Meligethes Aeneus Research Articles

The role of pollen odour in the attraction of pollen beetles to oilseed rape flowers

AbstractThe role of pollen odour in resource location by the pollen beetle, Meligethes aeneus (Fabricius) (Coleoptera: Nitidulidae), a pollen‐feeding insect regarded as a pest of oilseed rape, Brassica napus L., (Brassicaceae) crops, was investigated in a linear track olfactometer. Both male and female beetles were attracted to the odour of whole oilseed rape flowers, indicating that these insects can locate their host plants using floral odours as cues. The attractive odour of flowers was found to emanate from all floral parts tested: the petals/sepals, the anthers, and from pollen itself. Therefore, at least part of the attractive odour of oilseed rape flowers emanates from pollen. Beetles were more attracted to floral samples containing anthers than those without anthers when these odours were directly compared in a choice‐test, and this indicates that there were detectable differences between them. Anthers and pollen may therefore release distinctive odours that are quantitatively and/or qualitatively different from the odour of the rest of the flower. These experiments support the hypothesis that pollen‐seeking insects use pollen odour cues to locate this food source.

Effect of artificial culture media on germination, growth, virulence and surface properties of the entomopathogenic hyphomycete Metarhizium anisopliae

Culture media influenced the germination of conidia, appressorial development and mycelial growth of Metarhizium anisopliae. Although the addition of KC1 to Sabouraud dextrose agar (SDAM) significantly reduced germination of the conidia of three isolates in vitro and lowered germination and appressorial development on the cuticles of Myzus persicae and Meligethes aeneus , conidia grown on SDAM and minimal medium (MM) were more aggressive than conidia derived from SDA or yeast extract agar. No relationship was found between either germination or appressorial development and LT 50 values, suggesting that virulence depends on the speed of infection. Calcofluor staining and lectin binding showed that culture conditions also influenced the adhesion of Metarhizium anisopliae to insect cuticles through alteration of surface carbohydrates such as 1–4-β-glucans. Dark, pigmented conidia from MM bound less lectins and calcofluor whereas the paler conidia from the other media fluoresced more intensely. Conidia from SDAM fluoresced 20-fold brighter and adhered more readily to insect cuticle compared with conidia derived from MM. Although inocula from both media showed elevated pathogenic activity against aphid and pollen beetle, MM-grown conidia were more aggressive since high mortality was induced by comparatively fewer conidia. When fluorescent intensity of calcofluor-stained conidia was compared to the number of conidia adhering to the insect cuticles, the relationship was significant, indicating that this dye has practical implications for the assessment of fungal strains in screening programmes for commercial development.

Effects of host plant species on the interaction between the parasitic wasp Diospilus capito and pollen beetles (Meligethes spp.)

Summary1 Field studies were conducted in central Sweden to establish whether two host plants with high and low suitability for pollen beetles (Meligethes aeneus Fabr. [Coleoptera: Nitidulidae] and Meligethes viridescens Fabr.) affected the parasitoid Diospilus capito Nees [Hymenoptera: Braconidae].2 Samples of larvae were taken from fields with plots of white mustard, Sinapis alba L. and spring rape, Brassica napus L. in 1997 and 1998. Levels of parasitism and the survival and size of D. capito were measured to determine any influence of the host plant species. Survival and weights of pollen beetles from S. alba and B. napus were also measured.3 Levels of parasitism between 8% and 29% were recorded. There was a significantly higher likelihood of being parasitized by D. capito for beetle larvae developing on S. alba than on B. napus. We argue that semiochemical or morphological properties of plant species could be responsible for differential parasitism by D. capito.4 Neither survival of D. capito nor parasitoid size differed from hosts developing on S. alba or B. napus. Pollen beetle emergence was the same for both plant species, but beetles that developed on S. alba weighed less than those from B. napus. Results suggest that the partial resistance of S. alba to the pollen beetle will have no negative effects on the parasitoid D. capito.

The influence of host plant species on parasitism of pollen beetles (Meligethes spp.) by Phradis morionellus

AbstractField experiments were conducted at Ultuna in central Sweden in order to determine if parasitism of pollen beetle larvae (Meligethes aeneus Fabr. and Meligethes viridescens Fabr. (Coleoptera: Nitidulidae)), by Phradis morionellus Holmgr. (Hymenoptera: Ichneumonidae) differed between the host plants spring rape, Brassica napus L., and white mustard, Sinapis alba L. Parasitism by P. morionellus varied between 25% and 53%. There was no consistent difference between proportion parasitism on the two host plant species among the four studied fields. There was a significantly higher risk of parasitization on S. alba than on B. napus in two fields. The opposite occurred in a third field and in a fourth field no difference between parasitism on plant species was found. Density of host larvae affected parasitization by P. morionellus on B. napus, with lower parasitism at higher pollen beetle densities. The next most common parasitoid species, after P. morionellus, encountered on pollen beetle larvae was Diospilus capito Nees (Hymenoptera: Braconidae). The level of total parasitism by these two species was between 31% and 66%. Interspecific competition of P. morionellus with D. capito through multiparasitism was independent of both field location and plant species. The level of multiparasitism was, in all cases, less than 6.3%.

Effects of Metarhizium anisopliae on the pollen beetle Meligethes aeneus and its parasitoids Phradis morionellus and Diospilus capito

The efficacy of two Finnish strains of Metarhizium anisopliaeagainst the rape blossom beetle Meligethes aeneus (Coleoptera:Nitidulidae) and the effect on its parasitoids Phradismorionellus (Hymenoptera: Ichneumonidae) and Diospilus capito(Hymenoptera: Braconidae) were studied in laboratory and semi-fieldexperiments. Several bio-assays were performed using either directexposure of the host beetle to the pathogens, or pots containing soilinoculated with M. anisopliae conidia at a rate of 2 ×108 per ml soil. Parasitised and unparasitised M. aeneuslarvae were collected in the field and were placed for pupation in thetest soil. The effect of treatment on M. aeneus andD. capito was estimated after adult emergence, and the effecton P. morionellus was estimated by dissecting hibernating pupae.While both of the M. anisopliae strains were highly pathogenicto M. aeneus adults and larvae upon direct exposure, soiltreatment resulted in no significant difference between the number ofinsects emerging (or found) from the treated and untreatedpots. However, the treated insects appeared to be latently infectedand the actual infection rate had to be estimated after their deathand incubation in a moist chamber. The mortality of the beetles causedby direct exposure to the fungus was 85% (range 70–88%). The rateof latent infection following indirect exposure via treated soil indeep pots in laboratory was 80% (range 49–100%), while that inP. morionellus was only 17% (range 0–85%), and inD. capito significantly higher, 76% (range 72–100%).The results indicate that M. anisopliae is a potentially usefulcandidate to be used as a bio-insecticide to control the pollenbeetle, and that at least the most abundant parasitoid, Phradismorionellus, is less affected than the target pest itself. Thepossible ecological role of latent infections in the host as well asin some of the parasitoids needs to be clarified.

The making of a pest: recruitment of Meligethes aeneus onto oilseed Brassicas

AbstractPopulations of the rapeseed pollen beetle Meligethes aeneus F. (Col., Nitidulidae) from areas with 0–16 years of history of intensive rapeseed growing were compared for key ecological characters. During the first 16 years of rapeseed cultivation the reproductive success of M. aeneus increased 200–300% over that of the beetles living on the natural host plants, cruciferous weeds. The increase was linear over time and statistically highly significant, and it did not appear to be related to food quality or to the size of the beetles. During the same period the tolerance to intraspecific competition decreased, possibly due to the relative absence of such competition on the new crop. Furthermore, the optimum population density for M. aeneus to maximize the size of its next generation on summer turnip rape was determined to be 0.5–1.0 beetles/plant, which is slightly below the economic threshold for chemical control (1 beetle/plant). Therefore the practical protection of the rapeseed yield also ensures the highest possible pest population size for the next year. These mechanisms may in part explain the particular noxiousness of the species as a pest all over Europe. In general these data show that after the introduction of a new crop plant into a region, significant changes during the recruitment process in a pestiferous insect may take place, contributing to the future pest status of the insect. It is suggested that such genetic and ecological changes in insects may be a more common mechanism than previously thought in initiating and sustaining pest outbreaks, and that conventional pest management methods may enhance that effect.

Barbarea verna and Barbarea vulgaris as host plants for the pollen beetle (Meligethes aeneus)

During the introduction of a new crop, it is important to study and select for resistance to pests to ensure preservation of possible natural protection in the plant material. One accession of Barbarea verna (Mill.) Asch. and one of Barbarea vulgaris R. Br. was exposed to female pollen beetles, Meligethes aeneus Fab. (Coleoptera: Nitidulidae) in a greenhouse experiment. The experiment was initiated to reveal the extent to which the plant material is accepted as a host plant to this important Brassicaceae pest. The two species, B. verna and B. vulgaris, were used for oviposition by 26% and 21% of the females, respectively. The amount of oviposition and feeding injury on either of the two species throughout the experiment did not indicate any difference in acceptance of the species by the pollen beetle. For both species a majority of the egg-laying females oviposited only once during the 5-day period and laid less than 3 eggs per clutch. The females present on the Barbarea spp. were not more disposed to accept the species as the days passed and the mean clutch size did not increase with time since previous oviposition. Females laying eggs used large and small buds of B. verna to the same extent for feeding. On B. vulgaris there was a tendency for more frequent use of large buds for feeding than small. Acceptance for oviposition by the pollen beetle on the two accessions of Barbarea is low in this experiment. Injury caused was limited to a level probably tolerated by the plant material without causing economic damage. As the oogenesis of the pollen beetle is reduced by exposure to low-quality host plants the potential natural protection would endure despite an increasing occurrence of the species.

RESPONSE OF THE POLLEN BEETLE, Meligethes aeneus, TO TRAPS BAITED WITH VOLATILES FROM OILSEED RAPE, Brassica napus

The response of the pollen beetle, Meligethes aeneus, to yellow water traps baited with individual lures of 25 floral volatile compounds was studied in 17 field experiments. The compounds comprised seven nitrogenous amino acid derivatives, five nonnitrogenous amino acid derivatives, nine fatty acid derivatives, and four isoprenoids. Twenty compounds affected the trap catch of M. aeneus, but response was often dependent on release rate. Most compounds were attractive, but four fatty acid derivatives were repellent. 1-Hexanol was either attractive or repellent, depending on the release rate. It is suggested that M. aeneus responds to this large number of chemically diverse compounds because it is polyphagous.

The pollen beetle, Meligethes aeneus, changes egg production rate to match host quality.

Motivation-based models dominate current theory concerning host plant selection for oviposition by herbivorous insects. A female searching for a host plant will be more likely to accept a host which is of inferior quality for her offspring if motivation is high, e.g. a large eggload or long time since last oviposition. This implies that insects will accumulate eggs if exposed to hosts of low acceptability and after a time lay eggs on such hosts. An alternative strategy for insects when exposed to less acceptable hosts is to stop producing, instead of accumulating, eggs. Thus, resources would be saved until a more acceptable host is found. If this hypothesis is true, a herbivorous insect would cease egg production when exposed to hosts of low acceptability and resume egg production when exposed to hosts of high acceptability. Previous exposure should not affect oviposition rate when an insect encounters a new host of a different quality. In an earlier study pollen beetles, Meligethes aeneus (F.) (Coleoptera: Nitidulidae), did not accumulate eggs in the absence of high quality hosts. In this study we monitored the daily oviposition rate of female pollen beetles on hosts plants of low (Sinapis alba L.), intermediate (Brassica nigra Koch) or high (B. napus L.) acceptability over a 5-day period. Individuals were then switched to an oviposition resource of a different acceptability. Beetles moved from high- to low-acceptability plants reduced their oviposition rate considerably. In the opposite case, low to high acceptability, the rate of oviposition increased markedly after the switch. When M. aeneus were moved from the high-acceptability host to that of intermediate acceptability oviposition rate was modified accordingly. However, when moved to the intermediate host from a host of low acceptability oviposition on B. nigra was much less than would normally be expected. A possible mechanism for this finding is discussed. M. aeneus, by adjusting oviposition rate to host acceptability, maximizes the average host quality for offspring, even at the cost of a lower egg-laying rate.

Landscape structure and biological control in agroecosystems

Biological pest control has primarily relied on local improvements in populations of natural enemies, but landscape structure may also be important. This is shown here with experiments at different spatial scales using the rape pollen beetle (Meligethes aeneus), an important pest on oilseed rape (Brassica napus). The presence of old field margin strips along rape fields was associated with increased mortality of pollen beetles resulting from parasitism and adjacent, large, old fallow habitats had an even greater effect. In structurally complex landscapes, parasitism was higher and crop damage was lower than in simple landscapes with a high percentage of agricultural use.

INFLUENCE OF VISUAL CUES AND ISOTHIOCYANATE LURES ON CAPTURE OF THE POLLEN BEETLE, Meligethes aeneus IN FIELD TRAPS

The effect of trap design, trap color, and isothiocyanate (NCS) lures on the capture of the pollen beetle, Meligethes aeneus, was studied in field experiments. Unbaited yellow water and adhesive traps were equally attractive. A small sticky card trap with a yellow upper face, mounted at 45° to the vertical, was attractive throughout the year, but horizontal yellow cards were attractive only in spring, and vertical cards only in autumn. Yellow–green and white water traps were less attractive than yellow, while grass green, cream, and black were unattractive. M. aeneus was attracted equally to four alkanyl, three alkenyl, and to 2-phenylethyl isothiocyanate lures, when the release rates were in the range of 5–30 mg/day. Attraction to the most effective unbaited yellow traps was enhanced 1.7–3.3 times with the addition of a lure comprising a mixture of allyl, 3-butenyl, 4-pentenyl, and 2-phenylethyl NCS. Interactions were observed among the visual and odor cues. A yellow sticky card trap, baited with 2-phenylethyl NCS and mounted at 45° to the vertical, may be used throughout the year to monitor M. aeneus.

Honey-bee-mediated Infection of Pollen Beetle (Meligethes aeneus Fab.) by the Insect-pathogenic Fungus, Metarhizium anisopliae

Pollen beetles (Meligethes aeneus) are pests that feed and oviposit in the buds and flowers of oilseed rape. Honey-bees foraging from a hive fitted with an inoculum dispenser at the entrance effectively delivered dry conidia of the entomogenous fungus, Metarhizium anisopliae , to the flowers of oilseed rape in caged field plots. In both winter- and spring-sown rape experiments, a greater mortality of pollen beetles occurred in treated plots than in control plots. The mortality (61% on winter rape, 100% on spring rape) was greatest during peak flowering, when the feeding activity of both bees and beetles from the flowers was maximal, providing optimal conditions for inoculum dissemination and infection. Conidial sporulation occurred on a significant proportion of the dead pollen beetles. There was no evidence of any adverse effect on the honey-bee colonies.

Clutch Size and Larval Performance of Pollen Beetles on Different Host Plants

Discrepancies found between preference and performance for herbivorous insects are often explained using a variety of interpretations. In this study the relationship between clutch size and larval performance is considered in the context of preference. Sequential choice trials were used to determine the preference ranking of the pollen beetle (Meligethes aeneus) for five different plant species. Performance estimated as larval weights after one week was measured in a separate experiment on the same plant species. M. aeneus showed a strong preference for three Brassica species (Brassica napus, B. juncea and B. nigra) when compared to Sinapis alba and Eruca sativa. Overall larval performance on B. napus was highest followed by E. sativa, B. nigra, B. juncea and S. alba (in descending order). Thus discrepancies between preference and performance were demonstrated. Some of these differences could be explained when clutch size and larval competition were considered. Clutch size per bud differed among plant species, from the highest on E. sativa to the lowest on B. nigra. Competition among larvae in a bud was demonstrated on B. nigra, B. juncea and S. alba. On these species larvae from buds containing four or more larvae weighed less than larvae from buds with a single larva. Reduction in weight suffered by larvae from multiple clutches was correlated to the expected number of larvae per bud for each plant species. Larvae on plant species receiving small clutch sizes and sustaining high larval mortality suffered most from competition. On plant species with larger clutch sizes and little larval mortality no competition among larvae was demonstrated. Clutch size was correlated to mean bud size on the different plant species. It is suggested that the pollen beetle modifies clutch size according to expectation of larval competition on different host plant species.

