摘要:

Abstract.1. At their high‐altitude overwintering sites in Mexico, monarch butterflies frequently are subjected to sub‐zero°C temperatures during December‐March. Although monarchs have moderate supercooling ability, two ecological factors strongly influence their capacity to resist freezing: wetting and exposure to the clear night sky.2. As shown in Fig. 2, 50% of a population of butterflies with water on their body surfaces freeze at warmer sub‐zero temperatures (‐4.2°C) compared to butterflies with no water on their bodies (‐7.7°C). 100% mortality occurs, respectively, at −7.7°C and −15°C.3. Comparative measurements of rainfall within a large overwintering colony in Mexico indicated that the intact canopy acts as an umbrella that reduces butterfly wetting during winter storms.4. Variable experimental exposure of butterflies to the clear night sky indicated that openings in the forest canopy increases radiational cooling and causes monarch body temperatures to drop as much as 4°C below ambient air temperature. Monarchs under dense cover had body temperatures approximately the same as the ambient air temperature, but more exposed individuals had body temperatures below ambient in direct proportion to the degree of exposure. Consequently, forest thinning increases the probability that the butterflies will freeze to death.5. Whereas both wetting and exposure are increased by disturbance of the forest canopy, the interaction of these two factors exacerbates freezing mortality during winter storms: 50% of dry and unexposed butterflies froze at −8°C, whereas wetted and fully exposed butterflies froze at only −0.5°C.6. Butterflies inside and on the bottom of the fir bough clusters are better protected from wetting than those on the outside. This supports the hypothesis that the structure of the butterfly clusters has evolved through individual selection to avoid wetting.7. The data strongly reinforce previous evidence that forest thinning should be totally prevented within and adjacent to the overwintering sites in order to minimize both wetting and exposure of the butterflies that synergistically increase winter mortality at the ov

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01177.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. Hypotheses about declining populations of container‐inhabitingAedesmosquitoes following the invasion by additional species were tested.2. The larval competition hypothesis was studied experimentally in pure and mixed cultures ofAedes aegypti(L.),A.albopictus(Skuse) andA.triseriatus(Say). The experiments used decomposing leaf litter in the laboratory, as opposed to most previous research which used non‐natural food.3. Resistance to starvation is introduced as a new measure of larval performance and competitiveness. The hypothesis is that more successful larvae store larger energy reserves and resist the lack of food longer.4. Contrary to previous research showing better performance ofA.aegyptiin mixed cultures,A.albopictusdeveloped faster and had greater survival when natural food was used.5. Resistance to starvation was greater in the better performing species (i.e.A.aegyptiwith non‐natural food andA.albopictuswith leaf litter). Oxygen consumption by starved larvae was similar in the three container species, and in the ground‐water mosquito,A.taeniorhynchus(Wied.), whose resistance to starvation was comparatively v

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01178.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Summary.1. The length of time that a gall‐forming midge,Rhopalomyia califomica, was vulnerable to attack by four parasitoid species was measured in the field at two locations.2. The midge had a restricted window of vulnerability to each of the parasitoid species, but similar windows of vulnerability were found at the two sites.3. A stage‐structured model was used to illustrate that the length of the vulnerable window should have no effect on the fraction parasitized by a single parasitoid species if that species is the only parasitoid attacking the host in a coupled host‐parasitoid interaction. However, the length of the window of vulnerability can have a positive effect on the fraction parasitized by a species in competition with other parasitoid species.4. The length of the window of vulnerability can help explain the relative field abundance of four of the common parasitoid species ofR.califo

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01179.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. It has been proposed that herbivore populations are lower in diverse than in simple plant habitats because of greater abundance and/or higher efficiency of natural enemies in mixed plant stands. However, higher enemy colonization is expected in monorather than multispecific vegetation if the response of specialist natural enemies to habitat diversification is similar to that of monophagous herbivores.2. We used release‐recapture experiments to determine how the presence of maize (non‐host plant) influences the movement of the parasitoidPediobius foveolatusin the absence of hosts. We then assessed how vegetation diversity affects wasp reproduction (parasitism) and subsequent density in the presence of its hosts, Mexican bean beetle larvae.3. Fewer female wasps immigrated into and more emigrated out of a bean‐tall maize intercrop than bean monocultures. Bean plant density and the presence of maizeper sedid not significantly affect parasitoid immigration. Instead, maize height was the primary factor contributing to lower female immigration into the bean‐tall maize plots. However, tall maize plants did not impede the wasps' within‐habitat movement.4. When wasps were released outside the plots, higher parasitism was recorded in monocultures, irrespective of host density. In contrast, when wasps were released within the plots, significantly higher parasitism rates were found in the bean‐tall maize habitat.5. Results suggest that female wasps accumulate in the bean‐tall maize habitat in response to resources other than hosts and, ultimately, wasp density may be determined primarily by differential emigration rather than by immi

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01180.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. This study examined the invasion sequence ofDrosophilaspecies arriving at decomposing strawberries.2. It was found that the age of fruit affected the likelihood ofDrosophilaarrival, flies preferring relatively fresh fruit. However, there were no repeatable trends in invasion sequence ofDrosophilaspecies, different species responding in a similar manner to changes in fruit age.3. Although no systematic trends in invasion sequence were observed, species tended to be temporally separated due to stochastic differences in their arrival at the fruit.4. It is suggested that stochastic, as well as repeatable, temporal separation of species can have an important influence on subsequent community development.

