Impact of cocoon predation and parasitism on endemic populations of the common pine sawfly, Diprion pini (L.) (Hymenoptera, Diprionidae) in different forest types

1 Predation and parasitism on litter‐buried cocoons of the common pine sawfly Diprion pini (L.) were compared in different forest types with endemic sawfly populations by field exposure of laboratory‐reared cocoons during three consecutive years (1993–1995). 2 The impact of cocoon predation was dependent on season and forest type. The highest predation (up to 95%) was found during autumn in forest stands with a dense understory vegetation. 3 Cocoon parasitism varied between year, season and forest type. The highest parasitoid attack was observed in pure pine forests with more or less barren soils, but did not exceed 24% of exposed cocoons. 4 Cocoons were exposed in small patches. Predators tended to exploit all cocoons of a patch, whereas parasitoids only attacked a few cocoons of a patch. Predation was similar on cocoons placed in the litter and those buried more deeply in the soil, whereas parasitism of soil‐buried cocoons was rare. 5 These results indicate that predators can have a remarkable potential for limiting endemic sawfly densities, if habitat conditions in a forest maintain their population and support their foraging behaviour. A notable effect of parasitoids on sawfly cocoons deposited in the litter is obviously restricted to typical pure and barren pine forests, but may play there a similar role as predation.     

The effects of 20-hydroxyecdysone on breaking diapause of Diprion pini L. (Hym., Diprionidae)

Abstract .The pine sawfly, Diprion pini L, like almost all Diprionidae, has complex diapause phenomena which affect the prepupal stages (eonymph and pronymph) in the cocoon. Experiments using 20-hydroxyecdysone to break diapause of different populations of D. pini , were conducted. Doses of 2 and 10 μg/g of the hormone were injected into the insects in diapause, previously removed from their cocoons and placed in transparent capsules to observe their development. The experiments showed that 20-hydroxyecdysone may cause D. pini to resume development and may even totally break the diapause. However, this reactivation seemed to require large doses of 20-hydroxyecdysone. Moreover, the effects of applying this hormone varied according to the insect's origin, duration of the diapause maintenance and conditions of photoperiod and temperature present before and after the treatment. According to the cases, this resumption of development can affect a variable fraction of individuals and be more or less complete. It is suggested that the reaction to 20-hydroxyecdysone in standard conditions could be used to measure the intensity of the diapause of natural D. pini populations.     

Sex pheromone for monitoring flight periods and population densities of the pine sawfly, Diprion jingyuanensis Xiao et Zhang (Hym., Diprionidae)

Abstract:  The pine sawfly, Diprion jingyuanensis Xiao et Zhang, is a serious pest of Chinese pine ( Pinus tabulaeformis Carr.) in the northern parts of China. The general biology of this recently described sawfly species is little known and in the present study we used a synthetic pheromone to monitor its flight period and to evaluate the possibility to use pheromone traps as a tool for estimating and predicting population densities. The attractant pheromone of D. jingyuanensis is (1 S ,2 R ,6 R )-1,2,6-trimethyldodecyl propionate and in this study we used a four-isomer threo-mixture containing the active attractant. Both doses tested, 1 and 2 mg/trap, revealed the same seasonal flight pattern, the higher dosage demonstrating more clear flight peaks. The first flight peak of D. jingyuanensis occurred in mid-June during all 3 years, 1997–1999, and in 1997 and 1998 a second flight peak was also recorded in mid-July. The flight period was similar in time for populations located at higher (1400 m) and lower altitude (1100 m), from early June to late July or early August. Temperature was the main weather factor-affecting trap catches. Diurnal flight activity began at 9.00, peaked at 13.00 and lasted until 20.00. A series of tests with traps in different positions within stands and trees were conducted and the results demonstrated the importance of standardizing the trapping protocol in population monitoring studies. By using traps with 2 mg baits it is possible to detect sawfly occurrence at very low population densities, not detectable by other means. Strong positive correlations between trap catch and relative population density were found in 2000 and 2001, but not in 1998, when only few sites were monitored and the population was in a decreasing phase. The results are promising for future large-scale use of pheromone-based monitoring of D. jingyuanensis .     

Zur Kenntnis der Kleinen Kiefern-Buschhornblattwespe,Diprion (Microdiprion) pallipes (Fall.) (Hym., Diprionidae)

Contributions to the knowledge of Diprion (Microdiprion) pallipes (Fall.) (Hym., Diprionidae). 1. Systematics, distribution and morphology Studies were made on D. pallipes var. politum injurious to young pine stands near Munich as well in laboratory as in high-fens and pine cultures. The Tenthredinid is native within the Alps and their foreland and had migrated northward along the river Isar in last decades. Part 1 deals with problems of systematics, distribution and morphology. D. pallipes (Fall.) is to be found in two European areas: a nominate race (D. pallipes pallipes) in northern Europe and a southern race (D. pallipes politum) in the alpine region. The original food plant of the alpine race is Pinus mugo Turra. The most important morphological characteristics of the developmental stages of the subspecies are described.     

Altitudinal variation in foliar chemistry and anatomy of yunnan pine, Pinus yunnanensis, and pine sawfly (Hym., Diprionidae) performance

Host plant characteristics associated with the larval feeding performance of a pine sawfly were examined across an altitudinal gradient in south-western China. Neodiprion xiangyunicus (Xiao and Huang) larval performance (e.g. larval mass and potential fecundity) and defoliation intensity increased at higher altitudes along an altitudinal gradient in Sichuan, China. Needle chemistry and anatomy of yunnan pine, Pinus yunnanensis (Franch.), also varied across along an altitudinal gradient. In general, total sesquiterpene levels decreased with increasing altitude, oxygenated monoterpenes increased with increasing altitude, and total monoterpenes did not vary significantly across the altitudes. Concentrations for 16 of the 28 individual terpenes varied significantly between at least two of the three altitudes; the vast majority (75%) of these differences were for sesquiterpenes. Foliar nutrition, morphology and anatomy also varied by altitude. Foliar nitrogen and needle dry biomass were significantly lower in trees at the high altitude site. Needle epidermal thickness and diameters of resin ducts were significantly thicker and wider, respectively, in trees at the middle altitude than at the top and bottom altitudes. These results suggest that a combination of foliage chemistry and morphology rather than any single plant trait probably determines the observed patterns of insect herbivory.     

Variation in the suitability of Pinus sylvestris to feeding by two defoliators, Diprion pini (Hym., Diprionidae) and Graellsia isabellae galliaegloria (Lep., Attacidae)

Abstract: Feeding bioassays were conducted on several Pinus sylvestris clones to establish if there were any differences in suitability for two pine defoliators: the sawfly Diprion pini L. (Hym., Diprionidae), which causes considerable damage in Europe, and the rare and protected moth Graellsia isabellae galliaegloria Oberthür (Lep., Attacidae). There were significant differences in survival, weight, sex-ratio and female fecundity of D. pini on Scots pine clones. However, sawfly survival appeared to be the most stable variable in time and three clone categories were revealed by conducting feeding bioassays on 16 clones. The survival of G. isabellae galliaegloria until the formation of the chrysalis was significantly different among clones. It varied from 35 to 75% but no clear clone category was distinguished . The two clone classifications according to insect survival showed differences. In particular, one unsuitable clone for D. pini development appeared to be favourable to moth development. The 3-carene richness of foliage was significantly linked with sawfly survival and the monoterpene composition of foliage was not linked with G. isabellae galliaegloria survival.     

Interspecific competition between the principal larval parasitoids of the pine sawfly,Neodiprion sertifer (Geoff.) (Hym.: Diprionidae)

Summary Interspecific competition between the 3 principal larval parasitoids of the pine sawfly, Neodiprion sertifer, is of common occurrence when total larval parasitism, and hence multiple attack rates, are high. At the intrinsic level, the ectoparasitoid Exenterus abruptorius is superior to the 2 endoparasitoids, Lophyroplectus luteator and Lamachus eques, respectively, whereas L. luteator is superior to L. eques only. During mass outbreaks of the host competition between Exenterus and Lophyroplectus is most intense, whilst Lamachus fails to build up in the presence of its 2 competitors (Fig. 1). However, due to its superior host finding ability, Lamachus dominates in areas of low host densities where the 2 other species are less efficient (Table 1). L. luteator frequently sustains heavy losses in direct competition with E. abruptorius, but these can be largely compensated in the next generation because its fertility is about 5-times higher than that of Exenterus.     

Genetic diversity within and among populations of Diprion pini (Hym., Diprionidae) determined by random amplified polymorphic DNA-polymerase chain reaction of haploid males

Abstract: Based on haploid males, random amplified polymorphic DNA (RAPD) markers were used to study genetic variation within and among four French populations and one Finnish outgroup population of the common pine sawfly, Diprion pini (L.), representing a severe European forest pest associated with mass outbreaks. Taking into account that all multilocus haplotypes were detected, a total of 140 individuals were completely discriminated by means of 17 polymorphic markers (present or absent), which were amplified from nine selected random-decamer primers. All populations shared the same genetic types, but pronounced population-specific frequency distributions were found, indicating that on average 84% of the present genetic variation exists within populations and the remaining smaller part counts for interpopulational variation. The haplotype differentiation registered was able to distinguish between the class of lowland populations and the class of highland populations. Reproductive isolation and therefore limited altitudinal gene flow, indicated in case of a French highland population by an enlarged number of fixed markers, and hitchhiking effects with respect to selective processes at certain loci following local adaptation and speciation are discussed to explain the population structures found. A possible selection is indicated by five markers, showing significantly different frequency distributions between the class of highland populations and the class of lowland populations.     

Utilization of dry matter and bioelements in larvae of Neodiprion sertifer Geoffr. (Hym., Diprionidae) feeding on Scots pine (Pinus sylvestris L.)

