Megastigmus Spermotrophus Research Articles

Unexpected parasitism of Douglas-fir seed chalcid limits biocontrol options for invasive Douglas-fir in New Zealand

Douglas-fir seed chalcid (DFSC) Megastigmus spermotrophus, a small (3 mm long) host-specific seed-predatory wasp, was accidentally introduced into New Zealand in the 1920s. Concern over DFSC reducing Douglas-fir seed production in New Zealand led to an attempt at biocontrol in 1955 with the release, but failed establishment, of the small (2.5 mm long) parasitoid wasp, Mesopolobus spermotrophus. We investigated why DFSC causes little destruction of Douglas-fir seed in New Zealand (usually <20%) despite the apparent absence of major natural enemies. Douglas-fir seed collections from 13 New Zealand sites yielded the seed predator (DFSC) but also potential parasitoids, which were identified using morphology and partial COI DNA sequencing. DFSC destroyed only 0.15% of Douglas-fir seed. All parasitoids were identified as the pteromalid wasp, Mes. spermotrophus, the host-specific biocontrol agent released in 1955. Total parasitism was 48.5%, but levels at some sites approached 90%, with some evidence of density-dependence. The discovery of the parasitoid Mes. spermotrophus could indicate that the biocontrol agent released in 1955 did establish after all. Alternatively, Mes. spermotrophus could have arrived accidentally in more recent importations of Douglas-fir seed. The high level of parasitism of DFSC by Mes. spermotrophus is consistent with DFSC being under successful biological control in New Zealand. Suppression of DFSC populations will benefit commercial Douglas-fir seed production in New Zealand, but it also represents the likely loss of a potential biological control agent for wilding Douglas-fir.

Noi semnalari ale unor specii de insecte forestiere invazive in Romania

New records of ten invasive insect species in Romania are presented. The studied species are: Cameraria ohridella Deschka & Dimic 1986, Parectopa robiniella Clemens 1863, Phyllonorycter robiniella (Clemens 1859), Phyllonorycter issikii (Kumata 1963), Hyphantria cunea (Drury 1773), Obolodiplosis robiniae (Haldeman 1847), Leptoglossus occidentalis Heidemann 1910, Eopineus strobus (Hartig 1837), Megastigmus spermotrophus Wachtl 1893 and Harmonia axyridis Pallas 1773. The native range of each species, the first report and the present distribution in Europe and in Romania are discussed. The new records suggest that all the analysed species have established populations in our country and a more widespread distribution than that previously known. Some of them attain sometimes locally or zonally high population levels and are regarded as important pests. For the most species, new observations are necessary, both concerning their presence in the areas where they were not found so far, but also to assess the impact of insect populations on their hosts and on the recipient biocoenoses. A particular attention should be paid to the species H. axyridis, whose swarms invade the houses of the people during the autumn and could cause annoyance and possibly allergy. Citizen participation in observing and reporting of these new ”guests” is encouraged.

Transcriptome analysis provides insight into venom evolution in a seed-parasitic wasp, Megastigmus spermotrophus.

One of the most striking host range transitions is the evolution of plant parasitism from animal parasitism. Parasitoid wasps that have secondarily evolved to attack plants (ie gall wasps and seed-feeders) demonstrate intimate associations with their hosts, yet the mechanism of plant-host manipulation is currently not known. There is, however, emerging evidence suggesting that ovipositional secretions play a role in plant manipulation. To investigate whether parasites have modified pre-existing adaptations to facilitate dramatic host shifts we aimed to characterize the expression of venom proteins in a plant parasite using a collection of parasitoid venom sequences as a guide. The transcriptome of a seed-feeding wasp, Megastigmus spermotrophus, was assembled de novo and three putative venoms were found to be highly expressed in adult females. One of these putative venoms, aspartylglucosaminidase, has been previously identified as a major venom component in two distantly related parasitoid wasps (Asobara tabida and Leptopilina heterotoma) and may have originated via gene duplication within the Hymenoptera. Our study shows that M. spermotrophus, a specialized plant parasite, expresses putative venom transcripts that share homology to venoms identified in Nasonia vitripennis (both superfamily Chalcidoidea), which suggests that M. spermotrophus may have co-opted pre-existing machinery to develop as a plant parasite.

