Population density and area: the role of between- and within-patch processes

The relationship between population density and the size of host plant patches was investigated for the red milkweed beetle Tetraopestetraophthalmus inhabiting unmanipulated patches of Asclepias syriaca. The resource concentration hypothesis proposes that density-area patterns, specifically that of increasing herbivore density with patch size, are primarily a function of movement between host plant patches. This research investigated the degree to which movement accounted for density-area patterns. Poisson regression analysis of beetle abundance versus milkweed patch size revealed that beetle density tended to increase with patch size. The pattern of density and patch size resulted from local reproduction and residence time. The density of emerging beetles tended to increase with patch size while emigration rates were unrelated to patch size. Immigration rates were constant with patch size for male beetles, and decreased with patch size for female beetles. Net flux of beetles (immigration – emigration) did not vary with patch size for male beetles and decreased with patch size for female beetles. Comparisons are made between this system and previously studied systems where movement plays a significant role in forming density area patterns. Additionally, several hypotheses are presented which may account for greater in situ recruitment and residence time in large patches. Received: 23 February 1996 / Accepted: 8 January 1997     

The exploitation of an ant-defended host plant by a shelter-building herbivore

Larvae of a Polyhymno species (Lepidoptera: Gelechiidae) feed on the ant-defended acacia, Acacia cornigera, in the tropical lowlands of Veracruz, Mexico. Polyhymno larvae construct sealed shelters by silking together the pinna or pinnules of acacia leaves. Although larval density and larval survival are higher on acacias not occupied by ants, shelters serve as a partial refuge from the ant Pseudomyrmex ferruginea (Hymenoptera: Formicidae), which defends A. cornigera plants; thus, shelters provide Polyhymno larvae access to an ant-defended host plant. P. ferruginea ants act as the primary antiherbivore defense of A. cornigera plants, which lack the chemical and mechanical defenses of non-ant-defended acacias. Thus, defeating the ant defense of A. cornigera provides Polyhymno larvae access to an otherwise poorly defended host plant. Damage caused by Polyhymno larval feeding reaches levels which can kill A. cornigera plants. Received: 6 June 1996 / Accepted: 6 September 1996     

Acorn selection by Mexican jays: a test of a tri-trophic symbiotic relationship hypothesis

By caching acorns, jays serve as important dispersal agents for oak (Quercus) species. Yet little is known about which acorn characteristics affect selection by jays. In the traditional model of jay/oak symbiosis, large, brown, ripe acorns free of invertebrate parasites (e.g., Curculio acorn weevils) are selected by jays. Recently, it has been suggested that a tri-trophic relationship between oaks, jays, and weevils may have evolved to counter the negative dietary effects of acorn tannins. Under the tri-trophic model, jays would preferentially select acorns containing weevil larvae. We tested the assumptions that (1) acorns containing curculionid larvae exist in sufficient quantities to support jay populations and (2) jays can detect, and preferentially select, acorns containing weevil larvae, and investigated the cues by which jays select acorns. Captive Mexican jays (Aphelocomaultramarina) were presented Emory oak (Quercusemoryi) acorns in aviary feeding trials. Large, dense, viable acorns free of curculionid larvae were preferentially selected. Contrary to results of previous research, color did not affect selection. Acorn viability increased and curculionid larval occupancy decreased in adjacent savannas and isolated stands relative to existing oak woodland, perhaps favoring oak recruitment into adjacent lower-elevation grasslands. Our results compel us to reject the tri-trophic model for this system, and are consistent with the traditional jay/oak symbiosis model. Relatively long-distance dispersal of viable acorns favors Emory oak replacement, and spatial patterns of acorn viability and curculionid parasitism suggest expansion of Emory oak into adjacent low-elevation semi-arid grasslands. Received: 29 February 1996 / Accepted: 26 September 1996     

How symbiotic bacteria influence plant utilisation by the polyphagous aphid, Aphis fabae

