摘要:

Abstract.1The response of different clones of sand‐dune willow,Salix cordata, to herbivory by a specialist herbivore,Altica subplicata, was studied in three glasshouse experiments. Plants were caged and exposed to three herbivory treatments: no beetles, low number of beetles, and high number of beetles.2Plants consistently had significantly higher growth rates in the absence of herbivory than under conditions of low or high herbivory (1.5–6 times higher).3Herbivore treatment influenced mortality from drought stress; more plants from the low and high herbivory treatments (40% and 80%) died from drought stress than did control plants (0%).4Clone genotype significantly influenced growth rates and the susceptibility of plants to drought stress. However, clones showed similar growth responses to herbivory, suggesting a lack of genetic variation in tolerance or resistance to herbiv

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00246.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1Abundance of leaf‐mining larvae of the outbreak speciesEriocraniaspp. was monitored in northern Sweden in 1955–67 and 1984–92 in a mountain birch forest rejuvenated by anEpirrita autumnataoutbreak in 1954–55.2Eriocraniamine density fluctuated in a regular biennial pattern, probably due to a semivoltine life cycle. The alternate year fluctuations had shifted phase between the two study periods.3The density fluctuated at a significantly higher mean level and with a lower amplitude in thec.10‐year‐old forest in the 1960s than in the 30‐year‐old forest in the 1980s. However, no difference was found in mine density betweenc.30‐ and>60‐year‐old stands in the 1980s.4Significant correlations between the density of mines in high‐density years and date of budburst, and between rate of change between high‐density years and the time between snow‐melt and budburst indicate effects of weather. No correlation was found between yearly average mine density and date of budburst, precipitation or temperatures in May or June.5No correlation was found between the mean densities ofEriocraniamines andE.autumnatacaterpillars in the same and the two preceding years. However, in the first (1955) and highest of three studiedE.autumnatapeaks a negative effect on population density ofEriocraniawas indicated.6A significant, negative correlation between the number ofEriocraniamines andE.autumnatacaterpillars on single branches was found in three out of eighteen years. The separation of caterpillars and mines at branch level indicates an effect of avoidance at interm

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00247.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1The effects of resource limitation and the lethal and sublethal effects of a granulosis virus on a lepidopteran host, the Indian meal moth,Plodia interpunctella, were examined.2The food quality was manipulated by the addition of an inert bulking agent (methyl cellulose) which caused the size, development rate and fecundity of the moths to be reduced.3The resource quality had no effect on the mortality due to the virus. In contrast, sublethal effects of the virus on pupal weight were more apparent under conditions of resource limitation.4Considerable variation between the sublethal effects after challenge with different doses of the virus was found. The balance between deleterious sublethal effects of the virus and the selection of more robust individuals by the bioassays is proposed as a mechanism to explain this variation.5Implications for the dynamics of insect hosts and their pathogens are discussed.

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00248.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Summary1We examined the foraging behaviour of the parasitoid wasp,Closterocerus tricinctus(Ashmead) (Chalcidoidea: Eulophidae), as it visited larvae of the leaf‐mining moth,Cameraria hamadryadella(Lepidoptera: Gracillariidae), in an outbreak population.2We tracked females ofC.tricinctus, recording the time spent searching for mines and handling host larvae. The density of leaf‐mines (host larvae) and their condition were recorded for each leaf visited. A subset of leaves visited byC.tricinctuswas enclosed in fine mesh bags so that foraging success could be determined by rearing or dissection. The average density of mines and the average leaf‐area mined was estimated for a random sample of leaves from each tree.3The selection of leaves upon which to forage appears to be density‐dependent.C.tricinctusvisits leaves with leaf‐mine densities twice the average, and when switching leaves lands directly on leaf‐mines 5 times more often than expected assuming random landings.4The total time spent foraging on a leaf, the average time spent handling hosts, and the total search time within leaves tend to decline on leaves with many hosts, but the observed declines are not statistically significant.5The proportion of leaf‐mines visited within a leaf is strongly inversely density‐dependent. 30% of visits to leaf‐mines are re‐visits and 29% of handling time is spent re‐handling previously visited hosts. Furthermore, only 21% of visits to mines lead to successful parasitism. We suggest that self‐interference and the avoidance behaviour of the host may reduce the number of visits of leaf‐mines byC.tricinctuswithin a leaf.6The effect of the strongly inversely density‐dependent foraging investment within leaves is to offset the observed density‐dependent pattern of leaf visitation making the overall spatial pattern of visitation byC.tricinctusto mines ofC.hamadryadellainversely density‐dependent.7We suggest that the uncertainty ofC.tricinctussurviving on multiply mined leaves because of density‐dependent host mortality due to intraspecific competition in high‐density host populations, the rarity of high‐density host populations, and the rarity of multiply‐mined leaves in low‐density host populations combine to select against an aggregat

