Factors affecting calculation of nutritional induces for foliage-fed insects: an experimental approach

To determine how nutritional indices for insects fed leaves are affected by the experimental conditions and the physiology of the plant material, we used larvae of the buckmoth, Hemileuca lucina Hy. Ed. (Saturniidae) and their hostplant Spiraea latifolia Ait. Bork (Rosaceae). Under experimental conditions identical to those used to determine larval nutritional indices, we found that the age of leaves (new versus mature) significantly affected their metabolism and water loss, but simulated herbivory did not directly affect leaf metabolism. Over a 6-day test, nitrogen concentration showed an initial increase followed by a gradual decline, and was higher in new leaves compared to mature leaves. New leaves increased in protein concentration and then gradually returned to the initial level, whereas mature leaves changed little over the 6-day test. These changes in percent nitrogen and protein may largely reflect the disproportional changes in non-nitrogenous materials. Solitary and grouped larvae had similar growth rates on new leaves, but they differed on mature leaves. Deliberate manipulation of larvae during the course of an experiment significantly reduced relative growth rates by increasing duration of the stadium rather than by decreasing biomass gained. Two methods of estimating larval gut contents at mid-stadium were compared: weight of frass produced and weight of digestive tract and contents. After the end of the 4-day test period used to determine nutritional indices, the digestive tracts with food accounted for 10.8% of the larval dry weight. Larval frass produced in 24 h after the end of the test period comprised 9.3% of the larval dry weight. Correction factors for plant metabolism changed nutritional indices by 1 to 8%, while those for larval gut contents altered indices by 2 to 15%.     

The effects of enriched CO2 atmospheres on plant-insect herbivore interactions: growth responses of larvae of the specialist butterfly,Junonia coenia (Lepidoptera: Nymphalidae)

Summary Little is known about the effects of enriched CO₂ environments, which are anticipated to exist in the next century, on natural plant-insect herbivore interactions. To begin to understand such effects on insect growth and survival, I reared both early and penultimate instar larvae of the buckeye, Junonia coenia (Lepidoptera: Nymphalidae), on leaves from one of their major hostplants, plantain, Plantago lanceolata (Plantaginaceae), grown in either ambient (350 PPM) or high (700 PPM) CO₂ atmospheres. Despite consuming more foliage, early instar larvae experienced reduced growth on high CO₂-grown compared to ambient CO₂-grown leaves. However, survivorship of early instar larvae was unaffected by the CO₂ treatment. Larval weight gain was positively correlated with the nitrogen concentration of the plant material and consumption was negatively correlated with foliar nitrogen concentration, whereas neither larval weight gain nor consumption were significantly correlated with foliar water or allelochemical concentrations. In contrast, penultimate instar larvae had similar growth rates on ambient and high CO₂-grown leaves. Significantly higher consumption rates on high CO₂-grown plants enabled penultimate instar larvae to obtain similar amounts of nitrogen in both treatments. These larvae grew at similar rates on foliage from the two CO₂ treatments, despite a reduced efficiency of conversion of ingested food (ECI) on the low nitrogen, high CO₂-grown plants. However, nitrogen utilization efficiencies (NUE) were unaffected by CO₂ treatment. Again, for late instar larvae, consumption rates were negatively correlated with foliar nitrogen concentrations, and ECI was also very highly correlated with leaf nitrogen; foliar water or allelochemical concentrations did not affect either of these parameters. Differences in growth responses of early and late instar larvae to lower nitrogen, high-CO₂ grown foliage may be due to the inability of early instar larvae to efficiently process the increased flow of food through the gut caused by additional consumption of high CO₂ foliage.     