To explore the effect of rearing-plant species on the contribution of the symbiotic bacterium, Buchnera, to aphid performance, larvae of Aphis fabae that contained the bacteria (symbiotic aphids) and larvae experimentally deprived of the bacteria (aposymbiotic aphids) were reared on 16 plant species. Mortality of aphids was low on most plant species. The relative growth rate (RGR) of the larvae varied with plant species, and was generally depressed by elimination of the bacteria; the mean values of RGR varied between 0 and 0.29 μg μg⁻¹ day⁻¹ for symbiotic aphids and 0 and 0.17 μg μg⁻¹ day⁻¹ for aposymbiotic aphids. The extent to which RGR was depressed by aposymbiosis varied significantly between plant species, suggesting that aphid host plant may influence the contribution of the bacteria to plant utilisation. It is proposed that the bacteria may be particularly important on plants with phloem sap of high amino acid content of low quality, i.e. low concentrations of essential amino acids. Received: 18 August 1996 / Accepted: 13 January 1997     

Effects of changes in atmospheric carbon dioxide on the location of hosts by the moth, Cactoblastis cactorum

Sensory organs that detect CO₂ are common in herbivorous moths and butterflies, but their function has been unclear until now. As the CO₂ gradients in the vicinity of a host plant depend on its physiological condition, CO₂ could provide a sensory cue for the suitability of the plant as a larval food source. This study investigated whether changing the atmospheric CO₂ concentration affected oviposition by Cactoblastis cactorum on its host, the cactus Opuntia stricta. On host plants exposed to rapid fluctuations in CO₂ concentration, the frequency of oviposition was reduced by a factor of 3.2 compared to the control. As the fluctuations mask the much smaller CO₂ signals generated by the plants, this suggests that those signals constitute an important component of the host identification process. On host plants exposed to a constant background of doubled CO₂, oviposition was also reduced, by a factor of 1.8. An increased background reduces host signal detectability, partially as a consequence of a general principle of sensory physiology (Weber-Fechner's law), and partially due to other factors specific to CO₂-receptor neurons. Received: 4 October 1996 / Accepted: 16 January 1997     

Individual variation in two host plants of the ladybird beetle,Epilachna pustulosa (Coleoptera: Coccinellidae)

Individual variation in two species of host plants (thistle,Cirsium kamtschaticum, and blue cohosh,Caulophyllum robustum) of the herbivorous ladybird beetleEpilachna pustulosa was examined under laboratory conditions for their acceptability to adult beetles as a food resource, for adult preference and for larval performance. When clones of these plants were subjected to non-choice tests using posthibernating female beetles, there was found to be significant intraspecific variation among clones in terms of their acceptability, but interspecific variation was not detected. Significant intraspecific as well as interspecific variation were frequently detected in the two host plants when clones of these plants were subjected to choice tests using posthibernating female beetles; the magnitude of interspecific plant variation for beetle preference is not necessarily larger than that of intraspecific plant variation. Individual variation across plant species with respect to beetle larval performance was also significant. A positive correlation between adult preference and larval performance is suggested across the two taxonomically remote host plant species, thistle and blue cohosh, although this needs further investigation.     

Effects of plant hybridization on herbivore-parasitoid interactions

We studied the effects of host plant hybridization on the survival and mortality of the leaf-mining moth Phyllonorycter salicifoliella on hybrid and parental willow plants in the field and in a common garden experiment. P. salicifoliella survival differed significantly among three willow taxa in the field in 1994 but not in the field in 1995 or in the common garden. Parasitism by eulophid wasps differed significantly among taxa in 1994 and appeared to account for the variation in their survival. In the field in 1995, host feeding predation varied significant among taxa. The theory of tritrophic interactions predicts that plant genotype can affect natural enemy impact, and this study supports this prediction. Significant variation in survival and eulophid parasitism was also found among genotypes within taxa in the field in both years and in the common garden experiment. The common garden results show that genetic differences in plants affect the herbivore-parasitoid interaction. Variation among years in the patterns of survival and causes of mortality among field plants suggest that genotype by environment interactions may be important. Received: 1 March 1996 / Accepted: 4 November 1996     

Shade-induced changes in the branching pattern of a stoloniferous herb: functional response or allometric effect?