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00249.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1The gregarious larvae of the chrysomelid beetleMicrorhopala vittatamine the leaves of goldenrods (Solidagospp.). These mines serve both as food and as shelter for the larvae.2Life‐table data and experiments indicated that mine initiation and moves to secondary mines represented especially vulnerable stages during larval development. Leaf mines effectively protectedM.vittataagainst predators in the field.3Field experiments indicated that larvae hatching from larger clutches of eggs stood a greater chance of surviving to pupation, primarily because larvae hatching in groups proved more successful at initiating leaf mines. Once inside the leaf mine, however, larvae feeding in large groups attained lower adult masses, and were more likely to abandon the natal mine and did so earlier in development because large groups more rapidly destroyed a lea

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00250.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1Bioassays were carried out to examine differences in the feeding of a folivorous weevil,Stereonychus fraxiniDe Geer, on leaves from ash saplings with undamaged roots, and those with damaged roots. Beetles ate significantly more of the leaves of root‐damaged saplings in a choice experiment.2A separate study of the effect of root damage on leaf toughness was carried out on a second group of ash saplings. Saplings with damaged roots had less tough leaves than control saplings. Decreased leaf toughness in hypothesized as a mechanism to explain the preference of weevils for leaves from damaged trees.3Both chemical and physical changes may occur in the tissues of ash trees in response to environmental stress such as drought and root damage.4Root damage caused by agricultural disturbance is hypothesized as a mechanism making mature ash trees in hedgerow ecosystems more susceptible to insect herbivore

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00251.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1Gastrophysa viridulaDegeer (Coleoptera: Chrysomelidae) and the pathogenic rust fungusUromyces rumicis(Schum.) Wint. both occur on leaves ofRumex crispusL. andR.obtusifoliusL. Individual stages of beetle development, and egg laying, were compared on healthy and infected leaves of each plant species in the laboratory. Oviposition choice was investigated in the field and laboratory.2Beetles reared on infected leaves of each species had greater larval mortality and slower development than those reared on healthy leaves. Although larvae feeding on infected leaves consumed up to 2.5 times more dry weight than those reared on healthy leaves, they had a lower relative growth rate and pupated at a lower weight. These changes were consistent with the reduced nutritive quality of rust‐infectedRumexleaves.3Fecundity of beetles reared on infected leaves of both species was considerably reduced. Eggs laid by beetles feeding on infectedR.crispusleaves also had a reduced viability.4The beetle developed consistently poorer on healthyR.crispusthan on healthyR.obtusifoliusthroughout its life‐cycle. Differences in larval performance were greater between host species than between infected and healthy leaves.5Oviposition was similar on infected and healthyR.crispusin both the laboratory and field. However, adults consumed less, and laid fewer eggs on infected than on healthyR.obtusifolius.The pattern of egg laying on different aged leaves was affected by rust infection: a greater proportion of eggs was laid on the older, infected leaves, than on the equivalent aged leaves on the healthy plants. Few larvae survived from eggs laid on rusted leaves in the fi

