Long-term effects of exogenous methyl jasmonate application on Scots pine (Pinus sylvestris) needle chemical defence and diprionid sawfly performance

Scots pine [ Pinus sylvestris L. (Pinaceae)] trees with four different seed origins were exposed to exogenous applications of the elicitor, methyl jasmonate (MeJA), for three consecutive years. We studied the effects of MeJA on needle chemistry (including monoterpenes, sesquiterpenes, and tricyclic resin acids), plant growth, and the performance of two diprionid sawflies, the European pine sawfly ( Neodiprion sertifer Geoffr.) and the common pine sawfly ( Diprion pini L.) (both Hymenoptera: Diprionidae). In general, foliar MeJA application affected the whole range of needle secondary chemistry with significantly higher concentrations of two monoterpenes, β-pinene and limonene, in particular. Furthermore, for some seed origins the growth rates of N. sertifer and D. pini larvae were lower on needles of MeJA-treated plants with either high total terpene or high resin acid concentrations. However, inconsistencies in diprionid sawfly performance within each studied Scots pine origin suggest genetic variance in needle secondary chemistry. The differences between selected seed origins and notably variable responses to MeJA application imply that adaptation of the seed to new conditions may have had an impact on secondary chemistry and, thus, on insect performance. Finally, our results suggest that modification of Scots pine defence by a low-concentration exogenous elicitor affects the production of terpenoids in the newly growing needles, leading to poorer pine sawfly performance in origins with high terpenoid content, while not harming the growth of Scots pine trees.     

Pine sawfly defence and variation in host plant resin acids: a trade-off with growth

Abstract.

• 1 The defence capability of pine sawfly larvae reared on Scots pines with low or high concentrations of resin acids was determined in a field experiment, where larvae were exposed to predatory ants, and in a laboratory experiment in which the rate of defence droplet replenishment was measured.
• 2 Larvae on low resin-acid shoots disappeared faster than larvae on high resin-acid shoots when exposed to predatory ants.
• 3 Larvae fed high resin-acid needles produced 50% larger defence droplets than larvae fed low resin-acid needles.
• 4 In an additional experiment high predation pressure was simulated by repeatedly removing defence droplets. Larvae responded by feeding more on tissue rich in resin acids, but at the cost of a reduction in growth rate.
• 5 Based on these data and earlier findings that survival and development of sawfly larvae are negatively affected by high resin-acid concentrations, we conclude that pine sawflies face a dilemma of conflicting demands, i.e. although high needle resin-acid concentrations enhance defence capability, they also reduce growth and survival. Our data also suggest that it is not necessarily the ovipositing female that has to balance this trade-off: plasticity in larval feeding behaviour provides larvae with a means to at least partly solve the dilemma.

     

Influence of watering and trenching ponderosa pine on a pine sawfly

Summary Neodiprion autumnalis (Smith) larvae were caged for two successive years on root-trenched, watered, and untreated ponderosa pine (Pinus ponderosa Doug. ex Laws.) to determine effects of host moisture stress on larval feeding. Levels of moisture stress (as measured by the Scholander pressure chamber) differed significantly among treatment levels during 1984 and 1985 larval feeding periods. Differences in larval feeding success were not detected in 1984. In 1985, however, larvae on trenched (stressed) trees clipped and rejected more foliage, consumed more needles, had lower pupal weights, lower survival, and a longer feeding period than larvae on watered or untreated trees. Frass production did not differ among treatment levels. The length of the feeding period was shorter for larvae on watered trees than for larvae on untreated trees, but other measures of feeding success did not significantly differ between watered and untreated trees.     

Effects of predation risk on population variation in adult size in a stream-dwelling isopod

I used a combination of laboratory experiments and field surveys to examine the role that population-specific predation risk may play in shaping the life history strategy of a stream-dwelling isopod Lirceus fontinalis. Two focal populations were identified that were exposed to different predator types. The first population was exposed to larvae of the streamside salamander (Ambystoma barbouri) and the second to banded sculpin (Cottus carolinae). A laboratory experiment, in which different size classes of prey were offered simultaneously to individual predators, revealed that L. fontinalis suffered greatest mortality risk at small sizes with A. barbouri. Alternatively, with C. carolinae the risk of mortality was independent of size. Life history theory predicts that L. fontinalis from populations exposed to the gape-limited salamander larvae should be larger at maturity relative to individuals from populations exposed to C. carolinae. Field surveys on the two focal populations both within 1 year and across 4 years supported this prediction. Four other populations, two exposed to streamside salamander larvae and two to fish, provided additional support for the prediction. I concluded that L. fontinalis exhibited an adaptive response in size at maturity in response to population-specific predation risk. I then used gut content assays of the major predators to assess whether the population-specific life history strategies adopted by L. fontinalis were successful in avoiding predation.     

Growth and defence in young pine and spruce and the expression of resistance to a stem-feeding weevil

Defence in young trees has been much less studied than defence in older ones. In conifers, resin within ducts in bark is an important quantitative defence, but its expression in young trees may be influenced by developmental or physical constraints on the absolute size of the resin ducts as well as by differential allocation of resources to growth and resin synthesis. To examine these relationships, we used nitrogen fertilisation of 1- and 2-year-old pine and spruce to produce trees of different sizes and measured the effect on the number and size of resin ducts and the amount of resin they contained. All of these variables tended to increase with stem diameter, indicating a positive relationship between resin-based defence and growth of 1- and 2-year-old trees. In pine, however, the mass of resin flowing from severed ducts was much lower relative to duct area in 1- than in 2-year-old trees, suggesting that the older trees allocated a higher proportion of the carbon budget to resin synthesis. Resin-based defence in 1-year-old pines appears to be both positively related to growth and resource limited. In spruce, resin production was generally lower, and age-related differences were not observed, suggesting that resin-based defence is less important in this species. Bio-assays of 2-year-old trees with the pine weevil, Hylobius abietis, emphasised the importance of resin as a defence against this bark feeding insect. Nitrogen fertilisation had a limited influence on resistance expression. One-year-old trees remained susceptible because of their small size, low resin production and limited response to fertilisation. The strong growth response of 2-year-old trees to fertilisation increased resin-based defence, but most spruce trees remained susceptible, while most pines were resistant at all levels of fertilisation.     

Host stress influences oviposition preference and performance of a pine sawfly

Abstract.

• 1 The effect of ponderosa pine (Pinus ponderosa Doug]. ex Laws) water stress, measured by xylem water potential, on oviposition preference and egg development of Neodiprion fulviceps was determined by two laboratory experiments in 1987 and 1988.
• 2 In the first experiment, adult females were allowed to choose between foliage from water-stressed, watered, and untreated control trees.
• 3 Significant differences were found between treatments in xylem water potential in 1987 but not in 1988.
• 4 Females chose water-stressed trees and a positive correlation was found between female preference and xylem water potential in 1987 but not in 1988.
• 5 In the second experiment, females were placed in globe cages with a single branch from each treatment and allowed to oviposit.
• 6 Significant differences were found among treatments in egg survival in 1987 but not in 1988.
• 7 These results indicate that sawflies can distinguish between stressed and non-stressed trees and that this behaviour is adaptive in that egg survival was greatest on the preferred treatment.

