Persistence of experience effects in the parasitoid Trichogramma nr. brassicae

1. The importance of learning in parasitoids is likely to depend on how long learning effects persist, the number of learned stimuli that can be retained, and whether learning effects are confined to low ranked hosts.
2. The duration of learning of host-specific stimuli was tested in four strains of Trichogramma nr. brassicae (Hymenoptera: Trichogrammatidae). Both the persistence of learning of the rearing host and oviposition experience were tested in separate experiments.
3. The response to a non-preferred host, Sitotroga cerealella , was measured at different times following experience on that host or an alternative host, Heliothis punctigera . Both types of experience persisted for at least 5 days in all four strains.
4. The effect of oviposition in a highly preferred host, H. punctigera , following oviposition in S. cerealella was also tested. The increased acceptance of S. cerealella following oviposition on this host persisted despite a subsequent oviposition on H. punctigera .
5. The implications for host range at both individual and population levels are discussed.     

Oviposition by Danaus plexippus in relation to cardenolide content of three Asclepias species in the southeastern U.S.A.

Abstract. 1. Female monarchs were observed in the field ovipositing on a native North American milkweek host, Asclepias humistrata L. As in a comparable Australian study on an introduced novel host ( Asclepias fruticosa L.) we found post-alighting rejection of plants with low and high cardiac glycoside concentration (CG).
2. Most oviposition took place on plants with CG in the range 200–500 μg/0.1 g dry weight. Thin-layer chromatography showed no obvious qualitative difference in cardenolide types between accepted and rejected plants, excepting an indication that rejected plants may have a higher level of more polar cardenolides.
3. In a controlled laboratory experiment comparing oviposition on a low ( A.incarnata L.) vs high ( A.curassavica L.) CG host plant species we found no relationship between CG and oviposition on the low CG species, but a negative relationship in the high CG species. This corroborates our findings on oviposition on single host species in the field.
4. We also record the first indication of a physiological cost of monarch larvae feeding on plants high in CG. There was a significant negative relationship between survival of first instar larvae and CG levels in plants. This study opens the way for further work on the association of monarch butterflies and their toxic milkweed hosts.     

Oviposition acceptance and larval development of Chilo partellus stemborers in drought‐stressed wild and cultivated grasses of East Africa

Maternal host choices during oviposition by herbivorous insects determine the fitness of their offspring and may be influenced by environmental changes that can alter host‐plant quality. This is of particular relevance to ‘push‐pull’ cropping systems where host preferences are exploited to manage insect pest populations. We tested how drought stress in maize and companion plants that are used in these systems affect oviposition preference, larval feeding, and development of the spotted stemborer, Chilo partellus Swinhoe (Lepidoptera: Crambidae). Five host species were tested (all Poaceae): maize (Zea mays L.), Napier grass (Pennisetum purpureum Schumach), signal grass [Brachiaria brizantha (A. Rich) Stapf], Brachiaria cv. ‘Mulato’, and molasses grass [Melinis minutiflora (Beauv.)]. Under drought stress, maize experienced as much oviposition as control unstressed maize in choice and no‐choice experiments. Similarly, larval leaf damage was not significantly different in drought‐stressed vs. unstressed maize. In contrast, oviposition occurred less on drought‐stressed than on unstressed Napier and signal grass. Oviposition acceptance and leaf damage remained low in both drought‐stressed and unstressed molasses grass and Mulato. Larval survival and development remained high in drought‐stressed maize, but not in Napier, signal, and molasses grass and Mulato, where survival and development were low in both drought‐stressed and unstressed plants. Our results indicate that herbivore responses to drought‐stressed plants depend on the plant species and that drought stress can change host preference and acceptance rankings. In particular, trap‐crops such as Napier grass may not divert oviposition from the main maize crop under drought stress conditions.     

Insect oviposition plasticity in response to host availability: the case of the tephritid fruit fly Bactrocera dorsalis

1. Insect oviposition behaviour is ecologically and physiologically plastic. For tephritid fruit flies, Bactrocera dorsalis Hendel, host availability varies spatially and temporally. Females are expected to adopt adaptive oviposition strategies to maximise lifetime reproductive fitness, including survival. Bactrocera dorsalis oviposition tactics in response to different host availabilities were investigated. 2. This study includes three treatments: (i) variable host densities (host density varied according to a fixed cycle from day to day over values of 1, 5, 10 and 20 hosts per cage), (ii) a fixed high host density (20 hosts per cage), and (iii) a fixed low host density (1 host per cage). 3. Daily egg‐laying number per female over the course of 27 days was entirely independent of host density and highly dependent on female age. As host availability increased, females accepted significantly more hosts, generally laid small egg clutches, and more broadly distributed the eggs. 4. Tephritid fruit flies adaptively adjusted egg clutches in ways that reflected the variability of host availability. Egg‐ and time‐limitation constraints appeared to drive these adjustments. Female egg maturation was triggered by oviposition activity and reflected marked lifetime trade‐offs. Such strategies involved specific time schedules for egg laying. 5.This study defined the oviposition plasticity of the tephritid fruit fly. These results have general implications for the behavioural ecology of insect herbivores and parasitoids.     

Egg size and composition in an ageing capital breeder – consequences for offspring performance

1. Egg size is often used as a proxy of egg quality although size and composition may vary, e.g. in insects egg size usually decreases as female ages. Whether this decrease in size reflects reduced concentrations of essential nutrients such as lipids and proteins of eggs laid by ageing females, or does reduced size per se explain often observed lower fitness of later laid eggs is poorly explored. 2. Egg properties were compared with fitness parameters of offspring laid on the first and fourth night during the oviposition period of a capital breeding moth, Cleorodes lichenaria (Hufnagel). The study aim was to explore whether decreased egg size is caused by decreased provisioning into later laid eggs measured as egg protein and lipid concentration, and whether it results in lower fitness of later laid offspring. 3. The fresh and dry weight of eggs decreased over the oviposition period, but the protein and lipid concentration remained constant. Survival of larvae was lower among the fourth night laid offspring on a low quality host Parmelia sulcata Taylor compared to a high quality host Ramalina fraxinea (L.) Ach. No differences were observed in egg fertility or hatchability, neonate survival without food and pupal mass between the offspring produced on different nights. 4. Decreased survival of offspring produced later was rather attributable to absolute provisioning (i.e. lower weight of eggs) than relative provisioning (i.e. decreased concentrations of nutrients in eggs). It is argued that lower survival of later laid smaller eggs on low quality diet is likely attributable to physical and chemical characteristics of host lichens and/or physical properties of tiny neonate larvae.     

Influence of temperature, photoperiod and humidity on oviposition and egg hatch of the root-feeding flea beetle Longitarsus bethae (Chrysomelidae: Alticinae), a natural enemy of the weed Lantana camara (Verbenaceae)

The root-feeding flea beetle Longitarsus bethae Savini & Escalona, was introduced into South Africa as a candidate biological control agent for the noxious and invasive weed, Lantana camara L. As part of the study to predict the beetles' survival in its new range, the influence of climatic conditions on its egg development and reproductive performance were investigated in the laboratory. The threshold temperature (T degrees) and degree-days (DD) required for egg hatch were determined after exposing the eggs to various constant temperatures (12, 17, 22, 27 and 32 degrees C) in separate growth chambers. The DD required for egg hatch was 178.6, and the temperature threshold required for egg hatch was 11.3 degrees C. Survival of eggs varied from 27 to 56% at 32 and 17 degrees C, respectively, and was optimum between 17 and 25 degrees C. Oviposition was examined under high and low relative humidity (RH) regimes while egg hatch was determined at six RH levels, each maintained in a separate controlled growth chamber set at a constant temperature (25 degrees C). Whilst RH had no influence on oviposition, eggs were highly susceptible to aridity, and continuous exposure to relative humidity below 63% for more than three days was wholly lethal at 25 degrees C. Optimum egg hatch occurred at RH between 85 and 95% for up to 12 days. The effect of day length on oviposition and subsequent egg hatch was investigated under two photoperiod regimes. Neither oviposition nor subsequent egg hatch was influenced by photoperiod. The knowledge obtained will be useful for mass rearing as well as field release programmes for L. bethae.     

Shifting preference between oviposition vs. host-feeding under changing host densities in two aphelinid parasitoids

Destructive host-feeding is common in hymenopteran parasitoids. Such feeding may be restricted to host stages not preferred for oviposition. However, whether this is a fixed strategy or can vary according to resource levels or parasitoid needs is less clear. We tested the trade-off between host feeding and oviposition on two whitefly parasitoids under varying host densities. Females of two aphelinid parasitoids, Eretmocerus hayati and Encarsia sophia were exposed to nine different densities of their whitefly host, Bemisia tabaci, in single-instar tests to identify their functional response. Mixed-instar host choice tests were also conducted by exposing whiteflies at four densities to the parasitoids. We hypothesized that the parasitoid females can detect different host densities, and decide on oviposition vs. host-feeding accordingly. The results showed that both Er. hayati and En. sophia females tended to increase both oviposition and host-feeding with increased host density within a certain range. Oviposition reached a plateau at lower host density than host-feeding in Er. hayati, while En. sophia reached its oviposition plateau at higher densities. At low densities, Er. hayati parasitized most on first and second (the optimal ones), and fed most on third nymphal instars (the suboptimal one) of the whitefly host as theory predicts, while at high densities, both parasitism and host-feeding occurred on first and second instars which are preferred for oviposition. En. sophia parasitized most on third and fourth (the optimal ones), while fed on first instars (the suboptimal one) at low densities, and utilized third and fourth instars for both at high densities. In conclusion, oviposition vs. host-feeding strategy of parasitoid females was found to vary at different host densities. The balance between reserving optimal hosts for oviposition or using them for host-feeding depended on parasitoid life history and the availability of host resources.     

Post-secretory determinants of sugar concentration in nectar

Abstract. The concentration of sugar in the nectars of unprotected flowers of several species was measured and did not reach the high values that would be in equilibrium with the daytime humidities recorded outside the corolla, although the sugar concentration was highly correlated with ambient relative humidity. This paper examines features that maintain low nectar sugar concentrations at low ambient humidities. Post-secretory changes in concentration are influenced to a small extent by nectar composition but depend largely on physico-chemical and microclimatic effects. Factors contributing to the maintenance of steep gradients in water activity between the nectar and the ambient air include corolla morphology, sugar concentration gradients and waterproofing lipid monolayers on the nectar surface. This paper considers the relative importance of such features in relation to the pollination syndrome. A simple technique is described for the measurement of intrafloral relative humidity.     

The influence of oviposition phenology on survival in host races of Eurosta solidaginis

Phenological differences between host plants can contribute to allochronic isolation of host races of phytophagous insects. Host races of the gallmaking fly, Eurosta solidaginis, that live on different species of goldenrod (Solidago altissima and S. gigantea) exhibit different emergence phenologies. These differences could result from adaptation to corresponding phenological differences between the hosts in periods of optimal suitability for gall formation and survival to adulthood. In order to test this, some flies of each host race were allowed to emerge naturally while the emergence times of others were manipulated to correspond to the emergence and oviposition periods of the other host race. Percent gall formation and survival to adulthood were examined for three oviposition periods: the peak time of emergence and oviposition of the earlier-emerging host race (that from S. gigantea), that of the later-emerging host race (that from S. altissima), and a week after the peak emergence of the host race from S. altissima. Flies of both host races were allowed to ovipuncture plants of the appropriate species during each of these periods. Plant relative growth rates were measured during each of these periods. The experiment was repeated twice over a two-year period. Relative growth rates of both host species were highest during the earliest oviposition period (the period during which the host race from S. gigantea normally emerges). Percent gall formation was significantly correlated with plant relative growth rate, but the coefficient of determination was low. In both years of the study, percent gall formation of both host races was highest during the earliest oviposition period (the period during which the host race from S. gigantea normally oviposits). Likewise, percent survival to adulthood in both host races was highest during the earliest oviposition period. There was no significant effect of oviposition period on the percent of larval death due to parasitism by Eurytoma gigantea or predation by Mordellistena unicolor. These results suggest that the host race from S. altissima does not emerge at the time that its host is optimally suited for gall formation or survival to adulthood. Therefore, differences in emergence phenologies do not appear to be due to corresponding phenological differences between the host species in suitability for gall formation or survival to adulthood.     

Evolution of host acceptance and its reversibility in a seed beetle

1. Adapting to a low‐quality plant may require modification of an insect's digestive physiology, oviposition behaviour, or other host‐use traits. If colonising a marginal host entails a cost, a decay in adaptation would be expected after selection is relaxed, i.e. if populations on a novel host are reverted to their high‐quality ancestral host. 2. Replicate lines of the seed beetle Callosobruchus maculatus (F.) rapidly adapted to lentil seeds; larval survival rose from approximately 1 to ≥ 90%, and oviposition on lentil increased more than two‐fold. This study compared egg‐laying behaviour in lines that either remained on lentil or were reverted to the ancestral host, mung bean, for 22–62 generations. 3. Consistent with the trade‐off hypothesis, females from two reverted sublines showed decreased oviposition on lentil (estimated as lifetime fecundity), but host acceptance in a third subline was unchanged. In a short‐term assay, acceptance of lentil by newly emerged females was lower in each reverted subline than in the corresponding non‐reverted one. Because effective population sizes (determined from genome resequencing) were large throughout the experiment, this decline in host acceptance is unlikely to be explained solely by genetic drift. 4. Variation among replicates in the magnitude of the reversion effect was also observed in a previous study of larval survival. However, the pattern of variation for survival was not congruent with the pattern of variation for host acceptance in this study. Thus, genes mediating improved performance on lentil appear to be largely independent of those responsible for increased oviposition.     

Weather variability impacts on oviposition dynamics of the southern house mosquito at intermediate time scales

Oviposition is a major event in the life history of mosquitoes, shaping both individual fitness and vectorial capacity. Several exogenous factors have been shown as important for the dynamic forcing of oviposition at finely (hourly) and coarsely (monthly or season to season) grained temporal scales. However, field studies addressing the interplay of weather factors on oviposition dynamics at the intermediate (days to weeks) time scale are missing. Here, we present the results from a field study that showed the oviposition dynamics of the southern house mosquito, Culex quinquefasciatus Say (Diptera: Culicidae), to be: (i) primarily dictated by relative humidity; and (ii) disrupted by rainfall events that resulted in a modified sensitivity to relative humidity. Rainfall changed the concentration of ammonia, a major limiting resource of microbes used as food by mosquito larvae. Following major rainfall events, the importance of relative humidity in forcing the oviposition dynamics also changed. Finally, our results indicate that qualitative changes in oviposition habitats modify the importance of weather variables as predictors of mosquito oviposition dynamics.     

Clutch size in frugivorous insects as a function of host firmness: the case of the tephritid fly Anastrepha ludens

Abstract. 1. Optimal clutch size theory predicts that individuals will oviposit the number of eggs that increases their fitness. In Anastrepha ludens Loew (Diptera: Tephritidae), females oviposit larger clutches in unripe (firm) fruits than in ripe (soft) fruits. The following hypotheses were tested: (1) Using fruit firmness as an indicator of fruit quality, A. ludens females vary the number of eggs per clutch every time they reach an oviposition decision. (2) Maximising offspring survival with respect to either unripe or ripe fruit requires placing large clutches in firm fruit and smaller clutches in soft fruit. 2. Agar spheres were used as artificial hosts. Three agar concentrations resulted in three degrees of firmness. Mango fruits Mangifera indica L. served as natural hosts. Ripe and unripe fruits were used to test soft and firm host conditions respectively. Females laid significantly larger clutches in the firmer artificial hosts than in the softer hosts. They also laid significantly more eggs in artificial hosts without sugar than in hosts with sugar. Firm (unripe) mangoes also received significantly larger clutches than soft (ripe) mangoes. 3. When an individual female was first presented with a firm artificial host, it laid a large clutch. If subsequently offered a soft host, the female laid a significantly smaller clutch. Finally, if again offered a firm host, clutch size was increased significantly. 4. Possible trade‐offs in offspring fitness were explored in ripe and unripe mangoes by measuring offspring egg‐to‐adult survival, pupal weight, mean adult longevity, and fecundity. Despite the fact that larval survival was greater in soft fruit than in firm fruit, parameters such as pupal weight, mean longevity, and fecundity of adults stemming from both fruit types did not differ significantly. 5. A probable trade‐off between high offspring mortality caused by host unsuitability and low offspring and adult mortality caused by parasitism and predation is discussed as the reason for the exploitation of sub‐optimal hosts.     

Influence of plant genotype and environment on oviposition preference and offspring survival in a gallmaking herbivore

Summary Plant resistance to insect herbivores may derive from traits influencing herbivore preference, traits influencing the suitability of the plant as a host, or both. However, the plant traits influencing host-plant selection by ovipositing insect herbivores may not completely overlap those traits that affect larval survival, and distinct traits may exhibit different levels of genetic vs. environmental control. Therefore, resource supply to the host plant could affect oviposition preference and larval performance differently in different plant genotypes. To test this hypothesis, the effects of resistance level, plant genotype, and resource supply to the host plant on oviposition preference and larval performance of a gallmaking herbivore, and on various plant traits that could influence these, were examined. Replicates of four genotypes of Solidago altissima, grown under low, medium, or high levels of nutrient supply in full sun or with medium levels of nutrients in shade, were exposed to mass-released Eurosta solidaginis. The number of plants ovipunctured was significantly affected by plant genotype and the interaction between genotype and nutrient supply to the host plant: one susceptible and one resistant genotype were more preferred, and preference tended to increase with nutrient supply in the more-preferred genotypes. The growth rate of ovipunctured plants during the oviposition period was significantly greater than that of unpunctured plants. Bud diameter (which was strongly correlated with plant growth rate), leaf area, and leaf water content were significant determinants of the percentage of plants ovipunctured, explaining 74% of the variance. The number of surviving larvae was significantly affected by plant genotype, but no effect of nutrient or light supply to the host plant was detected. The ratio of bud diameter to bud length was positively related to the percentage of ovipunctured plants that formed galls, suggesting that the accurate placement of eggs near the apical meristem by ovipositing females may be easier in short, thick buds. No significant correlation was observed between oviposition preference and larval survival at the population level. These results suggest that the plant traits affecting oviposition preference may exhibit different magnitudes of phenotypic plasticity than those affecting larval survival, and that the degree of phenotypic plasticity in plant traits affecting oviposition preference may differ among genotypes within a species.     

Maternal effect determines drought resistance of eggs in the predatory mite Phytoseiulus persimilis

The ability of an organism to adapt to short-term environmental changes within its lifetime is of fundamental importance. This adaptation may occur through phenotypic plasticity. Insects and mites, in particular, are sensitive to changes in temperature and humidity, especially during the juvenile stages. We studied the role of phenotypic plasticity in the adaptation of eggs to different relative humidity conditions, in the predatory mite Phytoseiulus persimilis, used worldwide as a biological control agent of the spider mite Tetranychus urticae. The biocontrol efficacy of P. persimilis decreases under dry conditions, partly because P. persimilis eggs are sensitive to drought. We exposed P. persimilis adult females from two different strains to constant and variable humidity regimes and evaluated the hatching rate of their eggs in dry conditions, as well as the survival and oviposition rates of these females. Whereas the eggs laid by P. persimilis females exposed to constant high humidity did not survive in dry conditions, females exposed to constant low humidity started laying drought-resistant eggs after 24 h of exposure. Survival and oviposition rates of the females were affected by humidity: females laid fewer eggs under constant low humidity and had a shorter lifespan under constant high and constant low humidity. The humidity regimes tested had similar effects across the two P. persimilis strains. Our results demonstrate that transgenerational phenotypic plasticity, called maternal effect, allows P. persimilis females to prepare their offspring for dry conditions.     

三峡库区两种耐水淹植物的存活率和碳水化合物储备关系

野古草和秋花柳是三峡库区消落带两种强水淹耐受能力的植物物种。以往研究显示植物的水淹耐受性和体内碳水化合物储备有关。为了探明野古草和秋花柳水淹下的高存活率是否和碳水化合物储备有关, 研究了在室外6个月的模拟水淹条件下两个物种在不同水淹时间(40、90、120和180d)和不同水淹深度下(不水淹、根部水淹和完全淹没)的生物量积累、存活率和碳水化合物含量和分布。结果表明: (1)野古草和秋花柳对长期水淹具有很高的耐受性, 根部水淹植物6个月处理后完全存活; 而完全淹没条件下, 野古草仅在4个月, 秋花柳仅在6个月处理后才开始死亡; (2)碳水化合物主要储备在野古草的茎和秋花柳的茎与主根中, 野古草的根和秋华柳的细根中碳水化合物含量很低; (3)水淹深度和水淹时间对植物生物量积累和碳水化合物含量影响显著(P 0.05):与未水淹植株相比, 根部水淹仅略微降低了生物量积累以及可溶性糖和淀粉含量 (P 0.05), 且保持基本稳定或增加的趋势, 而完全淹没的植株生物量随水淹时间逐渐降低, 碳水化合物含量在前90天快速下降 (P 0.05), 之后缓慢下降或保持不变。研究结果表明, 野古草和秋花柳强的水淹耐受性是和它们高的碳水化合物储备以及水淹条件下对碳水化合物的动用能力有关, 后期的死亡率增加与碳水化合物储备消耗殆尽有关, 野古草和秋花柳对碳水化合物储备对水淹的响应的差异可能和它们的碳水化合物储备在不同组织中的分配模式有关。       

Orange-derived stimuli regulating oviposition in the Mediterranean fruit fly

Abstract:  Some visual and olfactory host stimuli influencing oviposition of the Mediterranean fruit fly ( Ceratitis capitata Wiedemann) in orange fruit were investigated in a laboratory-reared strain of this species. Mated females of C. capitata were found to be attracted to the same extent by fragrant orange fruits and odourless sham oranges, while unmated females were notably less attracted than mated females by the above objects. Mated females laid significantly more eggs in orange-coloured than in whitish (optically neutral) spherical wax dummies (diameter, 9.0 cm) as well as in both orange-coloured and whitish wax dummies supplemented with high internal humidity (80–100%) compared with respective wax dummies supplied with low internal humidity (30–50%). The orange-like colour together with high internal humidity provides basic stimuli supporting adequate oviposition of C. capitata in oranges. The oviposition rate of female C. capitata was not significantly changed when graded dosages ranging from 0.004 to 19.6  μ l of orange peel oil per cm² were added to orange-coloured wax dummies, while oviposition was considerably subdued by addition of 3.9  μ l orange peel oil and completely disrupted by addition of 9.8 or 19.6  μ l of orange peel oil per cm² of whitish wax dummies. Oviposition of C. capitata in ripe orange fruit may thus be interpreted by the predilection of this tephritid species for an orange-coloured, glossy pericarp, being capable of counteracting the deterrent effect of the essential oil found in orange flavedo.     

不同施氮量的寄主植物对棉铃虫发育与繁殖的影响

在光暗比(14,10)、温(27±1)℃、湿(60%一80%RH)环境模拟箱内,研究了取食低、中、高三种不同施氮量的棉花、小麦、玉米和大豆繁殖器官对棉铃虫Helicoverpaarmigera(Hubner)发育与繁殖的影响,并通过对不同施氮量寄主植物繁殖器官的成分分析,探讨了造成这些影响的原因。结果表明,四种寄主植物的C/N比均随施氮量的增加而降低,并且大豆的C/N比(分别为0.5、0.4和0.3)<棉花(分别为1.1、0.9和0.9)<小麦(分别为4.5、4.O和3.8)<玉米(分别为5.2、4.2和4.2)。取食不同的施氮量寄主植物对棉铃虫发育和存活的影响主要表现在低龄(1～3龄)幼虫期,即随寄主植物C/N比下降,低龄幼虫的发育历期缩短,存活率提高;并且取食C/N较低的寄主植物的发育速度和存活率高于取食C/N较高的寄主植物。随着施氮量的增加,取食不同寄主植物的棉铃虫成虫产卵量均呈上升趋势;取食较低C/N比寄主植物的种群增长指数呈先后升后降趋势,而取食较高C/N比的寄主植物则呈上升趋势。取食C/N比适中的寄主植物更有利于该害虫的繁殖及其种群增长,C/N比太高或太低均不太有利。不同施氮量的寄主植物对棉铃虫发育与繁殖的影响是由于其体内碳水化合物和蛋白质含量的差异及两者间比例的不同所致。     

Preference–performance linkage in the neotropical spittlebug Deois flavopicta, and its relation to the Phylogenetic Constraints Hypothesis

1. Predictions from the Phylogenetic Constraints Hypothesis were tested for the first time in a tropical system using the pasture pest Deois flavopicta Stal, which oviposits into the ground and not into plant tissues. The prediction that there is no oviposition preference–larval performance linkage was supported. The absence of such a link provides an evolutionary basis for eruptive population dynamics. 2. The effects of host species and host plant quality on the preference of ovipositing females of D. flavopicta and performance of their offspring on the selected host plants were tested at the population level. 3. Female oviposition behaviour was affected by the presence of the host plant. Females of D. flavopicta showed a strong preference to oviposit close to host plants. The number of eggs was higher in pots containing Brachiaria ruziziensis (121.88 ± 13.70) than in pots containing only the wet oviposition substrate (5.2 ± 1.98) or dry oviposition substrate (0.067 ± 0.067). Ovipositing females did not, however, discriminate between plants of Brachiaria decumbens and Axonopus marginatus and did not show a strong oviposition preference in relation to differences in plant quality (protein and fibre content). They did show oviposition preference for plants under the high watering regime. The mean number of eggs collected from pots with non water‐stressed plants was 60% higher than the mean number of eggs collected on pots with water‐stressed plants. 4. Although females did not show ovipositional preference, spittlebug larval performance, measured as percentage survival and duration of nymphal period, was better on plants of high protein and low fibre content. These results indicate that there is not a linkage between female oviposition preference and subsequent nymphal performance in relation to differences in protein and fibre content in the host plants. There was, however, a limited linkage between oviposition preference and nymphal performance in relation to plant water content. Females showed preference for moist sites that have high survival of newly hatched nymphs. 5. Evidence indicates that for D. flavopicta, the influences of natality and female oviposition behaviour in response to plant quality are not the major factors driving population outbreaks, which is in accordance with the Phylogenetic Constraints Hypothesis.     

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. Received: 5 October 1998 / Accepted: 20 April 1999     

Oviposition preferences and larval performance of a flower-feeding weevil,Coelocephalapion aculeatum, in relation to host development

The oviposition behaviour ofCoelocephalapion aculeatum Fall (Coleoptera: Apionidae), a host specific florivore exploiting an ephemeral resource, was studied in relation to host development. Immature inflorescences of its host, the weedMimosa pigra L. (Mimosaceae), were categorised into 5 developmental stages ranging from first appearance to one day before opening. The duration, weight, percent abortion, and nitrogen content of each stage were measured. Oviposition preferences ofC. aculeatum for all inflorescence stages were determined in multiple choice trials. Newly hatched, larvae were transferred into inflorescences of each stage to determine larval survival as a function of stage. Numbers of eggs laid into inflorescences generally correlated with larval survival except that oviposition was greater in stage 4 inflorescences than expected from the larval survival. Numbers of eggs laid into inflorescences generally correlated with their carrying capacity except that adults avoided stage 5 inflorescences in which larval survival was low. The evolution of oviposition preferences ofC. aculeatum appears to have been influenced by two opposing selective forces. The need to complete development before the inflorescence deteriorated would have selected for oviposition in young inflorescences. The greater availability of food and lower probability of abortion offered by older inflorescences would have selected for oviposition in these. The net outcome of these pressures has resulted in the observed oviposition strategy.