Shade-induced changes in the branching pattern of clonal plants can lead to conspicuous modifications of their growth form and architecture. It has been hypothesized that reduced branching in shade may be an adaptive trait, enabling clonal plants to escape from unfavourable patches in a heterogeneous environment by allocating resources preferentially to the growth of the main axis (i.e. linear expansion), rather than to local proliferation by branching. However, such an adaptionist interpretation may be unjustified if (1) branching frequency is a function of the ontogenetic stage of plants, and if (2) shading slows down the ontogenetic development of plants, thereby delaying branch formation. In this case, architectural differences between sun- and shade-grown individuals, harvested at the same chronological age, may not represent a functional response to changes in light conditions, but may be a by-product of effects of shade on the rate of plant development. To distinguish between these two alternatives, individuals of the stoloniferous herb Potentilla reptans were subjected to three experimental light conditions: a control treatment providing full daylight, and two shade treatments: neutral shade (13% of ambient PPFD; no changes in light spectral composition) and simulated canopy shade (13% PPFD and a reduced red:far-red ratio). Plant development was followed throughout the experiment by daily monitoring primary stolon growth as well as branch and leaf initiation. Biomass and clonal offspring production were measured when plants were harvested. At the end of the experiment shaded plants had produced significantly fewer branches than clones grown in full daylight. In all three treatments, however, initiation of secondary stolons occurred at the same developmental stage of individual ramets. Shading significantly slowed down the ontogenetic development of plants and this resulted in the observed differences in branching patterns between sun- and shade-grown individuals, when compared at the same chronological age. These results hence provide evidence that shade-induced changes in the branching pattern of clonal plants can be due to purely allometric effects. Implications for interpreting architectural changes in terms of functional shade-avoidance responses are discussed. Received: 16 August 1996 / Accepted: December 1996     

Non-consumptive effects of larval Salamandra on crustacean prey: can eggs detect predators?

Predators affect prey populations not only by prey consumption but also in nonconsumptive ways including modifying prey behavior. I tested the effects of fire salamander larvae (Salamandra infraimmaculata) on populations of co-occurring crustacean species in artificial outdoor pools. I also tested whether these effects were due entirely to prey consumption by Salamandra larvae or alternatively to some nonconsumptive effect. The soil (containing crustacean eggs) added to the artificial pools was collected from a dried-out temporary pool that is inhabited by Salamandra during the early part of the hydroperiod. I randomly assigned the pools to one of three treatments: control, free Salamandra, or caged Salamandra. Free salamander larvae could roam the entire pool and prey upon crustaceans. Caged salamander larvae were placed within a cage with having 250-μm mesh windows. They could not prey upon the crustaceans but could, for example, influence them by chemical cues. Densities of the three dominant crustacean species (Arctodiaptomus similis, Ceriodaphnia quadrangula and Cyzicus sp.) were drastically reduced in both salamander treatments compared to the control. Crustacean densities, however, were not significantly different between the two salamander treatments. One plausible explanation is that crustacean eggs can detect the presence of this predator via chemical cues and delay hatching. Received: 4 March 1996 / Accepted, 23 October 1996     

Maintenance of the supercooled state in overwintering pyrochroid beetle larvae, Dendroides canadensis : role of hemolymph ice nucleators and antifreeze proteins

Freeze-avoiding fire-colored beetle larvae, Dendroides canadensis, were monitored seasonally to explore the role of endogenous hemolymph ice nucleators and antifreeze proteins on the maintenance of supercooling. In preparation for overwintering, D. canadensis depressed hemolymph ice nucleator activity and increased thermal hysteresis activity [mean value circa 0. 5 °C (summer) versus circa 5 °C (midwinter)] resulting in decreased larval and hemolymph supercooling points [−7 °C (summer) versus −20 °C (midwinter)]. Results of gel filtration chromatography, flotation ultracentifugation and quantitative investigation of ice nucleator activity using hemolymph from summer and winter collected larvae strongly suggest that highly active protein and lipoprotein ice nucleators are removed in preparation for overwintering. Additions of either purified antifreeze proteins or midwinter hemolymph with high antifreeze protein activity to a mixture of protein or lipoprotein ice nucleators isolated from D. canadensis hemolymph inhibited the activity of these nucleators. This suggests that in addition to seasonal removal, inhibition of hemolymph ice nucleators by antifreeze proteins contributes to seasonal increases in hemolymph supercooling capacity. Accepted: 8 August 1996     

Interspecific competition and insect herbivory reduce bush lupine (Lupinus arboreus ) seedling survival

