Acidic precipitation increases egg survival in Neodiprion sertifer

The effects of simulated acidic precipitation on the egg viability of the European pine sawfly, Neodiprion sertifer Geoffroy (Hymenoptera: Diprionidae) were studied by spraying egg clusters with a mixture of sulphuric and nitric acid (1:1, pH 2, 3, 4, 5, 6, distilled water, natural precipitation) for a month in April–May. Egg viability analyses showed that the proportion of hatched first instar larvae increased with increasing acidity. The better survival of sawfly eggs in pine needles in more acidic conditions may be partly due to changes in needle physiology affecting egg mortality. However, when the eggs were reared without needles in Petri-dishes, the proportion of successfully hatched larvae likewise increased with increasing acidity indicating direct acid-induced changes in the eggs themselves. Acidic precipitation as a stress factor of the host plant apparently changes population dynamics of pine sawflies and may be an important factor in triggering outbreaks.     

Mating disruption of the pine sawfly Neodiprion sertifer (Hymenoptera: Diprionidae) in isolated pine stands

• 1 Previous applications of the mating disruption technique to Neodiprion sertifer resulted in decreased numbers of males caught in the treated area but no effect on sex ratio or overall population density.
• 2 The present study assessed the efficacy of mating disruption against N. sertifer outbreaks in pine stands surrounded by agricultural areas or pasture, and therefore isolated from other infested areas.
• 3 Pine stands were treated by placing dispensers with an erythro‐mixture of the acetate ester of 3,7‐dimethyl‐2‐pentadecanol every 10 m in a grid.
• 4 The efficacy of this technique was evaluated by comparing the number of males caught in sticky traps baited with synthetic pheromone, and by comparing the sex ratio and the population density of the sawfly in the subsequent generation between treated and control pine stands.
• 5 The number of males caught within treated areas was significantly lower than in the control area.
• 6 In the treated pine stands 46% of the egg clusters resulted in male cocoons only, compared to 3% of the clusters in the control stand.
• 7 A significant reduction of the sawfly population was observed in the treated pine stands. The results contrast to the earlier mating disruption attempts with N. sertifer and can probably be ascribed to the isolation of the experimental stands.

     

Monitoring the European pine sawfly with pheromone traps in maturing Scots pine stands

Abstract

• 1 During 1989–93, field studies were conducted in Finland to develop a method based on pheromone traps to monitor and forecast population levels of the European pine sawfly (Neodiprion sertifer Geoffr.) and tree defoliation.
• 2 Three traps per site were baited with 100 µg of the N. sertifer sex pheromone, the acetate ester of (2S,3S,7S)‐3,7‐dimethyl‐2‐pentadecanol (diprionol), in maturing pine stands in southern and central Finland. In addition, three different dosages (1, 10 and 100 µg) of the pheromone were tested in 1991–92.
• 3 The highest number of males was observed in traps baited with the highest dose. On average, there was a 10‐fold increase in trap catch between lure doses.
• 4 Density of overwintering eggs was used to evaluate the effectiveness of pheromone traps in predicting sawfly populations. The proportion of healthy overwintering eggs was determined each year. A model based on the number of current shoots on sample trees, diameter at breast height and tree height was formulated to estimate eggs per hectare.
• 5 Linear regression analysis produced high coefficients of determination between number of males in traps and density of total eggs in the subsequent generation, when populations were at peak densities. The relationships were not significant for low population densities. The results indicate a risk of moderate defoliation when the seasonal trap catch is 800–1000 males per trap or higher.

     

Aggregative oviposition varies with density in processionary moths—Implications for insect outbreak propensity

1. In gregarious insects, groups commonly originate from females laying eggs in masses and feeding groups are established as soon as larvae hatch. Some group-living insect species may aggregate beyond the individual parent level, such that offspring from two or more egg masses develop within a common resource.
2. Here we show that aggregative oviposition can vary with population density at oviposition and possibly be an important factor in outbreak dynamics of phytophagous insects.
3. We analysed density data with respect to egg mass aggregation for two species of pine processionary moths, Thaumetopoea pinivora (in Sweden 2005–2019) and T. pityocampa (in Spain 1973–1991). Both species lay their eggs in egg masses and feed in groups. During the study periods, insect population density for both species varied by at least an order of magnitude.
4. The two species showed strikingly similar patterns of egg mass aggregation. Egg masses were overdispersed at high population density, with few trees showing a high load of egg masses.
5. Our data suggest that aggregative oviposition can be important in explaining the previously documented higher propensity for outbreaks in insects laying eggs in clusters, compared with those laying individual eggs.

     

Local variation in overwintering stages ofPanonychus akitanus Ehara (Acarina: Tetranychidae)

The overwintering behaviour ofPanonychus akitanus Ehara was compared in a population (Sapporo) overwintering as the egg and the adult female and in a population (Tomakomai) overwintering in the egg stage only. In both populations, winter eggs were larger in size and darker in colour than summer eggs, and females laying winter eggs showed clear differences in body colour and behaviour from summer females. Both populations produced winter eggs in response to short photoperiods, and the critical photoperiod was between 13.0 and 13.5 h in the Sapporo population and between 13.5 and 14.0 h in the Tomakomai population. The hatchability of winter eggs of the Sapporo population collected from the field began to increase in November, and that of the Tomakomai population in December. In the Sapporo population, a 40-day or more exposure to 5°C promoted hatching of winter eggs, and a 50-day exposure of females which had previously laid some winter eggs to 5°C induced the production of summer eggs. Females of the Sapporo population laid winter eggs in autumn, and then themselves overwintered as well, and produced summer eggs in the following spring. These females were called winter-habit females.The winter eggs of both populations were diapausing. However, the winter-habit females of the Sapporo population were not reproductively inactive and their condition of diapause was not certain. The ecological significance of the winter-habit females in the Sapporo population is discussed.     

Interpopulation variation in egg size in the winter cherry bug relative to oviposition site selection

To investigate whether or not oviposition on substrates other than host plants (e.g., non‐host plants, abiotic materials) would affect the evolution of egg size in phytophagous insects, we studied the winter cherry bug, Acanthocoris sordidus (Thunberg) (Heteroptera: Coreidae), as a model organism for its interpopulation variation in oviposition preference. The rate of oviposition off host plants is much higher in the Amami Island population than in either the Kyoto or Kochi populations. We compared egg size and number among the three local populations from Kyoto, Kochi, and Amami Island. In addition, to evaluate the adaptive significance of larger eggs for offspring in terms of searching for host plants, we examined the relationship between egg size and first‐instar body size. We also searched for a relationship between egg size and starvation tolerance in the second instars because first instars can develop to second instars without food intake, and thus the substantial host‐searching stage is the second instar, when females lay their eggs off host plants. Females from the Amami population produced fewer larger eggs than females from either the Kyoto or Kochi population. Regardless of the local population, the body size of first instars that emerged from larger eggs was larger, and the second instars originating from larger eggs had a higher starvation tolerance. The larger body size and higher starvation tolerance should enable nymphs to disperse further, which may enhance the probability of successfully reaching host plants. These results suggest that egg size in A. sordidus may be determined in relation to its oviposition habits to maximize reproductive success, resulting in interpopulation variation in egg size.     

First use of pheromones to detect phenology patterns and density relationships of pine sawflies in German forests

1 Monitoring studies of pine sawflies with pheromone traps were performed for the first time in Germany. Pheromone traps baited with species‐specific pheromone substances were installed in pine forests at different locations in Bavaria, Brandenburg and Lower Saxony during two years. 2 It was possible to track the flight phenology of Diprion pini, Gilpinia pallida and Neodiprion sertifer reliably and to get information about the number of generations of these species in 1997 and 1998. 3 A clear relationship between trap catch and population density could not be detected, but qualitative changes in trap catch caused by different density levels were observed. 4 For D. pini, trap catches were different among endemic populations of different forest types. Furthermore, catches of males reflected the results from the regular cocoon collections by foresters during the previous winter. 5 For N. sertifer, trap catches in endemic populations were well separated from trap catches on sites with higher sawfly densities. However, no significant correlation between trap catch and sawfly density or defoliation level could be found. These results suggest that the efficacy of the pheromone traps probably varied with biological features (sex ratio, density level, immigration) of the particular population.     

Die Parasitenkomplexe europäischer Diprionidae in ökologisch-evolutionsbiologischer Sicht

Parasite community of European Diprionidae from an ecological-evolutionary point of view Neodiprion sertifer, the only Eurasian species of this nearctic genus, is systematically and, due to its hibernation in the egg stage and its aestivation in the cocoon-stage, also biologically an outsider in the European Diprionid-fauna (Fig. 1). It is assumed that the ancestors of N. sertifer immigrated from western North America via Beringia to Eurasia during the Pleistocene period In spite of its exceptional status, this sawfly species has a rich parasitoid complex, characterized by several highly specific larval parasites, which, in size und diversity, is equivalent to the parasite-spectra of autochthonous European Diprionids such as Diprion pini, Microdiprion pallipes, etc. (Tab. 1 and 2). Based on a comparative analysis of these parasite complexes, a trial is made to explain 1. from which hosts the parasites of N. sertifer have originated and 2. which biological adaptations have allowed them to colonize the new host niche and to synchronize with the strictly univoltine life-cycle of N. sertifer. The boreo-montane Murodiprion pallipes has apparently played a key rôle as a source for parasitoid species of N. sertifer. Adaptation to the new host has been achieved by the acquisition of an obligatory summer diapause in such highly specific larval parasites as Exenterus abruptorius, Lamachus eques, Lophyroplectus luteator (Fig. 5) and in the «sertifer-form» of the egg parasite Dipriocampe diprioni (Fig. 3), whereas in Chrysonotomyia spp. the ability to develop as facultative hyperparasites has enabled them to bridge the summer gap when fresh host eggs are not available (Fig. 3 und 4).     