     

Influence of wind conditions and intertrap spacing on pheromone trap catches of male european pine sawfly,Neodiprion sertifer

The spatial distribution of catches of male European pine sawfly,Neodiprion sertifer (Geoffr.) (Hymenoptera, Diprionidae), was studied using different pheromone trap arrays. In hexagonal trap groups, trap interference was evident by reduced catch in the central of seven traps spaced 10 to 40 m apart. When the trap spacings were either 5 or 80 m no significant reduction could be shown. The interaction was more pronounced for strong (100 μg) pheromone ([2S, 3S, 7S]-3,7-dimethyl-2-pentadecanyl acetate) lures than for weak ones (10 μg). Similarly, the inner traps caught less than the outer traps in grids of 6×6 traps spaced 20 or 50 m apart. Mark-release-recapture experiments in the grid array confirmed that a majority of the males originated from outside the trap group and were caught in the first trap they encountered. There were no differences between catches in downwind, crosswind and upwind traps. Thus, no ‘overshooting’ was evident during average conditions, i.e. the sawflies did not divert from the trap initially attracting them by flying to the upwind trap. However, the proportion caught in the upwind traps increased with increasing wind velocity, suggesting more overlapping pheromone plumes at higher wind speeds. Also, the recapture rate of released males increased with increasing wind velocity up to a daily average of 3.1 m/s at 2m, indicating that sawflies more easily find the pheromone source, probably due to a more stable pheromone puff trajectory during higher wind velocities. The application of pheromone traps in e.g. monitoring studies is discussed and a distance of at least 50 m between the traps with strong lures used in the present study is recommended to avoid trap interaction.     

Function of Sitka spruce stone cells as a physical defence against white pine weevil

Stone cells are a physical defence of conifers against stem feeding insects such as weevils and bark beetles. In Sitka spruce, abundance of stone cells in the cortex of apical shoot tips is associated with resistance to white pine weevil. However, the mode of action by which stone cells interfere with growth and development of weevil larvae is unknown. We developed a bioassay system for testing potential effects of stone cells, which were isolated from resistant trees, on weevil larvae. Bioassays using artificial diet and controlled amounts of stone cells focused on physical defence. We evaluated the effects of stone cells on establishment of neonate larvae, mandible wear and changes in relative growth rates of third instar larvae. Establishment of neonates and relative growth rates of third instars were significantly reduced by stone cells. Stone cells appeared to be indigestible by weevil larvae. Our results suggest that stone cells affect weevil establishment and development by forming a physical feeding barrier against neonate larvae at the site of oviposition, and by reducing access to nutrients in the cortex of resistant trees, which contain an abundance of stone cells in place of a more nutrient rich tissue in susceptible trees.     

Effects of food plant and group size on predator defence: differences between two co-occurring aposematic Lygaeinae bugs

1. The role of food plant and aggregation on the defensive properties of two aposematic sympatrically occurring seed bugs, Tropidothorax leucopterus and Lygaeus equestris (Lygaeinae, Heteroptera), was investigated. Larvae reared on seeds either of their natural host plant Vincetoxicum hirundinaria (Asclepiadaceae) or of sunflower Helianthus annuus were subjected to predation by chicks. 2. The two species differ in their dependency on the host for their defence. Lygaeus equestris was better defended on its natural host plant than on the alternative food, as indicated by fewer attacks, lower mortality, and predator avoidance after experience. No such effect of food plant could be found for T. leucopterus, suggesting the existence of alternative defences in this species. 3. The number of attacks was lower when host plant‐fed larvae of both species were presented in groups. 4. The discussion concerns how major components of an aposematic syndrome, such as host plant chemistry, insect colouration, and aggregation, are integrated with other life‐history traits to form alternative lifestyles in L. equestris and T. leucopterus.     

Gregariousness,field distribution and defence in the sawfly larvae Croesus varus and C. septentrionalis (Hymenoptera,Tenthredinidae)

Summary The larvae of Croesus varus are cryptic and those of C. septentrionalis aposematic. In both species, females lay eggs in groups, but the groups formed by young larvae are smaller in C. varus than in C. septentrionalis. In both species, group size decreases during successive larval instars, and the cryptic species becomes solitary, while the aposematic species remains gregarious during all larval instars. On Alnus shrubs in the field, the distribution of the groups ofC. septentrionalis larvae is more aggregative. They are found mainly on mediumsized shrubs (1.5–2.5 m high), whereas groups of C. varus larvae are also found on larger shrubs. Moreover, during the 2 years of observation, C. septentrionalis was present for a shorter time than C. varus, even when the former was much more abundant than the latter. Thus, conspicuousness of the aposematic species should be amplified by this spatial and temporal aggregation. The efficiency of both defensive strategies is compared and discussed, taking into account these particular field distributions, predation and some parasitism factors, and the chemical defences of both species.     