Seedlings suffer high mortality in most plant populations, with both competition and herbivory proposed as being important mechanisms causing seedling death. The relative strength of these factors, however, is often unknown. Here I ask how interspecific competition for light and insect herbivory jointly affect seedling survival of bush lupine (Lupinus arboreus), a native shrub common to coastal California. Bush lupine seedlings germinate in grasslands during winter, and throughout spring potentially compete for light with surrounding fast-growing annual grasses. By early summer, after grasses have died, seedlings can be defoliated by a locally abundant caterpillar, the western tussock moth (Orgyia vetusta). I examined the relative importance of competition and insect herbivory on seedling survival in two separate experiments. First, I compared seedling mortality in plots either exposed to or protected from tussock moth larvae. Plants were protected from herbivory by the judicious use of insecticide; control plants were sprayed with water. Tussock moth herbivory resulted in significantly greater (31%) seedling mortality. To determine the effects of interspecific competition for light on seedling survival, I manipulated the density of grass surrounding lupine seedlings. I removed all vegetation surrounding some individuals, and left intact vegetation surrounding others. Reducing competition resulted in a 32% increase in seedling survival from February to June, as well as changes in seedling growth. To determine whether there are interactive effects of competition and herbivory on seedling survival, I enclosed tussock moth larvae on half of all surviving seedlings within each of the two prior competition treatments, comparing growth and survival of defoliated and undefoliated seedlings. Defoliation in June led to an additional 50% mortality for individuals that had grown with competitors through spring, and a 53% additional mortality for seedlings that grew without competitors through spring. Thus, although competition and herbivory both caused substantial seedling mortality, there was no statistical interaction between these factors. Competition-free plants were not less vulnerable to herbivory than plants that previously grew with competitors. Taken together, these experiments indicate that competition and herbivory are both important sources of mortality for bush lupine seedlings. Received: 4 April 1996 / Accepted: 5 November 1996     

Host race formation in the meadowsweet and strawberry feeding leaf beetle Galerucella tenella

We investigated host race formation in Galerucella tenella, a leaf beetle which feeds preferentially on meadowsweet (Rosaceae) in its natural habitats, but has become an important pest on strawberry (Rosaceae) in agricultural areas. Beetles from two isolated sites (Skeppsvik in Sweden and Solf in Finland) were compared with respect to preference and performance. At Skeppsvik the beetles were found feeding on meadowsweet, while at Solf large populations only develop in strawberry plantations, despite the presence of meadowsweet.In reciprocal field transplantations and laboratory bioassays, beetles from both sites discriminated against their foreign host, using their natal plant to a significantly higher degree for both egg laying and feeding, but with one interesting exception, namely that beetles from Solf increased their use of and even preferred meadowsweet for feeding in the laboratory. This increased use of meadowsweet by adult Solf beetles in the laboratory (without conspecific competitors) may be attributed to a density-dependent strategy, whereby mothers avoid pre-emptying the nutritional resources of the plants that will host their offspring in cases where no competitors are present.Larval fitness did not differ significantly between host plants, although larval survival of Solf beetles was halved when reared on their non-natal host plant (meadowsweet). Although beetles from both sites preferred to oviposit on their local host plant, our results provide little evidence that this presumed adaptation has to do with the nutritional quality of the plants involved. Instead, we suggest that other factors associated with the plants, such as enemy-free space and resistance to drought may be more important selective agents, shaping host preference in the field.     

Construction of an 800-kb contig in the near-centromeric region of the rice blast resistance gene Pi-ta? 2 using a highly representative rice BAC library

We constructed a rice Bacterial Artificial Chromosome (BAC) library from green leaf protoplasts of the cultivar Shimokita harboring the rice blast resistance gene Pi-ta. The average insert size of 155 kb and the library size of seven genome equivalents make it one of the most comprehensive BAC libraries available, and larger than many plant YAC libraries. The library clones were plated on seven high density membranes of microplate size, enabling efficient colony identification in colony hybridization experiments. Seven percent of clones carried chloroplast DNA. By probing with markers close to the blast resistance genes Pi-ta ² (closely linked to Pi-ta) and Pi-b, respectively located in the centromeric region of chromosome 12 and near the telomeric end of chromosome 2, on average 2.2 ± 1.3 and 8.0 ± 2.6 BAC clones/marker were isolated. Differences in chromosomal structures may contribute to this wide variation in yield. A contig of about 800 kb, consisting of 19 clones, was constructed in the Pi-ta ² region. This region had a high frequency of repetitive sequences. To circumvent this difficulty, we devised a “two-step walking” method. The contig spanned a 300 kb region between markers located at 0 cM and 0.3 cM from Pi-ta ² . The ratio of physical to genetic distances (> 1,000 kb/cM) was more than three times larger than the average of rice (300 kb/cM). The low recombination rate and high frequency of repetitive sequences may also be related to the near centromeric character of this region. Fluorescent in situ hybridization (FISH) with a BAC clone from the Pi-b region yielded very clear signals on the long arm of chromosome 2, while a clone from the Pi-ta ² region showed various cross-hybridizing signals near the centromeric regions of all chromosomes. Received: 14 August 1996 / Accepted: 2 December 1996     