A North American invasive seed pest, Megastigmus spermotrophus (Wachtl) (Hymenoptera: Torymidae): Its populations and parasitoids in a European introduction zone

Megastigmus spermotrophus (Hymenoptera: Torymidae) is a seed pest of Douglas-fir ( Pseudotsuga menziesii) in its natural range of North America. Along with seeds, this invading species was accidentally introduced to Europe where it became the major pest in Douglas-fir seed orchards and stands. The high rates of seed parasitism observed in Europe were first ascribed to a reduced control by natural enemies which apparently did not followed M. spermotrophus from their native American range. However, our survey showed that the parasitoid censuses carried out so far were incomplete. Several other chalcid parasitoid species were present in the stands surveyed in Belgium. Four parasitoid species emerged from Douglas-fir seeds infested by the Douglas-fir seed chalcid. Two pteromalids of North American origin, Mesopolobus spermotrophus and Mesopolobus americus, were dominant whereas a few specimens of an eupelmid, Brasema urozonus, and of another pteromalid , Anogmus hohenheimensis, were also observed. Our results strongly suggest that the previous evaluations of parasitoids populations were biased by an inadequate timing in the censuses.

Parasitism of seed of Douglas fir (Pseudotsuga menziesii) by the seed chalcid, Megastigmus spermotrophus, and its influence on seed hormone physiology

Parasitism of Douglas fir (Pseudotsuga menziesii) megagametophytes by the seed chalcid, Megastigmus spermotrophus Wachtl, occurs naturally after pollination but before fertilization. In the absence of fertilization, the presence of insect larvae within the megagametophyte prevents abortion and the storage tissue continues to develop as if the seed had been fertilized. We investigated the effect of parasitism on the metabolism of abscisic acid (ABA), auxins, cytokinins, and gibberellins during early development of Douglas fir seeds. Hormones and hormone metabolites of infested and uninfested megagametophytes with or without pollination were analyzed by HPLC–ESI/MS/MS. At 1 week after Megastigmus introduction, the insect’s presence stimulated ABA accumulation in unpollinated megagametophytes compared to unpollinated, unparasitized megagametophytes. In pollinated material, parasitism did not stimulate ABA accumulation compared to levels present in unparasitized megagametophytes. In all four treatments, the metabolism of ABA occurred primarily through conjugation to the ABA glucose ester (ABAGE), while the 7′-, 8′- and, 9′-hydroxylation pathways were only minor. ABAGE levels declined with time in all treatments and this occurred to a greater extent in pollinated, parasitized megagametophytes, suggesting that the insect’s presence induced the dramatic decrease in ABAGE. Although there were temporal variations in the auxin, cytokinin, and gibberellin profiles of parasitized megagametophytes, the profiles were generally similar to those of unparasitized megagametophytes. Our results suggest that failure of parasitized megagametophytes to abort may be due to the insect inducing similar hormone profiles to those present during normal development of Douglas fir seed.

Multinucleate storage cells in Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) and the effect of seed parasitism by the chalcid Megastigmus spermotrophus Wachtl

Megagametophytes of Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) accumulated storage products following fertilization. As megagametophytes matured, the number of nuclei per cell rose, resulting in syncytial storage cells. Studies carried out on trees in France and Canada confirmed that such previously unreported, free nuclear cells were a normal part of late megagametophyte development. Unfertilized megagametophytes showed that some binucleate cells before degeneration resulted in empty seed. Insect parasitism prevented megagametophyte abortion in unfertilized ovules. Oviposition by a torymid chalcid wasp (Megastigmus spermotrophus Wachtl) early in megagametophyte development resulted in normal megagametophyte development. Around the time of plant egg maturation, binucleate and trinucleate cells were observed. As megagametophytes matured, multinucleate mature storage cells rich in proteins, lipids and starch were formed. The insect was able to induce identical nuclear behaviour in infested, unfertilized megagametophytes, as that of uninfested, fertilized megagametophytes.

Oviposition strategies of conifer seed chalcids in relation to host phenology

Insects are considered the most important predators of seed cones, the female reproductive structures of conifers, prior to seed dispersal. Slightly more than 100 genera of insects are known to parasitize conifer seed cones. The most diverse (i.e., number of species) of these genera is Megastigmus (Hymenoptera: Torymidae), which comprises many important seed pests of native and exotic conifers. Seed chalcids, Megastigmus spp., lay eggs inside the developing ovules of host conifers and, until recently, oviposition was believed to occur only in fertilized ovules. Ovule development begins just after pollination, but stops if cells are not fertilized. The morphological stage of cone development at the time of oviposition by seed chalcids has been established for many species; however, knowledge of ovule development at that time has been documented for only one species, M. spermotrophus. Megastigmus spermotrophus oviposits in Douglas-fir ovules after pollination but before fertilization. Unlike the unfertilized ovules, those containing a M. spermotrophus larva continue to develop, whether fertilized or not, stressing the need to broaden our understanding of the insect-plant interactions for this entire genus. To achieve this task, we reviewed the scientific literature and assembled information pertaining to the timing of oviposition and to the pollination and fertilization periods of their respective host(s). More specifically, we were searching for circumstantial evidence that other species of Megastigmus associated with conifers could behave (i.e., oviposit before ovule fertilization) and impact on female gametophyte (i.e., prevent abortion) like M. spermotrophus. The evidence from our compilation suggests that seed chalcids infesting Pinaceae may also oviposit before ovule fertilization, just like M. spermotrophus, whereas those infesting Cupressaceae seemingly oviposit after ovule fertilization. Based on this evidence, we hypothesize that all species of Megastigmus associated with Pinaceae can oviposit in unfertilized ovules, whereas those exploiting Cupressaceae cannot, and thus oviposit only in already fully developed fertilized seeds. Furthermore, we predict that the presence of a larva in unfertilized ovules of all Pinaceae will influence the development of the female gametophyte by preventing its abortion. This influence on the Pinaceae can be interpreted as an ability to parasitize any of the potential seeds present in a seed cone, and as such represents a much more efficient oviposition strategy than searching and locating only fertilized seeds. Concomitantly, this ability has likely led to an overestimation of the impact of the species of seed chalcid infesting Pinaceae on seed production.

Characterization of microsatellite loci in a seed chalcid, Megastigmus spermotrophus (Hymenoptera: Torymidae)

AbstractHighly polymorphic microsatellite markers can supply demographic information on founder events and range expansion following initial introduction of invasive insect species. Six microsatellite loci were isolated from a partial DNA library in order to study the invasion patterns of a seed chalcid, Megastigmus spermotrophus, introduced to Europe and New Zealand. Allelic diversity at all described loci was high, ranging from 17 to 30 alleles per locus. All six loci were successfully amplified in 15 congeneric species.

INTERACTIONS BETWEEN THE DOUGLAS-FIR SEED CHALCID, MEGASTIGMUS SPERMOTROPHUS (HYMENOPTERA: TORYMIDAE), AND THE WESTERN CONIFER SEED BUG, LEPTOGLOSSUS OCCIDENTALIS (HEMIPTERA: COREIDAE)