Effects of plant quality and ant defence on herbivory rates in a neotropical ant‐plant

1. Understanding the degree to which populations and communities are limited by both bottom‐up and top‐down effects is still a major challenge for ecologists, and manipulation of plant quality, for example, can alter herbivory rates in plants. In addition, biotic defence by ants can directly influence the populations of herbivores, as demonstrated by increased rates of herbivory or increased herbivore density after ant exclusion. The aim of this study was to evaluate bottom‐up and top‐down effects on herbivory rates in a mutualistic ant‐plant. 2. In this study, the role of Azteca alfari ants as biotic defence in individuals of Cecropia pachystachya was investigated experimentally with a simultaneous manipulation of both bottom‐up (fertilisation) and top‐down (ant exclusion) factors. Four treatments were used in a fully factorial design, with 15 replicates for each treatment: (i) control plants, without manipulation; (ii) fertilised plants, ants not manipulated; (iii) unfertilised plants and excluded ants and (iv) fertilised plants and ants excluded. 3. Fertilisation increased the availability of foliar nitrogen in C. pachystachya, and herbivory rates by chewing insects were significantly higher in fertilised plants with ants excluded. 4. Herbivory, however, was more influenced by bottom‐up effects – such as the quality of the host plant – than by top‐down effects caused by ants as biotic defences, reinforcing the crucial role of leaf nutritional quality for herbivory levels experienced by plants. Conditionality in ant defence under increased nutritional quality of leaves through fertilisation might explain increased levels of herbivory in plants with higher leaf nitrogen.     

Hexanoic acid protects tomato plants against Botrytis cinerea by priming defence responses and reducing oxidative stress

Treatment with the resistance priming inducer hexanoic acid (Hx) protects tomato plants from Botrytis cinerea by activating defence responses. To investigate the molecular mechanisms underlying hexanoic acid‐induced resistance (Hx‐IR), we compared the expression profiles of three different conditions: Botrytis‐infected plants (Inf), Hx‐treated plants (Hx) and Hx‐treated + infected plants (Hx+Inf). The microarray analysis at 24 h post‐inoculation showed that Hx and Hx+Inf plants exhibited the differential expression and priming of many Botrytis‐induced genes. Interestingly, we found that the activation by Hx of other genes was not altered by the fungus at this time point. These genes may be considered to be specific targets of the Hx priming effect and may help to elucidate its mechanisms of action. It is noteworthy that, in Hx and Hx+Inf plants, there was up‐regulation of proteinase inhibitor genes, DNA‐binding factors, enzymes involved in plant hormone signalling and synthesis, and, remarkably, the genes involved in oxidative stress. Given the relevance of the oxidative burst occurring in plant–pathogen interactions, the effect of Hx on this process was studied in depth. We showed by specific staining that reactive oxygen species (ROS) accumulation in Hx+Inf plants was reduced and more restricted around infection sites. In addition, these plants showed higher ratios of reduced to oxidized glutathione and ascorbate, and normal levels of antioxidant activities. The results obtained indicate that Hx protects tomato plants from B. cinerea by regulating and priming Botrytis‐specific and non‐specific genes, preventing the harmful effects of oxidative stress produced by infection.     