The birch tube-maker Acrobasis betulella in a fragmented habitat: the importance of patch isolation and edges

We examined the response of the birch tube-maker Acrobasis betulella Hulst (Lepidoptera: Pyralidae) to habitat patch isolation and edges. Density of A.␣betulella larva was higher on distant islands than on islands close to the shore. Following experimental removal of all larvae from the islands, adults were able to recolonize even the most distant islands, and larval density was again positively correlated with the degree of isolation. Larval density was not correlated with island size or the amount of birch present on the islands. Larvae on more distant islands did not have lower mortality than those in less isolated sites. Larvae were found more often on edges than in the interior of birch stands and developed faster on edges. This positive edge effect, coupled with the ability of the adults to disperse to even our most distant islands, suggests that A. betulella would be favored in an environment fragmented at the same scale as our island system. Received: 15 April 1996 / Accepted: 30 September 1996     

Role of dispersal ability in the phenotypic differentiation and plasticity of two marine gastropods

Species with contrasting abilities for dispersal may adopt different strategies in response to wide ranges of environmental conditions. These strategies were investigated here by comparisons of phenotypic differentiation and plasticity in the gastropods Austrocochlea constricta and Bembicium vittatum, which coexist in a range of intertidal habitats on the Abrolhos Islands and Albany in Western Australia. They differ in their potential for larval dispersal, A. constricta having a short mobile planktonic stage and B. vittatum developing directly from benthic eggs. Levels of among-population variation in shell shape and maximum size did not differ between the species, or between locations subject to contrasting levels of gene flow in the case of A. constricta. Results of a common garden translocation experiment, coupled with those of previous breeding experiments, suggest that variation in shape is predominantly genetically determined in B. vittatum, but plastic in A. constricta. Plasticity of shape in A. constricta was most pronounced at Albany, where previous data suggested the widest gene flow, and was greater than in B. vittatum at both locations. Change of shape in A. constricta subsequent to translocation was related to growth rate, a trait which was also more plastic in this species than in B. vittatum. The results agree with the notion that wide dispersal enhances physiological flexibility, while restricted dispersal promotes local genetic adaptation, but disagree with the hypothesis that wide dispersal promotes phenotypic homogeneity. Received: 31 May 1996 / Accepted: 30 September 1996     

Cloning and molecular analysis of the Arabidopsis gene Terminal Flower 1

The Arabidopsis gene Terminal Flower 1 (TFL1) controls inflorescence meristem identity. A terminal flower (tfl1) mutant, which develops a terminal flower at the apex of the inflorescence, was induced by transformation with T-DNA. Using a plant DNA fragment flanking the integrated T-DNA as a probe, a clone was selected from a wild-type genomic library. Comparative sequence analysis of this clone with an EST clone (129D7T7) suggested the existence of a gene encoding a protein similar to that encoded by the cen gene which controls inflorescence meristem identity in Antirrhinum. Nucleotide sequences of the region homologous to this putative TFL1 gene were compared between five chemically induced tfl1 mutants and their parental wild-type ecotypes. Every mutant was found to have a nucleotide substitution which could be responsible for the tfl1 phenotype. This result confirmed that the cloned gene is TFL1 itself. In our tfl1 mutant, no nucleotide substitution was found in the transcribed region of the gene, and the T-DNA-insertion site was located at 458 bp downstream of the putative polyadenylation signal, suggesting that an element important for expression of the TFL1 gene exists in this area. Received: 14 November 1996 / Accepted: 29 November 1996     

Quantifying the effects of distance and conspecifics on colonization: experiments and models using the loosestrife leaf beetle, Galerucella calmariensis