AbstractIn two laboratory experiments, adult western conifer seed bugs, Leptoglossus occidentalis Heidemann, did not feed on seed infested by the Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl. When presented with seed lots containing 0, 20, 40, 60, 80, or 100% chalcid-infested seed, seed bugs fed on uninfested seed only when the percent chalcid infestation was ≤ 40% (Exp. 1), ≤ 20% (Exp. 2, females), or ≤ 60% (Exp. 2, males). In a third experiment, nymphs exposed to similar seed lots did not feed on M. spermotrophus when sound seeds were present. However, when exposed to 100% chalcid-infested seed, nymphs in two of 10 replicates fed on M. spermotrophus. Feeding by seed bugs caused seeds to lose 51% of their weight on average, but 18% of 43 test seeds on which seed bugs had fed germinated. In three Douglas-fir orchards surveyed, both species exhibited a clonal preference, but the ranking of clones preferred by each species was not the same. These results suggest that the impacts of L. occidentalis and M. spermotrophus are segregated and additive.

Effect of temperature and photoperiod on diapause development in a Douglas fir seed chalcid, Megastigmus spermotrophus.

Douglas fir seeds infested by diapausing larvae of a chalcid, Megastigmus spermotrophus, were transferred under different temperature and photoperiod regimes at various dates during autumn and winter. Both winter diapause and prolonged diapause were compared to the diapause patterns observed in control lots kept under natural conditions. The results showed that exposure to low temperatures in autumn and winter is required for the completion of diapause development. Median adult emergence dates and the date of transfer of larvae to 20°C were negatively correlated. The frequency of prolonged diapause of larvae transferred to 20°C decreased significantly after November. When larvae were submitted to different constant temperatures, the longer the daylength, the higher and faster the reactivation. These results suggest that chalcid larvae can perceive daylength through the seed coat. This phenomenon is probably related to the fact that most of the infested seeds falling in autumn are usually covered by a layer of leaf litter during the winter.

Cone and Seed Insect Phrenology in a Douglas-Fir Seed Orchard During Three Years in Western Oregon

Seasonal patterns of cone and seed insect activity and seed loss relative to cone production and weather patterns were examined over a 3-yr period in a Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco, seed orchard in western Oregon. Six insect species showed distinct patterns of occurrence and cone or seed damage during this period. A weevil, Lepesoma lecontei (Casey); the Douglas-fir cone gall midge, Contarinia oregonensis Foote; and two lepidopterans, Barbara colfaxiana Kearfott and Dioryctria abietivorella (Grote), damaged cones or seed exposed during late April each year. The Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl, damaged seed in cones exposed during May and June; western conifer seed bug, Leptoglossus occidentalis Heidemann, was most damaging in cones exposed July-August.

Estimating Effect of Megastigmus spermotrophus (Hymenoptera: Torymidae) on Douglas-Fir Seed Production: The New Paradigm

In a pollen exclusion experiment performed on the cones of five Douglas-fir (Pseudotsuga menziesii [Mirbel] Franco) trees, the number of seeds infested by a seed chalcid, Megastigmus spermotrophus Wachtl, did not differ significantly between pollinated and unpollinated cones from the same tree. This finding led to a revision of the formula used to calculate M. spermotrophus effect on Douglas-fir seed production because the traditional formula, which is based on the assumption that only pollinated seeds are infested by these chalcids, exaggerates their effect. The relationship between the new formula and the traditional formula is nonlinear, varying with both pollination rate and infestation level. To assist other researchers in estimating the error in past chalcid studies, the discrepancies for a range of pollination rates were calculated. Past assessments were strongly biased only where pollination rates were <70% and chalcid attack rates were from 50 to 85% using the traditional formula. For low pollination rates, the discrepancy can exceed 50%. Evidence is presented to explain how the unfertilized female gametophyte, which is small and is normally resorbed in the absence of pollination, can support the development of chalcid larvae. In this study, there was a strong correlation (-0.93) between cone length and chalcid attack rate.

