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Keywords:

  • Conservation;
  • dispersal;
  • egg number;
  • egg size;
  • flight duration;
  • flight muscles;
  • flight willingness;
  • migration

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Abstract. 1. Life-history traits associated with colonisation ability were compared in the threatened tenebrionid beetle Oplocephala haemorrhoidalis and its common relative Bolitophagus reticulatus. Both species feed and breed exclusively in fruiting bodies of the wood-decaying fungus Fomes fomentarius.

2. The presence and status of flight wings, flight muscles, and mature eggs were determined by dissection. Flight willingness was studied in a field experiment, and flight duration in a flight-mill experiment.

3. Females of O. haemorrhoidalis had fewer but larger eggs in their abdomen than B. reticulatus females.

4. All beetles of both species had fully developed flight wings but a larger proportion of B. reticulatus than O. haemorrhoidalis had developed flight muscles.

5. Bolitophagus reticulatus was more willing to take off than O. haemorrhoidalis, however both species, especially O. haemorrhoidalis, were powerful fliers, with many individuals being able to fly several kilometres. Oplocephala haemorrhoidalis tended to make few flights of long duration whereas B. reticulatus made several, but mostly shorter, flights.

6. The results indicate that B. reticulatus has a suite of life-history traits that makes it better adapted than O. haemorrhoidalis to exploit the scattered trees with fruiting bodies present in managed forests. This may explain why O. haemorrhoidalis is restricted primarily to sites with a high density of suitable substrates that have been available continuously for a long time.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The ideal colonist should be able to move effectively, to locate suitable new habitats, and to build up populations rapidly once there (MacArthur & Wilson, 1967; Dingle, 1972; Safriel & Ritte, 1980). Thus, a high rate of reproductive output shortly after arrival at the new patch should be advantageous for a colonist (Southwood, 1977; Palmer & Dingle, 1989), however one of the most commonly described costs associated with dispersal in insects is lowered fecundity (Roff & Fairbairn, 1991; Rankin & Burchsted, 1992; Zera & Denno, 1997). Migrating insects therefore show different adaptations to compensate for this cost. Many insects first migrate then histolyse their flight muscles, using the released resources for egg production (Nair & Prabhu, 1985; Zera & Denno, 1997), while others produce smaller eggs (Solbreck, 1986). These processes allow the insects to concentrate egg production in a short period just after migration (Solbreck et al., 1990). Thus, in some colonist species, selection pressures have resulted in life-history traits that maximise both movement ability and reproductive output (Rankin & Burchsted, 1992). Simberloff (1981) found that a high proportion of the insect species colonising mangrove islands successfully were such super colonists, i.e. they had both a high immigration rate and a low risk of extinction during their first season in the new habitat.

Theoretically, habitats that are unpredictable in time and space should generally host species with greater colonisation ability than more predictable habitats (Southwood, 1962). The strongest empirical support for this theory has been obtained from studies of wing-dimorphic species, for which ephemeral habitats have been found to harbour a larger proportion of long-winged species (Wagner & Liebherr, 1992; Denno et al., 2001). Human activities often tend to make natural habitats less predictable, by disturbing them and fragmenting their distribution (Gaston, 1994). Being a good colonist should therefore increase the likelihood of species surviving in managed landscapes. In Fennoscandia, modern forest management has led to lower densities of most kinds of dead wood, and also to a more uneven distribution of this resource (Siitonen, 2001). Consequently, several saproxylic insect species have declined and become threatened (e.g. Gärdenfors, 2000; Rassi et al., 2000). Some seem to be restricted to stands with a high density and long continuity of suitable substrate (Økland, 1994; Nilsson & Baranowski, 1997; Siitonen & Saaristo, 2000; Jonsson et al., 2001; Jonsell & Nordlander, 2002); however, the extent to which these patterns are due to specific microhabitat demands that are only fulfilled in old growth forests, or to poor colonisation abilities of these species, is poorly understood (Nordén & Appelqvist, 2001).