The ability of an insect to disperse to new habitat patches is difficult to quantify, but key to the establishment and persistence of populations. In this study, we examined dispersal of the phytophagous chrysomelid beetle, Galerucella calmariensis, which is currently being introduced into North America for the biological control of purple loosestrife (Lythrum salicaria), an aggressive wetland weed. We used a mark, release, and recapture approach to determine how rates of colonization of host patches by this beetle are influenced by the distance of the patch from the source of dispersers, and by the presence of conspecifics at the patch. We released color-coded beetles at six distances from a long, linear patch of purple loosestrife that was divided into segments with and without conspecifics. We observed initial flight directions as beetles left the release points and collected all beetles that settled at the target patch. We found a bias in initial flight toward the target for distances up to 50 m. Over the 7 days of the experiment, beetles arrived at the target from all release points, including the farthest release point, 847 m away. G. calmariensis was strongly attracted to conspecifics when settling after dispersal; 86% of the 582 recovered beetles came from the segments inhabited by conspecifics. The probability of an individual arriving at the patch declined steeply with release distance. This relationship fits a model in which beetles move in a random direction and stop if they intercept the target patch, and where beetles are lost at a constant rate with distance travelled. The dispersal and patch-colonizing behavior of G. calmariensis is likely to have important consequences for the biological control program against purple loosestrife. Received: 23 January 1996 / Accepted:30 September 1996     

Neural correlates of habituation of the proleg withdrawal reflex in larvae of the hawk moth, Manduca sexta

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, exhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a single planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response decrement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and electrical stimulation of a body wall nerve facilitated the decremented response (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, when adaptation was eliminated by direct electrical stimulation of sensory neurons, the response in the proleg motor nerve still decreased significantly. Muscle recordings indicated that the response of an identified proleg retractor motor neuron decreased significantly during habituation training. Thus, habituation of the proleg withdrawal reflex includes a central component that is apparent at the level of a single motor neuron. Accepted: 20 December 1996     

Natal origin affects host preference and larval performance relationships in a tritrophic system

Many insects face the challenge to select oviposition sites in heterogeneous environments where biotic and abiotic factors can change over time. One way to deal with this complexity is to use sensory experiences made during developmental stages to locate similar habitats or hosts in which larval development can be maximized. While various studies have investigated oviposition preference and larval performance relationships in insects, they have largely overlooked that sensory experiences made during the larval stage can affect such relationships. We addressed this issue by determining the role of natal experience on oviposition preference and larval performance relationships in a tritrophic system consisting of Galerucella sagittariae, feeding on the two host plants Potentilla palustris and Lysimachia thyrsiflora, and its larval parasitoid Asecodes lucens. We firstly determined whether differences in host‐derived olfactory information could lead to divergent host selection, and secondly, whether host preference could result in higher larval performance based on the natal origin of the insects. Our results showed that the natal origin and the quality of the current host are both important aspects in oviposition preference and larval performance relationships. While we found a positive relationship between preference and performance for natal Lysimachia beetles, natal Potentilla larvae showed no such relationship and developed better on L. thyrsiflora. Additionally, the host selection by the parasitoid was mainly affected by the natal origin, while its performance was higher on Lysimachia larvae. With this study, we showed that the relationship between oviposition preference and larval performance depends on the interplay between the natal origin of the female and the quality of the current host. However, without incorporating the full tritrophic context of these interactions, their implication in insect fitness and potential adaptation cannot be fully understood.     

Influence of soil hydric parameters on the winter cold hardiness of a burrowing beetle, Leptinotarsa decemlineata (Say)

This investigation examined the influence of soil moisture and associated parameters on the cold hardiness of the Colorado potato beetle (Leptinotarsa decemlineata Say), a temperate-zone species that overwinters in terrestrial burrows. The body mass and water content of adult beetles kept in sand at 4 °C varied over a 16-week period of diapause according to the substratum's moisture content. Changes in body water content, in turn, influenced the crystallization temperature (range −3.3 to −18.4 °C; n = 417), indicating that environmental moisture indirectly determined supercooling capacity, a measure of physiological cold hardiness. Beetles held in dry sand readily tolerated a 24-h exposure to temperatures ranging from 0° to −5 °C, but those chilled in sand containing as little as 1.7% water (dry mass) had elevated mortality. Thus, burrowing in dry soils not only promotes supercooling via its effect on water balance, but may also inhibit inoculative freezing. Mortality of beetles exposed to −5 °C for 24 h was lower in substrates composed of sand, clay and/or peat (36–52%) than in pure silica sand (78%) having an identical water content (17.0% dry mass). In addition to moisture, the texture, structure, water potential, and other physico-chemical attributes of soil may strongly influence the cold hardiness and overwintering survival of burrowing insects. Accepted: 10 September 1996