Aerial Applications of Esfenvalerate to Suppress Contarinia oregonensis (Diptera: Cecidomyiidae) and Megastigmus spermotrophus (Hymenoptera: Torymidae) in Douglas-Fir Seed Orchards

Aerial applications of esfenvalerate significantly reduced populations of a Douglas-fir cone gall midge, Contarinia oregonensis Foote, and a Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl, in mature Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, seed orchards. Populations of the spruce spider mite, Oligonychus ununguis (Jacobi), increased significantly in treated areas. Our results demonstrated that aerial applications can be made under conditions in the Pacific Northwest and can reduce insect damage levels with between 10 to 20 times less metric insecticide than when high-volume orchard sprayers are used.

Response of Douglas-Fir Cone and Seed Insects to Implants of Systemic Insecticides in a Northern California Forest and a Southern Oregon Seed Orchard

The systemic insecticides acephate, dimethoate, and carbofuran were implanted into Douglas-fir trees, Pseudotsuga menziesii (Mirb.) Franco, at Oakridge, OR, and Willow Creek, CA, to evaluate their effectiveness in reducing seed and cone insect damage. The acephate-implant treatment significantly reduced seed damage by the Douglas-fir cone moth, Barbara colfaxiana (Kearfott) (Lepidoptera: Olethreutidae), and a cone gall midge, Contarinia oregonensis Foote (Diptera: Cecidomyiidae), and significantly increased the percentage of filled seed by 300% at the California site. Neither dimethoate nor carbofuran significantly affected any of the seed or cone insects encountered in this study. Acephate was not effective against a seed chalcid, Megastigmus spermotrophus Wachtl (Hymenoptera: Torymidae), nor the western conifer-seed bug, Leptoglossus occidentalis Heidemann (Heteroptera: Coreidae). The association observed between C. oregonensis damage and undeveloped seeds suggests that management strategies that include effective control measures should also include methods to monitor and reduce seed abortion.

Applying the IDS method to remove seeds infested with the seed chalcid, Megastigmus spermotrophus Wachtl, in Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco

Douglas-fir seedlots were treated with the Incubation Drying Separation (IDS) method to test whether seeds infested with the seed chalcid, Megastigmus spermotrophus Wachtl, could be separated from non-infested seeds. Seeds were soaked in distilled water for 24 h, drained, placed in plastic bags and incubated at 15 °C for 3 days. The seeds were dried for either 0.5, 1, or 2 h at 25 °C, and then separated into floaters and sinkers in a water column. An average of 97% of the infested seed floated. The drying period did not affect the separation of infested seeds but significantly fewer sound seeds floated in the seedlots dried for 0.5 h than those dried for 1 h. Germination capacity of IDS-treated seeds did not differ from that of untreated seeds but the germination rate was significantly faster for IDS-treated seeds when all lots were stratified.

Influence of Host Genotype on Douglas-Fir Seed Losses to Contarinia oregonensis (Diptera: Cecidomyiidae) and Megastigmus spermotrophus (Hymenoptera: Torymidae) in Western Oregon

Seed losses to the Douglas-fir cone gall midge, Contarinia oregonensis Foote, and Douglas-fir seed chalcid, Megastigmus spermotrophus (Wachtl), were measured in a Douglas-fir, Pseudotsuga menziesii (Mirbel) Franco, clonal seed orchard and in a Douglas-fir progeny plantation in western Oregon. Seed losses to both insects differed significantly (P < 0.05) among clones and among the progeny of selected parental crosses. Seed loss differed more than three times between least-infested and most-infested clones or progeny. Seed losses in the progeny plantation indicated that resistance to these two insects is a heritable trait, with greater resistance showing a tendency to dominate over lesser resistance. Insect responses to host genotype may be modified by factors associated with the position of the tree within the stand. Implications of these results for tree improvement programs and seed orchard management are discussed.

Trunk-Implanted Acephate to Protect Douglas-Fir Seed Crops on Individual Trees in Northern California

To reduce seed and cone insect damage, 0.875 g per capsule of acephate (technical grade) was implanted into individual Douglas-fir, Pseudotsuga menziesii (Mirb.) Franko, trees on 15 April or 6 May 1985. A spacing of one capsule per 10 cm of trunk circumference was used. Neither implant treatment affected actual infestation levels of a Douglas-fir cone gall midge, Contarinia oregonensis Foote, or a Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl. Both implant treatments, however, significantly reduced the damage of a fir coneworm, Dioryctria abietivorella (Grote), and the Douglas-fir cone moth, Barhara colfaxiana (Kearfott), by at least 62%. The latter implant significantly increased the number of filled seeds per cone by 154% compared with control trees, but the difference between implant treatment dates was not significant. Interspecific interaction between reduction of damage by Lepidoptera and the increase in C. oregonensis-damaged seeds and filled seeds suggests a negative association with reduced predation.

Inventory Monitoring for Estimating Impact of Insects on Seed Production in a Douglas-Fir Seed Orchard in Western Oregon

Impact of various factors on seed production was examined in a Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) seed orchard in western Oregon during 1984. Cones were examined monthly between April and September for mortality or evidence of insect damage. Loss of immature cones began early in the growing season and had a substantial impact on seed production. A newly discovered cone-feeding weevil, Lepesoma lecontei (Casey), caused a large proportion of this mortality. Unexplained cone mortality (possibly frost damage), undeveloped seed, Douglas-fir cone gall midge ( Contarinia oregonensis Foote), and Douglas-fir seed chalcid ( Megastigmus spermotrophus (Wachtl)) were other major causes of seed loss. Crown elevation was important for an accurate estimate of the importance of some seed-loss factors. An inventory monitoring system for Douglas-fir seed production based on early monitoring of immature cones, identifying specific seed loss factors, stratified crown sampling, and an adequate sample size, is suggested.

Single and Double Application of Fenvalerate and Dimethoate for Control of Douglas-fir Seed and Cone Insects1

Douglas-fir cones were sprayed once (13 April) or twice (13 April and 16 May) with a 0.025% concentration of fenvalerate or a 0.23% concentration of dimethoate to reduce infestation of seed and cone insects. Single application of either chemical did not significantly reduce incidence of fused seed caused by a Douglas-fir cone gall midge, Contarinia oregonensis Foote. The double application of fenvalerate significantly reduced (64%) the infestation of a Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl. Two applications of dimethoate significantly reduced infestation levels of both the cone gall midge and the seed chalcid by 78 and 39%, respectively. The increase in percentage of extractable filled seeds associated with all spray treatments was higher than that of the unsprayed control, but the difference was not significant.

EVALUATION OF FOUR CHEMICAL INSECTICIDES REGISTERED FOR CONTROL OF THE DOUGLAS-FIR CONE GALL MIDGE, CONTARINIA OREGONENSIS (DIPTERA: CECIDOMYIIDAE), AND THE DOUGLAS-FIR SEED CHALCID, MEGASTIGMUS SPERMOTROPHUS (HYMENOPTERA: TORYMIDAE), IN DOUGLAS-FIR SEED ORCHARDS

AbstractOxydemeton-methyl, azinphosmethyl, dimethoate, and acephate were evaluated in Douglas-fir seed orchards for control of Douglas-fir cone gall midge, Contarinia oregonensis Foote, and Douglas-fir seed chalcid, Megastigmus spermotrophus Wachtl. Results varied with infestation levels and specific chemicals. There was no significant reduction of infestation levels of either insect with the application of acephate. Oxydemeton-methyl injections, oxydemeton-methyl spray, and dimethoate significantly reduced cone gall midge damage. All four chemicals were ineffective against the seed chalcid for infestations of less than 10% of extractable seed. Oxydemeton-methyl injections, azinphosmethyl, and dimethoate significantly reduced the incidence of seed chalcid when the infestation was high (61%). There was no statistically significant difference (α = 0.05) for pairwise comparisons between chemicals which significantly reduced the infestation levels of either insect.