The saproxylic tenebrionid beetles Oplocepahala haemorrhoidalis (F.) and Bolitophagus reticulatus (L.) have similar biological characteristics in many respects. Both species feed and breed exclusively in fruiting bodies of the wood-decaying fungus Fomes fomentarius (L.: Fr.) Kickxs (Palm, 1951, 1959), and both are able to utilise this substrate in a variety of microhabitat conditions (Jonsell et al., 2001; Jonsson et al., 2001). Their larval development usually takes 1 year, but they may live for more than one season as adults (Nilsson, 1997a; Jonsson et al., 2001). Where both species are present, they often occur at similar densities (Palm, 1959; Jonsson et al., 2001), however O. haemorrhoidalis occurs primarily in forest stands and wooded pastures with a high density and long continuity of substrate (Jonsson et al., 2001; Jonsell & Nordlander, 2002), whereas B. reticulatus is also common in forest stands with low substrate density (Jonsson et al., 2001; Jonsell & Nordlander, 2002). Based on these observations, it has been suggested that O. haemorrhoidalis is a weaker coloniser than B. reticulatus (Jonsson et al., 2001). A study of the genetic structure between inhabited forest sites has given some support for this hypothesis (Jonsson, 2002).

In the work reported here, a number of life-history traits that determine the colonisation ability of O. haemorrhoidalis and B. reticulatus were compared. It was hypothesised that B. reticulatus should invest more in flight and have a more efficient colonisation strategy than O. haemorrhoidalis. Therefore, the presence of flight wings and flight muscles was determined by dissecting individuals, willingness to take off was studied in a field experiment, duration of flight was tested in flight-mills, and the number and size of mature eggs in females were studied using dissection.

Methods

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Dissection

To study the status of flight wings and flight muscles in both sexes of O. haemorrhoidalis and B. reticulatus, and mature eggs in the females, adults of the two species were sampled at 2-week intervals from March to September 2001 (Fig. 1) at Lake Hosjön, ≈ 50 km east of Uppsala (59°52′N, 17°39′E) in south-central Sweden. The sampled individuals varied in age, and some were probably overwintered adults from the previous year(s) (for details of life cycle, see Nilsson, 1997a). In addition, young individuals that had not gone through their whole teneral period were found throughout the season. In total, 60 males and 71 females of O. haemorrhoidalis, and 143 males and 145 females of B. reticulatus were collected. For each species, samples were of similar size between sampling occasions and always came from more than one tree. Beetles were put in separate Eppendorf tubes and were killed by freezing within a few hours of collection. Within 6 months of collection, each beetle was removed from the freezer and incubated for 30 min at room temperature to thaw and allow surface moisture to evaporate, before being weighed to the nearest 0.01 mg then dissected. During the dissection, it was noted whether they had fully developed flight wings and whether they had fully developed flight muscles, developing or partly histolysed flight muscles, or no traces of flight muscle. For each female, the number of mature eggs was counted and the length and width of one randomly sampled mature egg was measured to the nearest 0.1 mm. Flight muscles were considered fully developed if they were composed of thick flight muscle fibres that had a yellowish or pinkish colour and were attached firmly to the meso-alionotum. Eggs were considered mature if they were > 75% as long as the largest egg found in any female of the respective species during the dissections. As eggs of both species had a form similar to a rotation ellipsoid, egg volume was calculated from the formula V = 1/6(π × W2)L (Solbreck et al., 1990; Berrigan, 1991). Relative egg volume was calculated by dividing the volume of the measured egg by the wet weight of the female.

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Figure 1. Seasonal changes in the proportions of dissected individuals (both sexes) of O. haemorrhoidalis and B. reticulatus (a) with fully or partly developed flight muscles and with only fully developed flight muscles and (b) in the proportion of females that had mature eggs in their abdomens.

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Flight-mill experiment

The flight duration of O. haemorrhoidalis and B. reticulatus was estimated using flight-mills. Beetles for this experiment were collected from the same locality (Hosjön) as those for the dissections, but only newly eclosed adult individuals, i.e. adult specimens that had not gone through the whole teneral period, were included. After collection, all individuals were kept separately in glass containers together with pieces of their host fungus, to which water was added every couple of days. Flight-testing of the beetles was started within 2 weeks after they had completed their teneral period. Thus, flight-tested beetles were of similar age, unmated, and neither starving nor desiccating. The beetles were kept in the room where the flight tests were carried out from collection until the end of the test period. The temperature was kept constant at 25 ± 2 °C with a day length of 20 h. Flight tests were carried out on five flight-mills similar to that used by Forsse and Solbreck (1985), but with a horizontal arm of 12.5 ± 0.2 cm. Flight-mills were placed in an arena lit by five fluorescent tubes, placed ≈50 cm above them (TL20W/47 De luxe, Philips, Belgium), in a gentle wind stream of <0.12 m s−1 produced by a fan. When flight-tested, the pronota of the beetles were glued (Hobbylim, Casco AB, Stockholm) in a horizontal position to the tip of the flight-mill arm, so that elytral movement was not restrained. Five beetles were tested simultaneously. Each beetle was kept on the flight-mill for an initial 20 min. Any beetle that did not make any flight attempts during this period was dismounted and put back in its container. If a beetle carried out short bursts of flight, <10 s long, during the 20-min period, it was left on the flight-mill for another 10 min. If a beetle made a flight lasting >10 s, the duration of the flight was recorded. When an individual stopped flying, it was treated in the same way as beetles that had not flown, i.e. it was left on the flight-mill for a further 20 min and, possibly, another 10 min if it carried out more flight attempts. The beetles were allowed to fly as many times as they wanted within the limits of the above protocol, and no flights were interrupted. Each beetle was flight-tested every day for a maximum of 31 consecutive days or until it began to fly. Beetles that flew were taken out of the experiment if they had not flown for 6 consecutive days following the last day of flight because they were then considered to have ended their flight period. The order in which beetles were tested and on which of the five flight-mills they were flown was determined randomly for each day. Flight tests were carried out only during the day but started and ended at different times of day. Flight experiments were carried out during July and August 2000 and June and July 2001. Only individuals that survived the whole test period were included in the analysis. The samples comprised 41 O. haemorrhoidalis (20 females, 21 males) and 46 B. reticulatus (25 females, 17 males, four of unknown sex). Six of the O. haemorrhoidalis and 31 of the B. reticulatus were flight tested in 2000; 35 O. haemorrhoidalis and 15 B. reticulatus were tested in 2001. All beetles were frozen directly after the experiment then dissected to examine flight muscle development and, in the females, presence of mature eggs.

Flight speed was also estimated from the flights in the flight-mills. This was done by counting the number of laps that each beetle flew in 30 s for roughly each 10 min of flight. The distance flown during this period was then estimated by multiplying the number of laps by the length of the perimeter of the flight mill. The average flight speed for each beetle was determined by averaging the 30-s estimates, and the average flight speed for each species was determined by averaging the values recorded for each beetle representing it. The average flight speeds derived for each species were used to estimate the distances flown for each individual. Flight speed estimates of this kind might be biased, but the estimates were similar to those found among free-flying insects of a similar body size (Dudley, 2000).

Take-off experiment

A field experiment was carried out to study willingness to take off in O. haemorrhoidalis and B. reticulatus. Beetles were collected and treated in the same way as in the flight-mill experiment, except that during the 24 h preceding the last test day the beetles did not have access to food. Take-off willingness was tested in Petri dishes with a 1.5-cm high cone-shaped piece of cork in the middle. The Petri dishes were placed on five platforms 1 m above the ground in the middle of a large clear-cutting. No trees with fruiting bodies of the host fungus, F. fomentarius, were present within a 50-m radius of the take-off platforms, and probably not on the rest of the clear-cutting. The take-off experiment was carried out with all individuals on 13 different days during summer 2001: 26 and 31 July, 2, 6, 8, 10, 13, 15, 16, 17, 21, 22, and 23 August. Tests were carried out only on days with relatively sunny weather and little or no rain. If it started to rain during a test, the experiment was interrupted until it stopped raining. The duration of the take-off tests was similar to the duration of the flight-mill tests. Each beetle was tested for 20 min each day. Any beetle that made a flight attempt, i.e. a rapid burst of flight that did not take it out of the Petri dish, was left for another 10 min on the platform. The order of testing and the platforms used were randomised for each individual and day. Only individuals that survived throughout the whole test period were included in the analysis. This comprised 18 O. haemorrhoidalis (eight females, 10 males) and 19 B. reticulatus (nine females, nine males, one of unknown sex). Temperature, degree of cloudiness, wind speed and direction were all measured when the individuals took off. The take-off direction of the flying beetle was noted, and its flight path followed as far as possible.

Statistical analysis

No significant differences in any flight parameter were detected between the sexes in either of the two species so statistical comparisons of flight-related characteristics between species were carried out using data for sexes pooled.

Differences in the proportions of individuals with fully developed flight muscles, with fully or partly developed flight muscles, and the proportion of females with mature eggs in their abdomen were tested between species using the χ2-test. Relative egg volume and number of mature eggs per female were compared using the Mann–Whitney test. In the flight-mill experiment, the species were compared with respect to the proportion of individuals that flew (>10 s) during the test period, median flight time per uninterrupted flight, duration of the longest uninterrupted flight, number of uninterrupted flights, total duration of flight, and the estimated total distance flown by the fliers. Differences between the proportions of fliers were tested using the χ2-test, while the other comparisons were made using the Mann–Whitney test.

Results

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Dissections

All sampled individuals of both O. haemorrhoidalis and B. reticulatus had fully developed flight wings, however a considerable proportion of both species did not have developed flight muscles (Fig. 1a). Bolitophagus reticulatus had a significantly higher proportion of individuals with fully or partly developed flight muscles (χ12 = 39.4, P < 0.001) and a significantly higher proportion of individuals with fully developed flight muscles (χ12 = 7.2, P < 0.01) compared with O. haemorrhoidalis. Flight muscles were present in B. reticulatus individuals throughout the season but were observed primarily around mid-summer in O. haemorrhoidalis (Fig. 1a).

A significantly larger proportion of the females of O. haemorrhoidalis than of B. reticulatus had mature eggs in their abdomen (χ12 = 4.16, P < 0.05), but throughout a large part of the season some females of both species were found to be carrying mature eggs (Fig. 1b). Females of O. haemorrhoidalis contained significantly fewer mature eggs (median: 6) than those of B. reticulatus (median: 14) (U = 444, NB = 19, NO = 19, P < 0.05) (Fig. 2a). Females of O. haemorrhoidalis with mature eggs weighed less (15 ± 3 mg) than those of B. reticulatus (24 ± 5 mg) but their eggs were larger (O. haemorrhoidalis: 0.17 ± 0.07 mm3, B. reticulatus: 0.06 ± 0.01 mm3). Relative egg volume was thus considerably larger in O. haemorrhoidalis than in B. reticulatus (U = 231, NB = 21, NO = 19, P < 0.001) (Fig. 2b).

image

Figure 2. (a) Number of eggs and (b) relative egg volume among dissected females of O. haemorrhoidalis and B. reticulatus with mature eggs.

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Flight-mill experiment

The distributions of total flight times during the test period for O. haemorrhoidalis and B. reticulatus are presented in Fig. 3a. A significantly lower proportion of O. haemorrhoidalis (0.39) than B. reticulatus (0.74) beetles flew (χ12 = 10.22, P = 0.01). Overall, the fliers of O. haemorrhoidalis flew for a significantly longer time (median: 3 h 9 min) than those of B. reticulatus (median: 1 h 35 min) (U = 827, NB = 36, NO = 16, P < 0.05). The longest total flight time recorded during the test period for a single O. haemorrhoidalis beetle was 7 h 30 min, while the corresponding figure for B. reticulatus was 29 h 36 min. The average flight speed on the flight mills was estimated to be 1.06 m s−1 for O. haemorrhoidalis and 1.17 m s−1 for B. reticulatus. Using these estimates to determine flight distances gave the distributions of total flight distances for O. haemorrhoidalis and B. reticulatus shown in Fig. 3b. The estimated median distance flown by flying individuals was 12 km for O. haemorrhoidalis compared with 6.7 km for B. reticulatus (U = 838, NB = 36, NO = 16, P < 0.05). The longest-flying O. haemorrhoidalis and B. reticulatus individuals were estimated to have flown 28.7 and 124.7 km respectively in total.

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Figure 3. (a) Total flight time and (b) estimated flight distance for O. haemorrhoidalis and B. reticulatus in the flight-mill study.

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The longest uninterrupted flights made by those O. haemorrhoidalis individuals that flew (median: 1 h 52 min) were significantly longer, on average, than those of B. reticulatus (median: 29 min) (U = 795, NB = 36, NO = 16, P < 0.01) (Fig. 4a). The longest uninterrupted flight made by an O. haemorrhoidalis was 5 h 43 min, while the longest flight made by a B. reticulatus lasted 3 h 29 min. The median duration of uninterrupted flight for almost all of the flying O. haemorrhoidalis beetles was longer than the corresponding duration for any flying B. reticulatus (Fig. 4b). No statistical test of significance of this parameter was therefore considered necessary. The O. haemorrhoidalis individual with the longest median uninterrupted flight flew for 5 h 43 min (in just one flight) and the corresponding time in B. reticulatus was 33 min. The median number of uninterrupted flights per flying individual was significantly lower in O. haemorrhoidalis (median = 2) than in B. reticulatus (median = 9.5) (U = 1038, NB = 35, NO = 16, P < 0.01) (Fig. 5a). The flights took place during a median period of 1 day in O. haemorrhoidalis and 2 days in B. reticulatus (Fig. 5b).

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Figure 4. (a) The duration of the longest uninterrupted flight and (b) the median duration of uninterrupted flights for O. haemorrhoidalis and B. reticulatus in the flight-mill study.

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Figure 5. (a) Number of uninterrupted flights and (b) number of flight days for O. haemorrhoidalis and B. reticulatus during the flight-mill study.

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After the flight-mill experiment, 80% of O. haemorrhoidalis and 90% of B. reticulatus had fully developed flight muscles, and 98% of O. haemorrhoidalis and 97% of B. reticulatus had at least partly developed flight muscles. Of the individuals with fully developed flight muscles, 44% of O. haemorrhoidalis and 17% of B. reticulatus did not fly during the experiment. Two per cent of the O. haemorrhoidalis and 8% of the B. reticulatus had flown but did not have fully developed flight muscles after the experiment.

Take-off experiment

None of the 18 O. haemorrhoidalis beetles tested made any attempt to take off in the field. In contrast, 37% of the B. reticulatus beetles took off and another 26% attempted to do so. Thus, in total, 63% of B. reticulatus undertook some flight activity during the 13 test days. Three of the seven flying individuals flew on the last test day, when all of the beetles had been starved since the day before. Details of the weather conditions, take-off time and direction, and height of flight of the seven fliers are shown in Table 1. All flights took place during the warmest part of the day, i.e. between 12.00 and 19.00 hours. The temperature at take-off was between 23 and 28 °C, the weather was generally sunny, and the wind light. Two individuals took off at a steep angle and disappeared from view 5–7 m above ground, whereas the others took off in a more horizontal trajectory.

Table 1.  Details of flights by Bolitophagus reticulatus observed during the take-off experiment with respect to time of take-off, weather conditions, flight direction, and height at which the beetles flew out of sight.
Sex of flying individualFlight dateTime of flight (hours) Tem- perature (°C)Weather Wind speed (m s−1)Flight direction when lost from sight Flight height (m) with distance from take-off (m) when lost from sight parentheses
Unknown2 August18.5024Sunny 20% cloud Sunny <0.12Against wind (W)5–7 (15)
Female8 August12.1024No cloud Sunny <0.12Against wind (S) Almost1 (2)
Male10 August14.402520% cloud Sunny 0.7downwind (E)1 (2)
Male13 August16.002460% cloud Sunny <0.12Downwind (NE) Probably 1 (2)
Male23 August13.20285% cloud Sunny <0.12downwind (NW) Against 1 (2)
Female23 August14.402825% cloud Cloudy 1.4wind (E) Against 2 (7)
Female23 August17.102360% cloud0.5wind (W)5–7 (7)

Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

This study revealed significant differences between O. haemorrhoidalis and B. reticulatus in virtually all of the life-history traits that were measured, indicating that these two species have quite different dispersal strategies, although they inhabit the same kind of breeding substrate.