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01181.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract1. Egg parasitoids in the genusAnagrus(Hymenoptera: Mymaridae) are important mortality factors for grape leafhoppers (Erythroneura elegantula; Homoptera: Cicadellidae) in California vineyards, yet must overwinter in habitats external to these vineyards. Existing evidence suggests that French prune trees, which harbour the overwintering hostEdwardsiana prunicola, planted adjacent to vineyards may enhance early‐season abundance ofAnagrus.2.Anagrusoverwintering in French prune tree refuges were labelled with the trace element rubidium in four separate experiments. Rubidium‐labelledAnagruswere captured in adjacent vineyards in two of the experiments, confirming that French prune trees contribute to early‐seasonAnagruspopulations.Anagrusfrom refuges were captured at the most distant sampling positions, 100 m from refuges.3. Use of rare element labelling has, for the first time, enabled the relative contribution of different sources to early‐season colonization by this parasitoid to be quantified. Refuges contributed 1% and 34% ofAnagruscolonizing two of the experimental vineyards, respectively. The remainder originated from overwintering habitats external to the French prune/vineyard system.4. The spatial patterns ofAnagrusoriginating from external overwintering habitats suggest that the French prune trees are generating a ‘windbreak effect’.Anagrusdispersing within the windstream colonized vineyards at a higher‐than‐average rate immediately downwind of refuges.5. The amount of colonization byAnagrusfrom external overwintering habitats was apparently related to the distance to presumed overwintering habitats. These findings demonstrate that both the number of natural enemies emerging from a refuge and the composition of the surrounding landscape are important in determining the impact of local, small‐scale habi

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01182.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. Adult males of the two‐spot ladybird beetle,Adalia bipunctata, did not show a functional response to increase in aphid abundance and consumed markedly fewer aphids than do the females.2. At high densities of prey, females spent more time in area‐restricted search than when prey was scarce. Males were always less active than females and they did not respond to an increase in prey abundance by a change in searching behaviour.3. After a brief encounter with a female, a male showed area‐restricted searching behaviour. This behaviour occurred in response to encountering a female's elytra and in particular to a chloroform‐soluble component (sex pheromone) present on or in the elytra.4. Males needed to encounter a female in order to respond to her presence, which indicated the pheromone is a contact pheromone.5. The searching behaviour of males appeared to be mainly directed towards locating females; that of females towards locating aphids. This difference between the sexes should be taken into account when quantifying the predatory response of ladybirds to aphid abundance in th

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01183.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. The post‐emergence lateral migration of both sexes of eight stonefly species was examined in a dystrophic, fourth‐order forest river in eastern Finland.2. For this purpose, 7351 stonefly adults were collected with eighteen trunk funnels positioned in rows of six at distances of approximately 1, 15 and 60 m from the river. A further 1880 adults were also caught from the vegetation of the bank zone by sweep netting and with slit traps.3. The species could be grouped into two types with respect to migration distance:Isoperla difformis, I.grammaticaandLeuctra fuscatended to stay in the bank area, whereasNemoura flexuosa, N.avicularis, Amphinemura borealis, L.hippopusandN.cinereatended to disperse into the forest, so that the majority were found some distance away from the shore.4. The males of the leuctrids,N.flexuosaandN.cinereamigrated farther than the females.5. The sex ratio was significantly biased in all species exceptN.flexuosa.The isoperlids, leuctrids andA.borealisshowed a significant predominance of females in the trunk funnel catches, but males were significantly dominant inN.avicularisandN.cinerea.The material caught by other methods reversed the ratio forI.difformisandN.avicularis.Comparison of the sex ratios of the species with other reports revealed marked variation and deviation from unity, much of which could be attributed to bias introduced by the sampling methods.6. Lateral migration seems to be the first phase in the colonization cycle, although the latter as such was not studied here. Adults of Euholognatha species migrate farther than those of Systellognatha, a difference of which may be due to their ability to feed as adu

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01184.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. Previous studies on the arboreal termite community in coconut plantations of northern New Guinea showed thatMicrocerotermes biroiis the most abundant species, despite the fighting superiority of two competitor speciesNasutitermes princepsandN.novarumhebridarum.In this study we tested the hypothesis that the success ofM.biroiis due to its efficiency at colonizing new habitats following nuptial flights.2. We demonstrated experimentally the ability ofM.biroicolonies to replace their reproductives when removed, or to produce reproductives in satellite nests when isolated from the remainder of the colony. Replacement reproductives were always neotenics, derived from nymphs or workers.3. Despite the ability of neotenics to differentiate within their home colonies, 84% of field colonies were headed by dealated imagos. This value constitutes a minimum estimate of the proportion of field colonies founded independently by imagos after the nuptial flight.4. The monitoring of a young plantation during the first 31/2 years of its colonization by arboreal nesting termites revealed its invasion byM.biroi, which colonized 63% of the trees while neitherNasutitermesspecies appeared.5. Our results demonstrate thatM.biroiis actually a pioneer species, able to invade a new habitat by means of nuptial flights. They outline the importance of each species' reproductive strategy in shaping the arboreal‐nesting termite communit

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01185.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

Abstract.1. The effect of body size on different components of male fitness was studied forEpirrita autumnata, a geometrid known for its eruptive population dynamics. Body size is the main determinant of female fecundity in this species.2. Longevity of males was found to have a weak negative correlation with body size at low temperatures. No significant correlation was found at higher temperatures.3. We found no correlation between male size and female fecundity or egg size which is consistent with the small size of spermatophores in this species.4. Small and large males were equally successful when allowed to compete for females in laboratory conditions.5. In one or two field collections, males found mating were larger than males found singly. Large males also had an advantage in finding of virgin females, offered experimentally. No size‐assortative mating was recorded.6. We conclude that size‐dependent mate location ability is the factor accounting for most of the variance in male fitness inE.autumnata.The dependence of fitness on body size may well be equally strong in males and fema

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1996.tb01186.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY