 cm) prior to fire exclusion varied in shape but generally displayed an irregular unimodal distribution. We suggest that management objectives for the structural restoration of ponderosa pine forests of northern Arizona emphasize: (1) conservation and retention of all pre‐settlement (>130 years) trees; (2) reduction of tree densities with a restoration objective ranging between 50 and 150 trees/ha having a large‐tree component between 25 and 50% of the total trees per hectare, respectively; (3) manipulation of the diameter distribution to achieve a unimodal or irregular, uneven‐aged shape (possibly targeting a balanced, uneven‐aged shape on cinder soil types) through the use of harvest and thinning practices that mimic gap disturbances (i.e., individual tree selection system); and (4) retention of 3–11 snags and logs per hectare resulting from natural mortality.     

Spatial and temporal factors associated with nest survival of Gray Flycatchers in managed ponderosa pine forests

Gray Flycatchers (Empidonax wrightii) breed in a variety of habitats in the arid and semi‐arid regions of the western United States, but little is known about their breeding biology, especially in the northern portion of their range where they nest in ponderosa pine (Pinus ponderosa) forests. From May to July 2014 and 2015, we conducted surveys for singing male Gray Flycatchers along the eastern slope of the Cascade Range in Washington, U.S.A, monitored flycatcher nests, and quantified nest‐site vegetation. We used a logistic‐exposure model fit within a Bayesian framework to model the daily survival probability of flycatcher nests. During the 2 yr of our study, we monitored 141 nests, with 93% in ponderosa pines. Mean clutch size was 3.6 eggs and the mean number of young fledged per nest was 3.2. Predation accounted for 90% of failed nests. We found a positive association between daily nest survival and both nest height and distance of nest substrates from the nearest tree. Flycatchers that locate their nests higher above the ground and further from adjacent trees may be choosing the safest alternative because higher nests may be less exposed to terrestrial predators and nests in trees that are farther from other trees may be less exposed to arboreal predators such as jays (Corvidae) that may forage in patches with connected canopies. Nests in trees farther from other trees may also allow earlier detection of approaching predators and thus aid in nest defense.     

Relative Importance of Environmental Stress and Herbivory in Reducing Litter Fall in a Semiarid Woodland

We examined the impact of soil stress (low water and nutrient availabilities) and two keystone insect herbivores on pinyon pine (Pinus edulis) needle litterfall. We compared trees growing on two distinct soil types: volcanic cinders, which exhibit pronounced water and nutrient limitation, and sandy-loam soils, which have higher water-storage capacity and nutrient availability. Using two long-term herbivore removal experiments (15 and 18 years, respectively), we also examined the effects of the pinyon needle scale (Matsucoccus acalyptus, which attacks juvenile trees) and the stem-boring moth (Dioryctria albovittella, which attacks mature trees) on pinyon litterfall. These herbivores reach high densities on cinder soils but are absent or occur at much lower levels on sandy-loam soils. Four years of litterfall measurements showed four major patterns. First, independent of herbivory, needle litterfall was 20% lower under trees on high-stress cinder soils than on sandy-loam soils. Second, in agreement with the negative impact of scales on tree growth (that is, a 30% decline in stem growth), trees with scale infestations had 25% lower litterfall rates than trees resistant to scale; however, 15 years of scale-insect removal did not significantly increase needle litterfall. This implies possible intrinsic differences in litter production between scale-resistant and scale-susceptible trees. Third, in contrast with significant negative effects of moth herbivory on tree growth (that is, a 27% decline in stem growth), moth herbivory had no effect on needle litterfall. This, along with increased stem density in moth-susceptible trees, may be evidence of compensatory production. Fourth, there were strong year by soil type and year by scale herbivory interactions, such that in some years the effect on litterfall can be obscured or reversed by some other factor. In summary, soil stress has a strong and predictable effect on needle litterfall, whereas the relationship between insect herbivory and needle litterfall is weaker and depends on the individual herbivore. These effects, however, are mediated by other environmental factors that have considerable annual variation.