Response of the pollen beetle Meligethes aeneus to volatiles emitted by intact plants and conspecifics

AbstractThe response of the pollen beetle Meligethes aeneus Fab. (Coleoptera, Nitidulidae) to the volatiles of undamaged plants and conspecifics was tested in a Y‐tube‐olfactometer‐bioassay. Beetles that had hibernated preferred significantly the volatiles emitted by their most important host plant, oilseed‐rape (Brassica napus L. ‘Lorar’) in the early bud‐stage. However, the odour emitted by rye (Secale cereale L.), tomato plants (Solanum lycopersicum L.), and yarrow (Achillea millefolium L.) were also attractive when tested against the corresponding growing‐medium. Dock plants (Rumex obtusifolius L.) and touch‐me‐not (Impatiens parviflora L.) possessed no attractive properties. When tested against each other, oilseed‐rape was preferred significantly by M. aeneus above all other plants, with the exception of tomato. The results indicate that M. aeneus is able to locate its host plant by olfactory stimuli in the early bud‐stage, i.e. in that stage in which the infestation begins in the field and when the typical yellow colour and floral scent of oilseed‐rape are absent.Female pollen beetles avoided significantly the volatiles emitted by female conspecifics and an ether extract of conspecifics of mixed sex, while volatiles from males had no significant effect on females. Furthermore, males showed no preferences when responding to conspecific odour in the olfactometer. These results suggest the existence of an epideictic pheromone for M. aeneus.

Spatial Scales in Ecological Pest Management (EPM): Importance of Regional Crop Rotations

ABSTRACT The importance of spatial scales and landscape heterogeneity to insect populations is widely accepted in ecology and conservation biology. What are the applications to crop protection? Theoretically, insect pest species with certain characteristic metapopulation dynamics may effectively be managed by crop rotation, by applying the same tools as used in conservation. This strategy may in many cases necessitate regionally planned rotations that exceed single farm boundaries. The basic idea of pest management by regional crop rotation is presented, the theoretical background outlined and agroecological prerequisites discussed. Spatiotemporal patterns of both pest occurence and the host crop field allocation in the landscape are important. For example, the required degree of isolation between host crop patches cannot be achieved with regionally covering major crops, e.g. cereals. The dispersal ability of the pest determines the overall spatial scale at which the rotation is practised. Recruitment from wild hosts must not be excessive. These aspects are illustrated with example species, the cabbage root fly, Delia radicum, a possible target for regional management, and the rape pollen beetle, Meligethes aeneus, an unlikely target. Regional rotation and biological control are not conflicting strategies, as the crop-pest systems most promising for regional management are the least promising for biological control.

Characteristics of oviposition behaviour of the pollen beetle, Meligethes aeneus on four different host plants

AbstractOviposition behaviour of Meligethes aeneus F. (Coleoptera: Nitidulidae) is characterised and quantified on four different plant species. Six behavioural components are identified: W – walking, WA – walking with abdomen on surface, R – resting, B – biting, AOH – placing abdomen over the bite hole and OVI – oviposition. Comparison of host acceptance behaviours on Brassica napus L., Brassica juncea (L.) Czern, Brassica nigra (L.) Koch and Sinapis alba L. showed that S. alba was accepted as a host only after a long exposure to the plant. Behaviour on the Brassica species was similar, however on B. nigra beetles spent a high proportion of time actually ovipositing. We conclude that important cues for oviposition are located both on the bud surface and inside the bud.

The effect of flower colour and glucosinolates on the interaction between oilseed rape and pollen beetles

Pollen beetles (Meligethes aeneus) are important pests on Brassica oilseed crops and herbivores of wild crucifer populations. The adults feed on flowers and unopened buds in which they oviposit. The larvae feed mainly on pollen, but all parts of the flower may be destroyed. Adult pollen beetles will also feed on non-cruciferous taxa, in contrast to the majority of herbivorous insects on Brassica, which are specialist feeders on crucifers. Many of the specialist insects have been shown to be attracted by glucosinolates and their hydrolytic products. These metabolites also act as ovipositioning and feeding stimulants. Polyphagous non-insect herbivores such as molluscs and pigeons are deterred by glucosinolates (Giamoustaris & Mithen, 1995). Flower colour has been shown to be important in attracting pollinators to crucifers (Kay, 1976). It is not known whether variation in flower colour may also affect plant attractiveness to herbivorous insects.

Low oviposition stimuli reduce egg production in the pollen beetle Meligethes aeneus

Abstract. Gravid female pollen beetles, Meligethes aeneus (F.) (Coleoptera: Nitidulidae), were exposed at different intervals to oviposition sites that varied in acceptability.The egg load of dissected individuals which had been deprived of oviposition sites was not greater than individuals which had been frequently exposed to oviposition sites.However, the egg load of individuals which had been exposed to highly acceptable host plants was greater than those exposed to a host plant of low acceptability.Over the experimental period the total number of eggs which an individual produced was lower for those that were either deprived of an oviposition site or frequently exposed to a low‐quality host plant.There was no evidence of either increased oviposition probability or clutch size as the period since the last oviposition increased.It is concluded that low host quality and low host encounter rate reduce the egg production of M.aeneus. It is further concluded that the suppressed egg production reduces the accumulation of eggs, so that physiological motivation for the insect to oviposit is not increased.If low oviposition site quality acts to reduce oogenesis, as found here, then the probability of egg deposition on low‐quality species may not increase over time and the effect of antixenotic resistance may be enhanced.

Pollen beetle (Meligethes aeneus) oviposition and feeding preference on different host plant species

AbstractThe oviposition and feeding preferences of the pollen beetle, Meligethes aeneus, were determined in choice and no‐choice tests in field, semi‐field and greenhouse trials. Plant species used were Brassica napus, B. campestris, B. juncea, B. nigra, B. carinata, Sinapis alba and Crambe abyssinica. With respect to number of eggs laid, S. alba and C. abyssinica were inferior to the other species. Pollen beetles laid fewer eggs on B. nigra than on the other Brassica spp. in no‐choice tests, however this difference was partly due to fewer eggs laid per bud rather than fewer buds used for oviposition. Most eggs, for all plant species, were deposited in buds sized 2–3 mm. Feeding damage on all plant species was relatively similar. Pollen beetles seem to have a wider host range for feeding than for oviposition. There was good agreement in plant species ranking as oviposition hosts between the field, semi‐field and greenhouse trials.

Occurrence and dynamics of <i>Nosema meligethi</i> (Microsporida) in populations of <i>Meligethes aeneus</i> (Coleoptera, Nitidulidae) in Finland

A total of 221 samples of the beetle Meligethes aeneus Fabricius were collected from southern and central Finland in 1989-90, and 10 152 beetles were individually examined to determine the occurrence of the microsporidian disease Nosema meligethi Issi & Raditsheva. The dynamics of this disease was followed at seven sites throughout the season, and its ecological aspects were studied at one locality, Hartola. The level of N. meligethi infection steadily increased in M. aeneus populations during the season, with horizontal transmission appearing to take place in flowerheads, where the beetles aggregate to feed. Infected beetles migrated to overwintering sites earlier than did healthy beetles, and just prior to overwintering, infected beetles were lighter in weight than healthy ones. This fact may drastically affect the overwintering survival of infected beetles.