Summary In the laboratory the consumption (C), egestion (F), assimilation (A) and assimilation efficiency (A/C) of dry matter and of the bioelements C, N, P, K, Ca, Mg, S and Na were studied weekly during the whole feeding stage. The larvae were reared in clusters of 10 individuals (on average 3.5 and 6.5 ), fed one-year-old needles. The average values of (C), (F) and (A) were 2,950, 2,530 and 414 mg dw per cluster, respectively, implying a dry matter assimilation efficiency of only 14.0%. The mean production of tissue (P) per cluster amounted to 250 mg dw. The concentration of bioelements in faeces (in order as above) was 51.2, 0.80, 0.08, 0.36, 0.41, 0.11, 0.10 and 0.02% of dw, and in larval tissue 57.5, 7.60, 0.64, 0.85, 0.24, 0.22, 0.24 and 0.06% of dw, respectively. The concentration of N and P in larval tissue was 5.8 and 4.9 times higher than in food, that of K, Mg, S and Na about twice, that of C and Ca about equal or less. The assimilation efficiency of the bioelements was estimated at 14.5, 59.3, 47.4, 20.9, 4.6, 12.5, 13.8 and 44.4%, and the energetics (C), (F), (A), (P) and (R=A-P) at 61,900, 53,600, 8,320, 6,710 and 1,610 J per cluster, respectively. It is argued that in N. sertifer the high consumption rate, low efficiency of dry matter assimilation and low nitrogen accumulation, in all attained at a low maintenance cost, are adaptations to a food low in nitrogen content (1.3% of dw). The study is part of an investigation on the interaction between insect consumption and pine growth, performed within the Swedish Coniferous Forest Project.The authors are given in alphabetic order; for further information see Acknowledgements     

Areal distribution of a Neodiprion sertifer (Hym., Diprionidae) outbreak on Scots pine as related to stand condition

Analyses of a recent N. sertifer outbreak in southern Sweden were made during on-going attack, comprising a regional and local mapping of defoliation severity, as well as a study of intensity of larval consumption, abundance of cocoons and parasitation of larvae in relation to fertilizer application. It was found that (1) the outbreak was preceded by summer and autumn droughts, (2) it was limited to infertile soils, (3) pine stands aged 20–40 yr suffered most, (4) it was less severe in trees benefiting from a reduced inter-tree competition, as measured from tree-ring growth, and (5) less severe in a fertilized stand. The findings support the hypothesis on a fundamental effect of stand nutritive state on changes in number of the sawfly.     

Impact of the parasitoid Lathrolestes ensator (Hym., Ichneumonidae, Ctenopelmatinae) as antagonist of apple sawfly Hoplocampa testudinea (Hym., Tenthredinidae)

Abstract: Lathrolestes ensator (Brauns) was the only parasitoid species reared from larvae of apple sawfly Hoplocampa testudinea (Klug) collected in Dutch orchards. The life history parameters of apple sawfly and its parasitoid L. ensator were investigated in field and semi‐field experiments. Usually, the adult parasitoids emerge in synchrony with the preferred host stage, the second instar sawfly larvae. Parasitoid females carried 120–175 eggs, but never laid more than half this number. The mortality of sawfly eggs and larvae varied from 75 and 99%. The eggs and mining larvae are most vulnerable, as the older larvae survive for 90% on average. Three factors at least determine how many of the descending larvae become adult sawfly or parasitoid next spring. The failure of descending larvae to construct a cocoon varied from 7 to 31% and was highest in clay soil. Probably a similar mechanical hindrance prohibited more emerging adults, of both sawfly and L. ensator, from attaining the surface in heavier soils next spring. Relatively more sawfly prepupae than parasitoids died in the cocoon stage, from fungus disease or other causes, but more parasitoids than hosts stayed in prolonged diapause. Mainly due to this last factor, the overall result was a decrease of the parasitoid : host ratio during the first season underground. A life table based on survival rates during various life stages yields a net reproductive rate of up to 2.4 daughters per female sawfly. It indicates that 60% of the sawfly larvae need to be parasitized to stop population growth, or correspondingly less when more than 75% of the host eggs and young larvae are killed by predators or other causes.     

Fecundity of the European pine sawfly Neodiprion sertifer (Hymenoptera, Diprionidae) related to cyclic outbreaks: Invariance effects

At the end of ontogenesis, individual variability of the fecundity of females of Neodiprion sertifer constitutes 30–120 eggs (with rare fluctuations from 16 to 140 eggs). Average fecundity as an adaptive index in a certain biocenosis, however, depends on many factors, including the structural organization of the sawfly population. The presence of a long diapause in some individuals results in the cleavage of the offspring into groups with different dates of reactivation. At the same time, mainly heavy eonymphs, giving rise to large females, stay in the diapause. Their participation in the reproduction together with females, raised in other years under different conditions, equalize the dimensional structure of the breeding fraction of the population. During population waves, revealed during 35 years of my continuous observations, fecundity fluctuated closely to the average value with deviations constituting 19–25%. It hardly depends on the population density of the phyllophagan. The intraspecific structuredness of the population in relation to the size of females and its transformation by the moment of their emergence promotes the evolutionary stability of the degree of fecundity of the sawfly.