Multiple effects of temperature,photoperiod and food quality on the performance of a pine sawfly

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Effects of nitrogen fertilization on pine needle chemistry and sawfly performance

Summary Changes in needle nitrogen and resin acid concentrations in young Scots pine trees fertilized with ammonium nitrate were followed over 3 years. Sawfly larvae (Neodiprion sertifer) were reared on fertilized and control trees the year after fertilization. Both nitrogen and resin acid concentrations increased in fertilized trees. The fact that resin acid concentrations increased contradicts predictions of the carbon/nutrient balance hypothesis. We suggest that needle resin-acid concentrations are limited more by the size of the resin ducts than by the availability of substrate for resin acid synthesis, and that the formation of resin ducts is limited by the availability of nitrogen. A modification of the carbon/nutrient balance hypothesis, relating compartment formation to allelochemical synthesis, is discussed. Performance of sawfly larvae was not affected by fertilization treatment, probably because concentrations of nitrogen (positively affecting performance) and resin acids (adversely affecting performance) increased simultaneously in fertilized trees. Thus, the results of this study do not support the notion that fertilization increases the resistance of trees to needle-eating insects.     

Influence of host plant vs. natural enemies on the spatial distribution of a pine sawfly, Neodiprion autumnalis

Abstract .1. The pine sawfly, Neodiprion autumnalis , infests ponderosa pine, Pinus ponderosa , growing at low densities near the bottom of an altitudinal gradient in Arizona, U.S.A. The relative importance of host-plant quality vs. natural-enemy effects in determining the spatial distribution of this sawfly was examined over a 3-year period.
2. Field and laboratory bioassays were conducted on all life stages of N. autumnalis at two forest stand densities (high ≥ 23 m² ha^–1, low ≤ 7 m² ha^–1) and at two elevations (bottom slope = 2390 m, top slope = 2540 m). These experiments were used for constructing life tables of N. autumnalis that compared the effects of host-plant quality on oviposition preference and progeny performance with the effects of natural enemies at different tree densities and elevations.
3. Life-table analyses determined that mortality attributed to host-plant effects during the egg and larval stages had the largest impact on fitness between tree densities and elevations.
4. Natural enemies caused a significant reduction in progeny survival, but their effects were similar across all tree densities and elevations during egg and larval life stages. However, cocoon-stage survival did vary between tree densities and elevations due to natural-enemy effects.
5. It was concluded that the observed oviposition preference for, and higher progeny performance on, trees at low densities and bottom slope elevations were caused primarily by host-plant effects.
6. These results further the argument that heterogeneity at the resource level (i.e. bottom-up forces) determines potential outcomes of multitrophic level interactions.     

Effects of light on direct and indirect defences against herbivores of young plants of Mallotus japonicus demonstrate a trade-off between two indirect defence traits

Background and Aims

Although most studies on plant defence strategies have focused on a particular defence trait, some plant species develop multiple defence traits. To clarify the effects of light on the development of multiple defence traits, the production of direct and indirect defence traits of young plants of Mallotus japonicus were examined experimentally under different light conditions.

Methods

The young plants were cultivated under three light conditions in the experimental field for 3 months from May to July. Numbers of ants and pearl bodies on leaves in July were examined. After cultivation, the plants were collected and the developments of trichomes and pellucid dots, and extrafloral nectaries (EFNs) on the leaves were examined. On plants without nectar-collecting insects, the size of EFNs and the volume of extrafloral nectar secreted from the EFNs were examined.

Key results

Densities of trichomes and pellucid dots did not differ significantly among the plants under the different light conditions, suggesting that the chemical and physical defences function under both high and low light availability. The number of EFNs on the leaves did not differ significantly among the plants under the different light conditions, but there appeared to be a trade-off between the size of EFNs and the number of pearl bodies; the largest EFNs and the smallest number of pearl bodies were found under high light availability. EFN size was significantly correlated with the volume of extrafloral nectar secreted for 24 h. The number of ants on the plants was smaller under low light availability than under high and moderate light availability.

Conclusions

These results suggest that direct defence traits function regardless of light conditions, but light conditions affected the development of indirect defence traits.