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00252.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1The survival of adult and first‐instarMyzus persicaereared at 20°C and 10°C was investigated after brief (1 min) exposure in the absence of plant material to temperatures between −5°C and −25°C, and extended exposures on plants of 1–10 days at a constant 5°C, 3°C and −5°C and a 24 h cycling regime between 5°C (18 h) and −5°C (6 h).2Life stage, rearing temperature, period of exposure and temperature regime all had a significant effect on the ability of aphids to survive cold. The effects of life stage and rearing temperature were most noticeable following exposure to cycling temperatures and extended exposures at −5°C, and least apparent after 1 min exposures at lower sub‐zero temperatures.3Mortality following exposure to temperatures cycling between −5°C and 5°C was greater than that at 3°C (the mean of the cycling temperatures) and less than at a constant −5°C, suggesting that when temperatures fluctuate by a few degrees around 0°C the minimum temperature may affect survival to a greater extent than the mean.4These results suggest that an overwintering population of acclimatedM.persicaewould persist without significant mortality after a period of 7

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00253.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1Newly‐emerged, second‐instar jack pine budworm (Choristoneura pinusFreeman) establish spring feeding sites preferentially in the pollen cones of their host tree,Pinus banksianaLamb.2Laboratory studies showed that the rate of establishment and survival of jack pine budworm on pollen cones was high throughout the entire spring emergence period of the insect.3In contrast, the rate of establishment and survival of jack pine budworm on vegetative buds was very poor early in the spring. Vegetative buds were only acceptable as feeding sites to the jack pine budworm for a relatively brief period in late spring.4Field studies showed that the change in population density of jack pine budworm during the spring emergence stage, as expressed byk‐values, was a function of the abundance of pollen cones in the stand. Population reduction was greatest in those stands with the fewest pollen cones.5Direct measurement of spring dispersal by jack pine budworm showed that dispersal and consequent losses to the budworm population were greatest in stands with the fewest pollen cones.6We conclude that changes in the density of jack pine budworm are strongly influenced by production of pollen cones in the host stand. Because pollen cone production is related to previous years of defoliation by the jack pine budworm, we propose that pollen cones act as a density‐dependent factor governing the density of early‐stage jack pine budworm.7The resulting dynamics are compared to those of other budworm species and used to explain observed regional and temporal patterns of jack pine budworm

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00254.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

Abstract.1The solitary parasitoidAnagrus delicatusattacks hosts (plant‐hopper eggs) that exist in distinct batches and that are readily detectable to the foraging parasitoid. However,Anagrus delicatusattacks only a small fraction of the available hosts within a batch of eggs and then disperses (Cronin&Strong, 1993a). Here we address the question: Why isA.delicatusabandoning seemingly high quality patches?2A parasitoid with an imperfect ability to discriminate between unparasitized hosts and hosts that it has attacked earlier within the same patch experiences a risk of self‐superparasitism when attacking multiple hosts within a single patch. Self‐superparasitism can incur costs in the form of lost time and eggs. Early patch leaving can be favoured as a means of avoiding the costs of self‐superparasitism.3A simple static model demonstrates that patch leaving is favoured by low costs of travelling to a new patch, high error rates in discriminating previously self‐parasitized hosts, and high levels of parasitism in the currently occupied patch.4A more detailed dynamic state variable model, parameterized forA.delicatus, demonstrates that this parasitoid's seemingly enigmatic behaviour can be explained under our hypothesis. In order for this to be the case, we predicted thatA.delicatuscannot recognize previously parasitized hosts. Subsequent to our prediction, Cronin&Strong (1993b) demonstrated that experiencedA.delicatusdo not avoid ovipositing in previously self‐parasitized hosts.5Optimal patch leaving rules can be highly sensitive to even very low host discrimination error rates, which may be widespread among

ISSN:0307-6946    

DOI:10.1111/j.1365-2311.1994.tb00255.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY