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 parasitoid Pediobius foveolatus in 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 maize per se did 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 immigration rates. 相似文献
The effects on grain weight of removing spikelet and floralorgans, other manipulations of the florets or spikelets andcovering intact or treated spikes to reduce the amount of lightwere tested. Removal of organs which exposed the developing grain to increasedlight intensity resulted in a reduction in grain size. The restrictionin grain growth was already apparent 2 weeks after anthesisand was accompanied by a more compact pericarp. Covering thetreated spikes with opaque bags resulted in normal pericarpstructure and restoration of grain weight. Only a small portionof the restoration effect could be attributed to the increaseof humidity under the covers. Treatments which exerted physical constraint on the developinggrain reduced its size and affected its shape. The size of thefloret cavity and the transmission of light through floral bractsare shown to be two of the factors controlling grain growth. Triticum destivum L., Triticum spelta L., wheat, morphogenesis, histogenesis, grain development, glume, lemma, palea 相似文献
SUMMARY. The diet and feeding habits of natural and cultured populations of Mesocyclops leuckarti were studied. Nauplii and copepodid stages I–III are herbivorous. Copepodid stages IV–V and adults preferentially preyed upon Ceriodaphnia and Diaphanosoma whereas Bosmina was not preyed upon. Rate of predation on Ceriodaphnia and newborn Artemia nauplii was higher between 15–22°C than between 22–27°C. Under starvation conditions, the survival time for males was shorter than for females. 相似文献
Abstract. 1. This study first measured the effect of plant (radish, pepper, and wheat), aphid–plant combination (the green peach aphid Myzus persicae on pepper and radish, and the bird cherry-oat aphid Rhopalosiphum padi on wheat) and the host on which Aphidius colemani was reared (the cotton aphid Aphis gossypii on cucumber, M. persicae on radish and pepper, and R. padi on wheat) on host choice behaviour of the parasitoid and the performance of its offspring. Then, the effect of predator presence ( Coccinella undecimpunctata larvae) on host preference of the wasps was tested. 2. When reared on M. persicae on either radish or pepper , wasps preferred the aphid–plant combination from which they had emerged. Wasps reared on A. gossypii (naïve to all hosts offered) and R. padi preferred to parasitise M. persicae on radish and M. persicae on either radish or pepper, respectively. Rhopalosiphum padi on wheat was the least preferred and also the least suitable host, as determined by offspring body size. 3. Contrary to expectations, the presence of predators did not influence the host choice of A. colemani , even when predator-free plants were offered nearby. Results indicate that wasps are more likely to remain in some host systems but not in others, even when facing risk of predation. 4. Results are discussed in respect to field data and the relative importance of risk of predation and host preference to wasp fitness. 相似文献
The abundance of Ceriodaphnia reticulata (Jurine) in Lake Kinneret is restricted every year to the period March to June with a peak in May. The most constant parameter accompanying the peak is temperature with a range of 20–22° C. An attempt was made to clarify whether the estival high average temperatures observed in the epilimnion of this lake (27–28° C) could explain the decrease of the Ceriodaphnia population. Three physiological parameters: food intake, ammonia excretion and respiration rates, were measured experimentally at three temperatures (15, 22 and 27° C). The dry weight of individuals of the winter and summer populations were measured, and the percentages of egg-bearing females in the populations were calculated. It was found that temperature increase accelerates the rates of ammonia excretion and food ingestion. The acceleration was greater in the lower temperature range (15–22° C) than in the higher one (22–27° C). Conversely, the enhancing of respiration rate is higher in the upper temperature range. The resulting deficit of energy at high temperatures causes a reduction in body size and a significant diminution of egg production, which leads to a progressive decline of the population. 相似文献
REDUCED glutathione (GHS) is maintained in substantial concentration (3 × 10?3 M) within neurones1. The availability of new thiol-oxidizing agents like diamide2,3, which freely enter cells and the principal action of which is to reduce the GSH concentration by conversion to GSSG (equation 1), promoted an investigation of the effects of these agents on release of transmitter at frog myoneural junctions. illustration
Abstract. 1. In studies of insect-host plant interaction it is often suggested that insects preferentially colonize host plants (or sites within plants) on which their fitness is maximized (a positive covariance of preference and performance). This suggestion stems from the assumption that natural selection has driven the system toward optimal use of resources. 2. Our study of the galling aphid Smynthurodes betae Westw. demonstrates that the distribution of galls on leaves is not due to preference, and can be altered by manipulating the aphid arrival time or the shoot growth rate. 3. We found no correlation between gall density and performance (aphid clone size) at different positions along the shoot. 4. Because leaves on the growing shoot are not equally responsive to aphid stimulation, the colonizers have no choice but to settle on leaves that are at the right stage when they arrive. 5. S.betae colonizers did not discriminate between shoots of their host and a congeneric non-host, on which their fitness is invariably zero. 6. Synchronization between galler and host plant phenologies seems to be the key to the observed distribution of galls on the tree. The data give no support to the preference-performance hypothesis. 相似文献
Summary Concurrent morphological, anatomical and physiological changes took place during the first reproductive cycle in the Australian red-claw crayfish Cherax quadricarinatus, which prepared the female for spawning and holding of the newly deposited eggs. The endopod became longer and wider than the exopod and developed a mixture of plumose and long thin simple (ovigerous) setae. Small oocytes (0.24±0.05 mm) were present in the immature ovary. The growing ovary contained two distinct oocyte populations: one consisted of small (0.55±0.07 mm), barely growing oocytes, while the other consisted of large oocytes, which increased in size continuously (0.73 to 2.55 mm) until egg laying took place. A gradual change in the relative abundance of ovarian polypeptides occurred until the late vitellogenic stage (large oocytes < 1.8 mm). Three predominant female-specific, SDS-PAGE separated, polypeptides were observed (103, 78 and 73 kDa) that may represent vitellin subunits. The most abundant carotenoid in the ovary was astaxanthin, while β-carotene was present at a lower concentration. The strong correlation between the increasing diameter of the oocyte and the concentration of astaxanthin in the ovary and in the hemolymph suggested an association of astaxanthin with vitellin and vitellogenin. 相似文献