Larval host plant affects fitness consequences of egg size variation in the seed beetle Stator limbatus

Egg size variation often has large effects on the fitness of progeny in insects. However, many studies have been unable to detect an advantage of developing from large eggs, suggesting that egg size variation has implications for offspring performance only under adverse conditions, such as during larval competition, periods of starvation, desiccation, or when larvae feed on low-quality resources. We test this hypothesis by examining the consequences of egg size variation for survivorship and development of a seed-feeding insect, Stator limbatus, on both a low-quality (Cercidium floridum) and a high-quality (Acacia greggii) host plant. Our results are consistent with the hypothesis. S. limbatus larval performance was affected by egg size only when developing on the poor-quality host (C. floridum); larvae from large eggs survived better on C. floridum than those from small eggs, while there was no evidence of an effect of egg size on progeny development time, body weight, or survivorship when larvae developed on A. greggii. These results indicate intense selection for large eggs within C. floridum-associated populations, but not in A. greggii-associated populations, so that egg size is predicted to vary among populations associated with different hosts. Our results also support this hypothesis; females from a C. floridum-associated population (Scottsdale) laid larger eggs than females from an A. greggii-associated population (Black Canyon City).     

Cocoon predation on diprionid sawflies: the effect of forest fertility

Predation by small mammals is thought to be one of the main regulators of outbreaking sawfly species. It has been suggested that predation may be lower in poor and dryish forests, and this is the reason why outbreaks often begin from this type of environment. We studied experimentally how fertility of the forest site affects cocoon predation experienced by two sawfly species, the common pine sawfly Diprion pini (Linnaeus) and the European pine sawfly Neodiprion sertifer (Geoffroy). We applied a fertilization treatment to selected pine-dominated barren forest sites in Finland, and 2–4 years later monitored predation on the sawfly cocoons in fertilized and control areas. The results did not support the idea that forest fertility was related to cocoon predation. We also could not verify that small mammal abundance was related to fertility of the forest. The most obvious pattern we observed was that the two sawfly species differed dramatically in predation experienced. N. sertifer has its cocoon phase in mid-summer and experienced only moderate predation (37%) whereas D. pini, with its cocoon phase in autumn, suffered from very heavy predation (96%). Our observations suggest that if predation is important in controlling the population dynamics of the species, its impact depends more on the sawfly species and season than on the fertility of the forest site. Received: 1 March 1998 / Accepted: 25 May 1998     

Egg size manipulation in the migrant skipper, Parnara guttata guttata (Lepidoptera: Hesperiidae), in response to different host plants

Female adults of the migrant skipper, Parnara guttata guttata, lay different-sized eggs on different host plants in different generations. Moreover, P. g. guttata maintains large egg size variation either in the population or in the individual. Why such phenotypic plasticity in egg size is maintained has not been clarified. In the present study we performed a series of experiments to verify whether or not females of P. g. guttata discriminate between the different host plants, i.e., rice plant Oriza sativa in the first and second generations and cogon grass Imperata cylindrica in the third (overwintering) generation and manipulate egg size accordingly on these host plants. When a caged female was allowed to lay eggs alternatively on soft-leafed rice plant and tough-leafed cogon grass, the size of the eggs laid on cogon grass was significantly larger than that on rice plant. When a caged female was allowed to lay eggs on the two host plants that were supplied on alternate days, the size of eggs laid on cogon grass was also significantly larger than that on rice plant. A preliminary experiment using crude extract from the plants suggested that females did not manipulate egg size in response to chemical stimulants alone. The skipper female was able to lay different-sized eggs instantaneously after assessing the host plant. However, the reaction norm to different host plants differed among females. Eggs of various sizes were matured in each ovariole, and egg size variation at the lowest part of the ovariole ranged from the size of the larger eggs laid on cogon grass to that of the smaller eggs laid on rice plant, although how the female chose the appropriate-sized egg at each occasion is unknown.     

Neodiprion sertifer defoliation causes long-term systemic changes of oxidative enzyme activities in Scots pine needles

The aim of the experiments reported here was to study possible long-term effects of Neodiprion sertifer Geoff. (Hymenoptera: Diprionidae) herbivory, or artificial defoliation, on oxidative enzyme activities in Scots pine (Pinus sylvestris L.) needles as a consequence of induced defense responses. During year 1 (the first season), defoliation by N. sertifer, which feeds on previous season’s needles, did not result in statistically significant changes in polyphenol oxidase activity in the current year’s needles. In contrast, defoliation did lead to increased peroxidase activity in those needles. In the second season (year 2) N. sertifer defoliation of pine seedlings, also defoliated in the previous season either by larvae or artificially, resulted in a decrease of peroxidase activity in the current year’s needles. No significant differences between treatments carried out in year 1 were found in year 2 for peroxidase activity in the previous year’s needles. However, defoliation in year 1 by N. sertifer resulted in decreased needle consumption, and higher mortality of larvae, in year two. These results indicate the existence of long-term changes in needle oxidative enzyme activities as a consequence of N. sertifer feeding.     

Mechanical-Statistical Modeling in Ecology: From Outbreak Detections to Pest Dynamics

Knowledge about large-scale and long-term dynamics of (natural) populations is required to assess the efficiency of control strategies, the potential for long-term persistence, and the adaptability to global changes such as habitat fragmentation and global warming. For most natural populations, such as pest populations, large-scale and long-term surveys cannot be carried out at a high resolution. For instance, for population dynamics characterized by irregular abundance explosions, i.e., outbreaks, it is common to report detected outbreaks rather than measuring the population density at every location and time event. Here, we propose a mechanical-statistical model for analyzing such outbreak occurrence data and making inference about population dynamics. This spatio-temporal model contains the main mechanisms of the dynamics and describes the observation process. This construction enables us to account for the discrepancy between the phenomenon scale and the sampling scale. We propose the Bayesian method to estimate model parameters, pest densities and hidden factors, i.e., variables involved in the dynamics but not observed. The model was specified and used to learn about the dynamics of the European pine sawfly (Neodiprion sertifer Geoffr., an insect causing major defoliation of pines in northern Europe) based on Finnish sawfly data covering the years 1961–1990. In this application, a dynamical Beverton–Holt model including a hidden regime variable was incorporated into the model to deal with large variations in the population densities. Our results gave support to the idea that pine sawfly dynamics should be studied as metapopulations with alternative equilibria. The results confirmed the importance of extreme minimum winter temperatures for the occurrence of European pine sawfly outbreaks. The strong positive connection between the ratio of lake area over total area and outbreaks was quantified for the first time.     

Absence of clinal variation in virgin retention capacity in Australian Drosophila melanogaster

The ability of virgin Drosophila melanogaster adults to retain eggs is thought to be an adaptation to persisting in temperate areas, based on differences in this trait between European and African populations, and based on seasonal changes in this trait in France. By retaining eggs in the absence of males and under conditions of poorer nutrition (conditions common in temperate areas during colder months), females reduce the wastage of resources and increase their probability of surviving spring into summer, enabling them to initiate summer population expansions. To test for variation in virgin egg retention along a climatic gradient, we characterized clinal variation in strains collected from eastern Australia extending from temperate Tasmania to tropical northern Queensland. Despite testing a large number of strains and repeated testing of the cline ends, we did not detect any evidence for clinal variation in virgin egg retention. Therefore although D. melanogaster in temperate Australia overwinter at the adult stage, there is no evidence for selection on virgin retention capacity producing clinal patterns. This contrasts with other evidence for clinal variation in egg production patterns over winter.     

Egg load and adaptive superparasitism in Anaphes nitens, an egg parasitoid of the Eucalyptus snout-beetle Gonipterus scutellatus

We studied egg production and the occurrence of adaptive superparasitism in Anaphes nitens, an egg parasitoid of the Eucalyptus snout beetle Gonipterus scutellatus. First, we determined whether A. nitens females were synovigenic or pro‐ovigenic. Newly emerged females were allowed to lay eggs alone during 3 days on six fresh egg capsules. A first group of females (n = 25) were killed by freezing and the remaining females (n = 21) were maintained during two extra days with food, but without hosts. Their fecundity was measured by dissection of host eggs and females’ ovarioles. We found that the second group of females increased their fecundity by about 20%, suggesting they were weakly synovigenic. To test for the occurrence of adaptive superparasitism in relation to competitors’ density, we compared the oviposition behaviour of females kept alone, in pairs, or in groups of four during patch visit. Results indicated that the females superparasited significantly more often in this last treatment. Synovigeny and the ability to modulate the use of superparasitism could be mentioned as important attributes that allow A. nitens to efficiently control the pest population.     

Spatial patterns at host and forest stand scale and population regulation of the pine processionary moth Thaumetopoea pityocampa

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Geographical variation in reproductive traits and trade‐offs between size and number of eggs in the king ratsnake,Elaphe carinata

We collected gravid king ratsnakes (Elaphe carinata) from three geographically separated populations in Chenzhou (CZ), Lishui (LS) and Dinghai (DH) of China to study the geographical variation in female reproductive traits and trade‐offs between the size and number of eggs. Not all reproductive traits varied among the three populations. Of the traits examined, five (egg‐laying date, post‐oviposition body mass, clutch size, egg mass and egg width) differed among the three populations. The egg‐laying date, ranging from late June to early August, varied among populations in a geographically continuous trend, with females at the most northern latitude (DH) laying eggs latest, and females at the most southern latitude (CZ) laying eggs earliest. Such a trend was less evident or even absent in the other traits that differed among the three populations. CZ and DH females, although separated by a distance of approximately 1100 km as the crow flies, were similar to each other in most traits examined. LS females were distinguished from CZ and DH females by the fact that they laid a greater number of eggs, but these were smaller. The egg size–number trade‐off was evident in each of the three populations and, at a given level of relative fecundity, egg mass was significantly greater in the DH population than in the LS population. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 104 , 701–709.     

Aerial application of mixed virus formulations to control joint infestations of Panolis flammea and Neodiprion sertifer on lodgepole pine

An aerial spray using mixed formulations of two viruses, Panolis flammea nuclear polyhedrosis virus (NPV) and Neodiprion sertifer NPV was conducted against mixed populations of the two pests, P. flammea and N. sertifer on lodgepole pine. These were compared with single virus formulations sprayed against their respective host pests and an unsprayed control. One hectare plots of 14 yr old pine were sprayed by helicopter using ultra low volume techniques. Larvae from each plot were assessed weekly for virus and this showed that the mixed formulations gave the same level of control as separate applications of each NPV. Levels of infection in P. flammea reached 95 – 100% 9 wk post-spray, and in N. sertifer reached 100% 6 wk post-spray. Later P. flammea pupal sampling showed no viral infection in the control but significant levels in sprayed plots, which ranged from 37–9% in the plot sprayed with the high dose of N. sertifer NPV to 75% in the plot sprayed with the low dose of NsNPV.     

Influence of induced water stress in ponderosa pine on pine sawflies

Summary Three levels of water stress were induced on pole-size ponderosa pine (Pinus ponderosa) to determine the influence of plant moisture stress on oviposition, survival, and growth of two species of pine sawfly (Neodiprion fulviceps and N. autumnalis). It was found that water stress affected oviposition and subsequent egg survival but not larval development or survival. Stress had a negative effect on early season oviposition (N. fulviceps) and a positive effect on late season oviposition (N. autumnalis). Egg hatch was different between species and years and among treatment levels. Larval development, feeding, and survival were not affected by water stress. Overall, the effect of stress was not sufficient to explain population outbreaks of sawflies. Several hypotheses are presented as possible explanations for the observed experimental results.     

Phenotypic divergence in reproductive traits of a moth population experiencing a phenological shift

Allochrony that is reproductive isolation by time may further lead to divergence of reproductive adaptive traits in response to different environmental pressures over time. A unique “summer” population of the pine processionary moth Thaumetopoea pityocampa, reproductively isolated from the typical winter populations by allochronic differentiation, is here analyzed. This allochronically shifted population reproduces in the spring and develops in the summer, whereas “winter” populations reproduce in the late summer and have winter larval development. Both summer and winter populations coexist in the same pine stands, yet they face different climatic pressures as their active stages are present in different seasons. The occurrence of significant differences between the reproductive traits of the summer population and the typical winter populations (either sympatric or allopatric) is thus hypothesized. Female fecundity, egg size, egg covering, and egg parasitism were analyzed showing that the egg load was lower and that egg size was higher in the summer population than in all the studied winter populations. The scales that cover the egg batches of T. pityocampa differed significantly between populations in shape and color, resulting in a looser and darker covering in the summer population. The single specialist egg parasitoid species of this moth was almost missing in the summer population, and the overall parasitism rates were lower than in the winter population. Results suggest the occurrence of phenotypic differentiation between the summer population and the typical T. pityocampa winter populations for the life‐history traits studied. This work provides an insight into how ecological divergence may follow the process of allochronic reproductive isolation.