In both O. haemorrhoidalis and B. reticulatus, all randomly sampled adults from the field (which were of varying age) had fully developed flight wings but only a minority had functional flight muscles. Thus, for much of their long adult lives, these beetles are incapable of flight. A large majority of the individuals of both species that were flight tested in the flight-mills had fully developed flight muscles at the end of the test period. These were all young individuals, suggesting that flight mostly takes place early in life. Only one of the young flight-tested females (an O. haemorrhoidalis female) had mature eggs in its abdomen, whereas a high proportion of the field-sampled females, which were of varying age, had mature eggs during the summer season. It is therefore likely that both O. haemorrhoidalis and B. reticulatus generally conform to the oogenesis-flight syndrome, with dispersal preceding reproduction, a common pattern among insects (Johnson, 1969). Furthermore, it is very likely that both O. haemorrhoidalis and B. reticulatus are capable of histolysing their flight muscles to use the resources thus released for egg production. A couple of individuals of both species did not have fully developed flight muscles after the flight-mill study, although they had previously flown.

The dissections showed that the number of mature eggs is larger in B. reticulatus than in O. haemorrhoidalis, indicating that B. reticulatus has a higher potential rate of population increase than O. haemorrhoidalis, which should be advantageous during colonisation (Southwood, 1977; Palmer & Dingle, 1989). To investigate this further, however, traits like egg production and development time to first reproduction also need to be considered. Despite its smaller body size, eggs of O. haemorrhoidalis were twice as large as those of B. reticulatus. The large eggs of O. haemorrhoidalis may be advantageous under crowded conditions with high levels of competition, as the larvae that hatch from them will be larger.

Bolitophagus reticulatus was considerably more willing to take off than O. haemorrhoidalis , both in the field experiment (in which no individual of O. haemorrhoidalis even attempted to take off ) and in the flight-mill experiment. An interesting observation was that three out of seven flying B. reticulatus individuals took off on the last day of the experiment, which was the only day on which the beetles had been starved. This suggests that shortage of food triggers take- off when beetles are in the right physiological phase for movement by flight. A great majority of the individuals of both species had seemingly functional flight muscles at the end of the flight-mill experiment, suggesting that their differential take-off frequency was determined behaviourally.

The flight-mill study showed that both species, especially O. haemorrhoidalis, are very potent fliers once on their wings. The total duration of flight of both species was of the same magnitude as durations recorded for certain saproxylic pest species that are recognised as good fliers (Solbreck, 1980; Forsse & Solbreck, 1985; Jactel & Gaillard, 1991; Akbulut & Linit, 1999). For example, Solbreck (1980) flight tested pine weevils Hylobius abietis for 10 consecutive days and found that ≈12% of the beetles flew for >2 h. In comparison, >30% of both O. haemorrhoidalis and B. reticulatus flew for >2 h in the present study. Calculations of flight distances suggest that the median flier of O. haemorrhoidalis is able to fly >12 km and the median flier of B. reticulatus can cover almost 7 km. These estimates of flight distance must be treated with caution, however, as they are based solely on laboratory experiments. There is a risk that flight-mill experiments may overestimate the flight duration of insects (Auerswald et al., 1998) but flight speeds could also be underestimated by this approach if the insects normally fly downwind in nature, as migrating insects commonly do (Solbreck, 1980; Gatehouse, 1997).

In the flight-mill study, O. haemorrhoidalis and B. reticulatus differed in the distribution of their total flight time between flights. Oplocephala haemorrhoidalis generally tended to make fewer flights, but of longer duration, than B. reticulatus, although several individuals of B. reticulatus occasionally undertook flights of long duration and some O. haemorrhoidalis made several short flights. Migratory flight has been defined as a persistent, straightened out movement accompanied by and dependent on an inhibition of responses to vegetative stimuli (such as food and oviposition sites) that will eventually arrest movement (Kennedy, 1961). Tethered flight studies have often been used to estimate the migratory tendency of different species and populations (e.g. Dingle & Evans, 1987; Rankin et al., 1994; Kent & Rankin, 2001). Most of the flights of O. haemorrhoidalis and some flights of B. reticulatus observed in this study were clearly of such long duration that they should be defined as migratory movements. Migration is often made even more efficient because the insects fly high above the ground with wind assistance (Gatehouse, 1997). The observation that two B. reticulatus beetles took off at a steep angle and were lost from view 5–7 m above ground indicates that individuals of this species do indeed fly above the tree canopy. Although these two individuals flew against the wind until they were lost from view after a few metres, it seems likely that this was just a temporary pattern. Other migrants have been observed to fly generally downwind after an initial flight against the wind (e.g. Solbreck, 1980).

The results indicate that O. haemorrhoidalis, in comparison with B. reticulatus, is less well adapted to a life in which frequent colonisation of new substrate by flight is necessary. A smaller proportion of O. haemorrhoidalis beetles had flight muscles, they were less prone to take off, and the females of this species had fewer (but larger) mature eggs in their abdomens compared with B. reticulatus. Instead, these traits indicate that O. haemorrhoidalis is adapted primarily to life in a fairly stable environment, where movement by flight is seldom needed. This could explain why O. haemorrhoidalis occurs mainly at sites with a high density of breeding substrates that have been available continuously for a long time (Jonsson et al., 2001; Jonsell & Nordlander, 2002). The deciduous forest phase following forest fires as well as wooded pastures are types of habitat in which high densities of deciduous trees with fruiting bodies of F. fomentarius are often present and O. haemorrhoidalis has probably thrived in the past (Esseen et al., 1997; Jonsson et al., 2001). Such habitats have always been more or less patchily distributed in the landscape, so occasional long-distance dispersal was presumably necessary to colonise them. This might explain the long flight duration of O. haemorrhoidalis once in flight. Similarly, Forsse (1989) found a high incidence of long migratory flights in bark-beetle species that utilise substrates that often occur in large but widely spaced patches.

The results indicate that B. reticulatus is better adapted to colonise scattered fruiting bodies than O. haemorrhoidalis. A higher proportion of B. reticulatus had flight muscles than O. haemorrhoidalis, it was more prone to take off, and females of the species contained more (but smaller) mature eggs. In addition, the tendency to make many flights of shorter duration suggests that B. reticulatus is well adapted to fly around within a forest searching for single trees with fruiting bodies. The fact that B. reticulatus has good colonisation ability is also supported by the low levels of genetic differentiation found between inhabited forest patches by Jonsson (2002), and by its similar average incidence per fruiting body at sites differing in substrate density and continuity (Jonsson et al., 2001; Jonsell & Nordlander, 2002). Within sites a few hundred square metres in extent, however, B. reticulatus tends to be more common where trees with suitable fruiting-bodies are located close to each other (Nilsson, 1997b; Rukke & Midtgaard, 1998; Sverdrup-Thygeson & Midtgaard, 1998). Several authors have suggested that these small-scale isolation patterns indicate that B. reticulatus has a relatively poor dispersal ability (Nilsson, 1997b; Rukke & Midtgaard, 1998; Sverdrup-Thygeson & Midtgaard, 1998; Knutsen et al., 2000). The results, however, show clearly that most B. reticulatus beetles would be able to fly far beyond even the most isolated of the trees in the cited studies. According to dissections, B. reticulatus mostly has reduced flight muscles except for a period shortly preceding its first reproduction. This fits well with the suggestion by Nilsson (1997b) that B. reticulatus often moves between trees by walking on the ground. Thus, trees that are located close to others may receive both young individuals colonising by flight and older individuals that colonise them by walking from trees nearby. This may be one reason why, within stands, densely grouped trees with the host fungus often have a higher incidence of B. reticulatus than more widely spaced trees.

To conclude, the results from this study suggest that B. reticulatus has a suite of life-history traits that make it better adapted than O. haemorrhoidalis to exploit the scattered trees with fruiting bodies present in managed forests. Despite being a powerful flier, O. haemorrhoidalis probably needs stands with a high density of breeding substrate to persist in a landscape in the long term. Other saproxylic insects that are restricted primarily to old-growth conditions (as described by authors such as Økland, 1994; Siitonen & Saaristo, 2000; Jonsell & Nordlander, 2002) may also have suites of life-history traits that make their exploitation of the scattered resources in managed forests inefficient.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The research on which this paper was based was financed by the Oscar and Lili Lamm Memorial Foundation. I thank Frauke Beutelmann, Beatrice Lindgren, Kajsa Lindström, and Åsa Nordlander for assistance in the laboratory and in the field. Göran Nordlander, Christer Solbreck and Lars-Ove Wikars are thanked for valuable comments on previous versions of the manuscript.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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