The effect of variation in host plant on the growth of an oligophagous insect, Malacosoma americanum and its polyphagous relative, Malacosoma disstria

Larvae of Malacosoma americanum (F.)(Lepidoptera: Lasiocampidae) an oligophagous species that feeds primarily on Prunus and other rosaceous trees, were compared to larvae of the more highly polyphagous congener M. disstria Hb., with respect to their sensitivity to variation in the foliage of a common host plant, Prunus serotina Ehrh. Pupal weight and time to pupation were measured on larvae reared on foliage from open-grown and from shaded saplings. The difference in foliage had a pronounced effect, but no difference was evident between the species in their response to the variation in foliage. This comparison implies that sensitivity to intraspecific variation in host quality does not differ between host-specific and generalized species. However, results from other species suggest that some species of insects do differ in this respect.

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Zusammenfassung Raupen von Malacosoma americanum (F.), einer oligophagen Art, die sich vor allem auf Prunus und andern baumartigen Rosaceen entwickelt, wurden verglichen mit Raupen der polyphageren Verwandten M. disstria Hb. und zwar im Hinblick auf deren Empfindlichkeit auf Unterschiede im Blatt ihrer gemeinsamen Wirtspflanze, Prunus serotina Ehrh. Das Puppengewicht und die Entwicklungszeit bis zur Verpuppung wurden gemessen bei Raupen, welche auf Blättern von freiwachsenden und von beschatteten Jungpflanzen gezüchtet worden waren. Die Blattunterschiede hatten eine ausgesprochene Wirkung, aber es gab keine Unterschiede in der Reaktion der beiden Arten. Dieser Vergleich lässt vermuten, dass die Empfindlichkeit auf intraspezifische Unterschiede der Wirtspflanzenqualität bei wirtsspezifischen und polyphagen Arten gleich ist. Indessen dürften sich laut anderen Resultaten einige Insektenarten anders verhalten.

     

Recycling of nitrogen in herbivore feces: plant recovery,herbivore assimilation,soil retention,and leaching losses

Herbivores directly and indirectly affect ecosystem functioning in forests. Feces deposition is a direct effect that supplies ephemeral N pulses to soils. Herbivore-mediated changes in plant N allocation and uptake are indirect effects that can also influence soil N availability. These effects may interact if defoliation influences the ability of plants to recover fecal N, and this may affect subsequent generations of herbivores. We added ¹⁵N-enriched insect feces (frass) to a series of replicated red oak, Quercus rubra, mesocosms that had been damaged experimentally and then followed the frass N over the course of 2 years. In the first season, some frass N was mineralized in the soil and leached in organic form from the mesocosms within 1 week of deposition. Within 1 month, frass N had been acquired by the oaks and enriched the foliage; late-season herbivores assimilated the frass N within the same growing season. In the second season, herbivore damage from the previous year lowered total leaf N contents and ¹⁵N recovered in the foliage. A subsequent cohort of early-season herbivores fed on this foliage consequently derived less of their N from the previous year’s frass, and feral leaf rollers colonized fewer of these saplings. The 0- to 5-cm soil fraction was the largest N sink measured, and 42% of the frass N was recovered in the soil. The results demonstrate that: (1) some frass N can be recycled rapidly into foliage and assimilated by successive cohorts of herbivore within the same season; (2) damage can affect N allocation in the following year’s foliage, influencing N availability to and host selection by herbivores; and (3) leaching losses occur soon after deposition but are buffered by soil pools, which are the largest sinks for frass N.     

Fluctuating asymmetry and survival ability in the forest tent caterpillar mothMalacosoma disstria: implications for pest management

Fluctuating asymmetry of the first tarsal segment of the proleg of the forest tent caterpiller mothMalacosoma disstria Hbn. (Lepidoptera: Lasiocampidae) was significantly inversely related to survival ability in the lab. The monitoring of population levels of fluctuating asymmetry could have important implications in pest management of this and other species by providing an indication of the health of a population.     

A modified method for the detection of microbial proteases on agar plates using tannic acid

In routine assay for the screening of microbes producing proteases, 10% trichloroaceticacid (TCA) is flooded on the milk agar plates after inoculation and required incubation to precipitate the protein. However, the clarity of the hydrolyzed zone is not very sharp and distinct. We herein present an improved assay for detecting the presence of extracellular protease from microorganisms on agar plates. In this method 10% tannic acid is flooded on the milk agar plate (in place of, TCA) to observe the zone of hydrolysis. Tannic acid sharply increases the colour intensity of the plate, as it favours the precipitation of the unhydrolyzed protein in the plate, thereby improving the contrast between the intact zones and the enzymatic lyses zones of the substrate. Our results indicate that this method is useful to detect extracellular proteases produced by both fungi as well as bacteria. The method used in the present study is sensitive, and can be easily performed for screening of large number of microbial cultures. This is the first report on the use of tannic acid for the detection of microbial proteases.     

The antioxidant effect of tannic acid on the in vitro copper-mediated formation of free radicals

Tannic acid (TA) has well-described antimutagenic and antioxidant activities. The antioxidant activity of TA has been previously attributed to its capacity to form a complex with iron ions, interfering with the Fenton reaction [Biochim. Biophys. Acta 1472, 1999, 142]. In this work, we observed that TA inhibits, in the micromolar range, in vitro Cu(II) plus ascorbate-mediated hydroxyl radical (*OH) formation (determined as 2-deoxyribose degradation) and oxygen uptake, as well as copper-mediated ascorbate oxidation and ascorbate radical formation (quantified in EPR studies). The effect of TA against 2-deoxyribose degradation was three orders of magnitude higher than classic *OH scavengers, but was similar to several other metal chelators. Moreover, the inhibitory effectiveness of TA, by the four techniques used herein, was inversely proportional to the Cu(II) concentration in the media. These results and the observation of copper-induced changes in the UV spectra of TA are indications that the antioxidant activity of TA relates to its copper chelating ability. Thus, copper ions complexed to TA are less capable of inducing ascorbate oxidation, inhibiting the sequence of reactions that lead to 2-deoxyribose degradation. On the other hand, the efficiency of TA against 2-deoxyribose degradation declined considerably with increasing concentrations of the *OH detector molecule, 2-deoxyribose, suggesting that the copper-TA complex also possesses an *OH trapping activity.     

Detoxification of oxalic acid by pseudomonas fluorescens strain pfMDU2: implications for the biological control of rice sheath blight caused by Rhizoctonia solani

Rhizoctonia solani isolates varying in their virulence were tested for their ability to produce oxalic acid (OA) in vitro. The results indicated that the virulent isolates produced more OA than the less virulent isolates. In order to isolate OA-detoxifying strains of Pseudomonas fluorescens, rhizosphere soil of rice was drenched with 100 mM OA and fluorescent pseudomonads were isolated from the OA-amended soil by using King's medium B. These isolates were tested for their antagonistic effect towards growth of R. solani in vitro. Among them P. fluorescens PfMDU2 was the most effective in inhibiting the mycelial growth of R. solani. P. fluorescens PfMDU2 was capable of detoxifying OA and several proteins were detected in the culture filtrate of PfMDU2 when it was grown in medium containing OA. To investigate whether the gene(s) involved in OA-detoxification resides on the plasmids in P. fluorescens PfMDU2, a plasmid-deficient strain of P. fluorescens was generated by plasmid curing. The plasmid-deficient strain (PfMDU2P-) failed to grow in medium containing OA and did not inhibit the growth of R. solani. Both PfMDU2 and PfMDU2P- were tested for their efficacy in controlling sheath blight of rice under greenhouse conditions. Seed treatment followed by soil application of rice with P. fluorescens strain, PfMDU2, reduced the severity of sheath blight by 75% compared with the control, whereas PfMDU2P- failed to control sheath blight disease.     

Foliar quality influences tree-herbivore-parasitoid interactions: effects of elevated CO<Subscript>2</Subscript>, O<Subscript>3</Subscript>, and plant genotype

This study examined the effects of carbon dioxide (CO₂)-, ozone (O₃)-, and genotype-mediated changes in quaking aspen (Populus tremuloides) chemistry on performance of the forest tent caterpillar (Malacosoma disstria) and its dipteran parasitoid (Compsilura concinnata) at the Aspen Free-Air CO₂ Enrichment (FACE) site. Parasitized and non-parasitized forest tent caterpillars were reared on two aspen genotypes under elevated levels of CO₂ and O₃, alone and in combination. Foliage was collected for determination of the chemical composition of leaves fed upon by forest tent caterpillars during the period of endoparasitoid larval development. Elevated CO₂ decreased nitrogen levels but had no effect on concentrations of carbon-based compounds. In contrast, elevated O₃ decreased nitrogen and phenolic glycoside levels, but increased concentrations of starch and condensed tannins. Foliar chemistry also differed between aspen genotypes. CO₂, O₃, genotype, and their interactions altered forest tent caterpillar performance, and differentially so between sexes. In general, enriched CO₂ had little effect on forest tent caterpillar performance under ambient O₃, but reduced performance (for insects on one aspen genotype) under elevated O₃. Conversely, elevated O₃ improved forest tent caterpillar performance under ambient, but not elevated, CO₂. Parasitoid larval survivorship decreased under elevated O₃, depending upon levels of CO₂ and aspen genotype. Additionally, larval performance and masses of mature female parasitoids differed between aspen genotypes. These results suggest that host-parasitoid interactions in forest systems may be altered by atmospheric conditions anticipated for the future, and that the degree of change may be influenced by plant genotype.     

Surface antigens of Xenorhabdus nematophila (F. Enterobacteriaceae) and Bacillus subtilis (F. Bacillaceae) react with antibacterial factors of Malacosoma disstria (C. Insecta: O. Lepidoptera) hemolymph

Previous research established different interactions of the insect pathogen, Xenorhabdus nematophila and nonpathogen, Bacillus subtilis, with antimicrobial hemocytes and humoral factors of larval Malacosoma disstria [Giannoulis, P., Brooks, C.L., Dunphy, G.B., Mandato, C.A., Niven, D.F., Zakarian, R.J., 2007. Interaction of the bacteria Xenorhabdus nematophila (Enterobacteriaceae) and Bacillus subtilis (Bacillaceae) with the hemocytes of larval Malacosoma disstria (Insecta: Lepidoptera: Lasicocampidae). J. Invertebr. Pathol. 94, 20-30]. The antimicrobial systems were inhibited by X. nematophila and stimulated by B. subtilis. The bacterial surface antigens participating in these reactions were unknown. Thus, herein the effects of lipopolysaccharide (endotoxin) from X. nematophila and lipoteichoic acid from B. subtilis on the larval M. disstria immune factors, the hemocytes and phenoloxidase, were determined. Endotoxin elevated the level of damaged hemocytes limiting the removal of X. nematophila from the hemolymph and enhancing the rapid release of bacteria trapped by nodulation. Similar effects were observed with the lipid A moiety of the endotoxin. The effects of lipopolysaccharide and lipid A on the hemocyte activities were abrogated by polymyxin B (an antibiotic that binds to lipid A) confirming lipopolysaccharide as the hemocytotoxin by virtue of the lipid A moiety. Lipoteichoic acid elicited nodulation and enhanced phenoloxidase activation and/or activity. Although lipoidal endotoxin and lipid A inhibited phenoloxidase activation they enhanced the activity of the enzyme. Apolipophorin-III precluded the effects of lipopolysaccharide, lipid A, and lipoteichoic acid on the hemocytes and prophenoloxidase until the antigens exceeded a critical threshold.     

Larval Sociality in Three Species of Central-place Foraging Lappet Moths (Lepidoptera: Lasiocampidae): A Comparative Survey

We studied three species of Lasiocampidae with social, tent-building caterpillars in Northern Bavaria, viz. Eriogaster lanestris, Eriogaster catax, and Malacosoma neustria. We used key life-history data (number of larval instars, sizes and weights of eggs, caterpillars, and moths, size of egg clutches) as well as behavioral data (activity patterns, tent-building behavior, trail following behavior) for a comparative study. Although larvae of all three species are active only in spring, show overlapping habitat requirements, and use the same major host-plant (Prunus spinosa) with only minor differences in phenology, they show markedly different life-history and behavioral strategies.E. catax lays comparatively few but large eggs while E. lanestris lays more but smaller eggs. M. neustria lays the smallest eggs but large clusters. E. lanestris caterpillars build a large tent with an accessible interior while those of E. catax build a small tent that is only used as a resting and molting platform. M. neustria shows a flexible behavior, may abandon the primary tent and build a new one several times. M. neustria colonies also subdivide and reunite regularly while Eriogaster colonies stay together until larvae become solitary. In E. lanestris all tentmates of a colony are highly synchronized while foraging or resting. Instead, in E. catax small subgroups leave the tent for foraging while at every time the majority rests on the tent. M. neustria caterpillars forage more or less individually and only synchronize by night. Results are discussed in relation to other species of the genera Eriogaster and Malacosoma and with regard to the evolution and diversification of caterpillar sociality.     

Soldier bug predation on swallowtail caterpillars (Lepidoptera: Papilionidae): Circumvention of defensive chemistry

All larvae in the family Papilionidae possess osmeteria, eversible glands that release defensive chemicals upon disturbance. These chemicals have been shown to be repellent to ants, mantids, and other predaceous arthropods with chewing mouthparts. In this study, we demonstrate that the pentatomid Podisus maculiventris,an important predator of the black swallowtail Papilio polyxenes,is capable of piercing the body wall of the caterpillar and consuming body fluids without eliciting osmeterial eversion. Tenebrio molitorlarvae coated with osmeterial secretions of Papilio polyxeneswere universally rejected by Podisus maculiventris,suggesting that the predator is in fact repelled by the swallowtail defensive chemistry. By circumventing the defensive behavior of the caterpillar, the pentatomid is able to utilize an otherwise chemically unsuitable prey species.     

Effects of plant nutrient availability and host plant species on the performance of two Pieris butterflies (Lepidoptera: Pieridae)

We assayed the interaction on the availability of plant nutrient and species of host plant on the performance of two species of Pieris butterfly. The results indicated that constant application of different levels of fertilizers to the four different host plants resulted to an increase in their content of plant nutrients. The chemical analysis showed that the added nutrients increased foliar nitrogen and water contents, but there was no effect on the level of glucosinolates. Larvae that fed on highly-nutritious foliage increased their growth rates and showed a shorter development period. The results of feeding trials revealed that the 4th-instar larvae, which had fed on host plants with higher levels of fertilization had a shorter duration of development, less consumption rate, higher growth rate and food processing efficiency. To summarize, this research revealed that both the availability of plant nutrient and species of host plant can strongly influence the physiology and foliar chemistry of host plants. Moreover, the changes of phytochemical in the host plants may play an important role in affecting the performance (growth and food utilization efficiency) of both species of Pieris butterflies.     

A study of spatial variability of domoic acid in razor clams: recommendations for resource management on the Washington coast

Domoic acid (DA) levels in Washington State razor clams (Siliqua patula) have been extremely variable and unpredictable, resulting in emergency closures of harvest areas in 1991, 1998, and 1999. Information concerning toxin variability relative to sampling location is important in developing a more reliable plan for managing DA outbreaks. In November 1998, record levels of DA in razor clams (up to 295 ppm) were reported at Kalaloch beach, Washington. A razor clam resource survey conducted there during the summer of 1999, along with the long retention time of DA in the clams, permitted a study of DA levels as a function of tidal elevation and north-south beach location. During the summer low tides (28–31 July 1999) razor clams were collected from six east-west transects, approximately 1.6 km apart. Each clam was individually analyzed for DA to determine the distribution of toxin between (interspecific variability) and within (intraspecific variability) transects. While average DA levels were similar between transects, toxin levels varied substantially among individual clams. The coefficient of variation among all samples (n=445) was 122%, indicating that harvest closures based upon composite analyses of only six clams could be in error. Here we recommend a sampling strategy of razor clams for regulatory purposes that will result in DA measurements more representative of total population toxin values.     

Model systems for the immunolocalisation of cis,trans abscisic acid in plant tissues

To explore the feasibility of immunolocalisation of endogenous abscisic acid (ABA), model systems were developed for testing quantitatively the sensitivity of the second antibody peroxidase/antiperoxidse (PAP) method for immunolocalisation of ABA on plant tissues. Exogenous (±)ABA was fixed to carrot sections on glass slides or to homogenised pea cotyledon material on microtitre plates, either directly by carbodiimide fixation or by glutaraldehyde fixation of ABA-protein conjugates linked through the C1 carboxyl by 1-ethyl-3(3-dimethyl-amino-propyl) carbodiimide hydrochloride (EDC). Backgrounds were decreased by including 0.1% normal goat serum in the incubations, by including 0.1% Triton X-100 as a wetter, by including glycine in the rinses after EDC fixation and by using low-pH rinses after incubation with the primary antibody. Serum antibodies recognising the peptide bond between the protein and abscisic acid were removed by preincubating the serum with acetic acid conjugated to protein. Positives were only accepted when they could be eliminated by adding an excess of ABA-protein conjugate in the primary antiserum. By using a soluble peroxidase reaction product to facilitate quantitation, the limit of reliable exogenous ABA detection was found to be only of the order of 1 pmol. For the histochemical immunolocalisation of endogenous ABA, better antisera and lower backgrounds will be required.The efficiency of fixation of exogenous ABA was determined using [³H] or [¹⁴C]ABA. When aqueous EDC or di-isopropyl carbodiimide (IPC) were used the fixation efficiency was low (up to 5%), but much higher efficiencies (up to 80%) were obtained using IPC vapour with freeze-dried material. Similarly efficient fixation of endogenous ABA in pea cotyledon material, as determined by gas chromatography-mass spectrometry analysis, was obtained using the same technique. The PAP method failed to detect fixed endogenous ABA in pea cotyledons, even though the total tissue amounts present exceeded 1 pmol, evidence that not enough of the ABA was accessible to the antibody.Abbreviations ABA abscisic acid - ACE-ALP acetic acid-alkaline phosphatase - EDC 1-ethyl-3(3-dimethyl-amino-propyl) carbodiimide hydrochloride - GC-MS gas chromatographymass spectrometry - IgG Immunoglobulin G - HSA humanserum albumin - IPC dinsopropyl carbodiimide - LINK goat anti-rabbit IgG - OD optical density - PAP peroxidase/rabbit antiperoxidase complex     

Dynamics of the phycotoxin domoic acid: accumulation and excretion in two commercially important bivalves

Batch cultures of the toxigenic diatomNitzschia pungens Grunow f.multiseries Hasle were fed to blue mussels (Mytilus edulis) and deep sea Atlantic scallops (Placopecten magellanicus) to elucidate conditions under which domoic acid (DA) was accumulated and excreted (depurated). Mussels accumulated the toxin to a maximum level of 13 g g^-1, at rates of 0.21 to 3.7 g h^-1 g^-1, dry weight. Accumulation efficiency (the proportion of accumulated DA to estimated net uptake) ranged from 1–5%. The highest filtration rate of 1.71 h^-1 occurred at concentrations of 4–8 × 10⁶ Nitzschia cells 1^-1 with no formation of pseudofeces. Depuration rates between fed and starved mussels over a 2 h test period were the same. The depuration rate of domoic acid was about 17% d^-1 and did not account for the low uptake efficiencies, so it is suggested that most of the DA is lost from mussels in the solution during the feeding process. Domoic acid accumulation in mussels was dependent on the amount of toxin available, which in turn was a function of the density and growth phase of theNitzschia population. Changes in filtration rate withNitzschia concentration and depuration rate with time can account for the DA levels of mussels collected during toxic episodes in Cardigan Bay, Prince Edward Island, Canada in 1988 and 1989.Scallops accumulated DA (0.39–1.3 g h^-1 g^-1, more slowly than mussels, however, accumulation efficiencies ranged from 5–100%. Filtration rates remained relatively low and constant at 0.081 h^-1. Scallops retained domoic acid longer than mussels, a fact which must be considered in the marketing of whole scallops for human consumption.     

Differential responses of populations of the copepod Acartia hudsonica to toxic and nutritionally insufficient food algae

Nutritional insufficiency and toxicity are deleterious effects of phytoplankton on grazers. We hypothesize that toxic food is likely to have stronger evolutionary selective effects on grazers than nutritionally insufficient food. We explore this hypothesis in comparative studies of egg production and egg hatching of the copepod Acartia hudsonica challenged with both a toxic and a nutritionally insufficient alga. Experiments lasting 6 days, in which mixtures of different proportions of the suspect and a control alga were offered as food to female copepods, showed that the dinoflagellate Alexandrium fundyense, which bears paralytic shellfish toxins, was toxic to A. hudsonica. In contrast, the diatom Phaeodactylum tricornutum was nutritionally insufficient to A. hudsonica. In another set of experiments, the effects of A. fundyense and P. tricornutum, respectively, as sole foods on egg production and egg hatching success of two geographically separated populations (Maine and Connecticut) of the copepod A. hudsonica were examined in common-environment experiments, after being raised under identical conditions for two generations. The location in Maine regularly experiences toxic blooms of Alexandrium sp. whereas the location in Connecticut does not. During a 6-day period, A. fundyense reduced the egg production rates of the Connecticut copepod population, but not of the Maine population. In contrast, the diatom P. tricornutum reduced the egg production of both populations. These results of this study are consistent with the hypothesis of local adaptation to toxic food, but not to nutritionally insufficient food.     

Synergistic effect of entomogenous fungi on some insecticides against Bihar hairy caterpillar Spilarctia obliqua (Lepidoptera: Arctiidae)

A number of fungal parasites infect a wide range of insects and cause epizootics from time to time. Beauveria bassiana (Balsamo) Vuillemin and Metarhizium anisopliae (Metschnikoff) Sorokin are two of the major disease-causing fungi in insects. Investigations were carried out to study the effect of these fungi on the toxicity of endosulfan, imidacloprid, lufenuron, diflubenzuron, dimethoate and oxydemeton methyl against 10-11 days old larvae of Spilarctia obliqua (Walker). For some products the combination treatments showed higher dose mortality response than the sole treatment of fungal conidia or the insecticide. The combination of insecticides with B. bassiana showed 1.26-35.8 fold increase in toxicity of insecticides over sole treatment, while the increase was 1.05-72.0 fold in case of M. anisopliae. Imidacloprid 17.8 SL and oxydemeton methyl 25EC may be used in combination with these fungi for management of S. obliqua.     

Differential requirements of two insect cell lines for growth in serum-free medium

Summary The development of a serum-free medium that supports the growth of cells from a Spodoptera frugiperda and a Lymantria dispar cell line is reported. A yeast hydrolysate provided the B-vitamin complex, and a combination of a meat hydrolysate and tryptose provided most of the free amino acids required for cell growth. Supplemental cystine and methionine were required to achieve maximum cell growth. The serum or serum replacements used in earlier formulations were replaced with commercial lipid preparations and increased levels of iron salts. Although the cell growth cycle had a somewhat extended lag phase and the population doubling time of the S. frugiperda cells was longer than on serum-containing medium, the saturation densities were much higher. Spodoptera cells grown in this medium replicated the Autographa californica nuclear polyhedrosis virus well, producing 8.71 × 10⁶ TCID₅₀ extracellular virus and 4.4×10⁸ polyhedra/ml culture. The specific activity of the polyhedra was somewhat less than that of polyhedra produced in insects.     

单宁酸对高原鼠兔和根田鼠生殖激素的影响

高原鼠兔和根田鼠是生存于青藏高原的小哺乳动物,它们所取食的高原植物含有多种次生化合物。为了证明食物中的次生化合物对高原鼠兔和根田鼠繁殖相关激素的影响,探讨它们与取食植物间的协同进化,分别对两种动物灌服0、5、10 和20 mg/ kg BW 剂量单宁酸1 d、3 d 及5 d 后,测定其下丘脑促性腺激素释放激素、血浆中睾酮和血浆雌二醇的含量。结果表明,单宁酸对高原鼠兔的LD₅₀ 为112 ±27.72 mg/ kg BW,对根田鼠的LD₅₀ 为117 ± 17.37 mg/ kg BW;单宁酸可以使高原鼠兔和根田鼠血浆睾酮和雌二醇水平升高,但对下丘脑促性腺激素释放激素水平没有明显的影响。说明单宁酸可能会促进这两种动物的性成熟,有利于它们的繁殖。     

Comparing the effects of the exotic cactus-feeding moth, Cactoblastis cactorum (Berg) (Lepidoptera: Pyralidae) and the native cactus-feeding moth, Melitara prodenialis (Walker) (Lepidoptera: Pyralidae) on two species of Florida Opuntia

This study examined the effects of the native cactus moth borer, Melitara prodenialis, and the invasive cactus moth borer, Cactoblastis cactorum, on two common cactus species, Opuntia stricta and O. humifusa at coastal and inland locations in central Florida. Opuntia stricta were present only at coastal sites and O. humifusa were present at coastal and inland sites. Throughout the duration of the study, coastal plants were subject to damage solely by C. cactorum and inland plants solely by M. prodenialis. Results showed marginally significantly higher numbers of eggsticks on O. stricta than O. humifusa and significantly higher numbers at coastal sites than at inland sites. There was also significantly higher moth damage on O. stricta than O. humifusa and at coastal sites than inland sites, but not significantly so. However, there was a higher level of plant mortality for O. humifusa than for O. stricta and a significantly higher level of cactus mortality at inland sites when compared to coastal sites. This increased mortality may be due to increased attack by true bugs, Chelinidea vittiger, and by Dactylopius sp., combined with attack by M. prodenialis. Inland plants also tended to be smaller than coastal plants and could be more susceptible to the combined effects of all insects. Further long-term research on coastal cactus survival when attacked and unattacked by Cactoblastis is necessary to fully determine the effects of this moth on Opuntia survival.     

Associational effects of plant defences in relation to within- and between-patch food choice by a mammalian herbivore: neighbour contrast susceptibility and defence

A basic idea of plant defences is that a plant should gain protection from its own defence. In addition, there is evidence that defence traits of the neighbouring plants can influence the degree of protection of an individual plant. These associational effects depend in part on the spatial scale of herbivore selectivity. A strong between-patch selectivity together with a weak within-patch selectivity leads to a situation where a palatable plant could avoid being grazed by growing in a patch with unpalatable plants, which is referred to as associational defence. Quite different associational effects will come about if the herbivore instead is unselective between patches and selective within a patch. We studied these effects in a manipulative experiment where we followed the food choice of fallow deer when they encountered two patches of overall different quality. One of the two patches consisted of pellets with low-tannin concentration in seven out of eight buckets and with high concentration in the remaining bucket. The other patch instead had seven high- and one low-tannin bucket. We performed the experiment both with individuals one at a time and with a group of 16–17 deer. We found that the deer were unselective between patches, but selective within a patch, and that the single low-tannin bucket among seven high-tannin buckets was used more than a low-tannin bucket among other low-tannin buckets. This corresponds to a situation where a palatable plant that grows among unpalatable plants is attacked more than if it was growing among its own kind, and for this effect we suggest the term neighbour contrast susceptibility, which is the opposite of associational defence. We also found that the high-tannin bucket in the less defended patch was less used than the high-tannin buckets in the other patch, which corresponds to neighbour contrast defence. The neighbour contrast susceptibility was present both for individual and group foraging, but the strength of the effect was somewhat weaker for groups due to weaker within-patch selectivity.