外源氮添加对高寒草地门源草原毛虫种群密度的影响

近几十年以来,中国陆地生态系统的大气氮沉降持续增加。这种外源氮输入会影响植物多样性和生产力,进而影响植食性昆虫的种群动态。门源草原毛虫(Gynaephora menyuanensis)是青藏高原东北部高寒草地的主要植食性昆虫,种群数量爆发时会造成巨大的生态经济损失。为了探明草原毛虫种群密度如何响应氮沉降加剧,依托青海海北高寒草地“外源氮添加梯度”控制实验平台(0、25、50、100 kg N hm^-2 a^-1),详细调查了门源草原毛虫的种群密度及其相关的食物数量、质量和栖息地环境变化。结果表明：(1)外源氮添加显著影响门源草原毛虫的虫口密度(ANOVA:F=3.29,P=0.04),且草原毛虫虫口密度随氮添加量的增加呈线性增加趋势(R~2=0.31,P=0.005)。(2)外源氮添加梯度下,食物质量(叶片氮含量),而非食物数量(植物地上生物量和禾草地上生物量)和栖息地环境(土壤温度、湿度和光照条件),是草原毛虫虫口密度变化的关键影响因素。基于上述结果,可以预测：未来氮沉降的加剧可能促进青藏高原高寒草地门源草原毛虫的虫害爆发。研究将为全球变化背...     

The toxicity and biological effect of Pedalium murex L. extracts on the tobacco cutworm,Spodoptera litura (Fabr.) larvae

Petroleum ether, acetone, water and aqueous extract of Pedalium murex L. (Pedalaceae) leaves, root and fruit were tested against fourth instar larvae of the tobacco cutworm, Spodoptera litura (Fabr.) under laboratory conditions using leaf dip method. Larval mortality; larval, pupal and adult periods; pupal weight, pupation and adult emergence; larval and pupal deformities of S. litura were recorded. All the tested solvent extracts of P. murex were effective against S. litura life stages by causing mortality in a dose dependent manner. However, the efficacy was more significant with respect to acetone (leaf and root) and petroleum ether (root) at higher concentrations (0.8%) which leads to 100% larval mortality. The water and aqueous extracts of root caused 86.6 and 88.0% larval mortality respectively at 4% concentration. Among the plant parts tested, root had more importance followed by leaf and fruit. Plant extracts extended the larval, pupal and adult periods; reduced the pupal weight, pupal and adult emergence and caused larval–pupal deformities. Total life time was highly prolonged in acetone extracts of P. murex root. Though all the extracts were found to have insecticide activity, acetone and petroleum ether extracts of P. murex root can be used as an effective alternative to modern synthetic insecticides. Bioactive principles from these extracts can be isolated, identified and integrated in S. litura management.     

Acid stress in Rumex hydrolapathum (Polygonaceae) and its influence on the phytophage Lycaena dispar (Lepidoptera; Lycaenidae)

Summary The relationship of insect growth to host condition was tested on host plants grown by hydroculture with a standard nutrient solution, but under different acidities, ranging from pH 3.5–7.5. The indirect effect of the host on the development of the phytophage was tested in the pupal stage. The host plant Rumex hydrolapathum had its highest nitrogen content in the range pH 5.5–6.5; pupal weight of Lycaena dispar was correlated with nitrogen concentration, ash weight and water content. There is a remarkable difference in element concentration between young and old leaves of the host, which varies with acidity of the rooting substrate.In this experiment a negative relationship was detected between reproduction of the phytophage and stress of the host.     

Change of pupal size of Panolis flammea (Lepidoptera; Noctuidae) and Bupalus piniarius (Geometridae) in response to concentration of industrial pollutants in their food plant

Summary Larvae of Panolis flammea and Bupalus piniarius were reared in the laboratory on needles of Scots pine affected by industrial air pollutants in Finland. Needles were collected at different distances from a distinctive source of emission along two 9-km-long transects, and from independent control plots. The elemental composition of the needles used as larval food was analysed. Pupal weight, length and width were negatively correlated wiht the distance from the source of emission. The elemental composition of the pine needles explained 24–53% of the variation in pupal weight. Most of the explained variation was assoicated with the concentration of heavy meals in the pine needles.     

Trophic links,nutrient fluxes,and natural history in the Allium ursinum food web,with particular reference to life history traits of two hoverfly herbivores (Diptera: Syrphidae)

The food web centering on Allium ursinum (Liliaceae) in a beech forest (Germany) is described, and temporal variation of active trophic links is related to species' life cycles. The most important insect herbivores are Cheilosia fasciata (a larval leaf miner) and Portevinia maculata (a larva bulb miner) (Diptera: Syrphidae). Energy, carbon and nitrogen flow in the food chain (Allium-Cheilosia-Phygadeuon ursini) are investigated and analysed with respect to differences in resource allocation by the leaf miner and its hymenoptereous parasitoid. In C. fasciata nitrogen is likely to be the limiting resource, while growth in Phygadeuon ursini appears energy-limited. Larval feeding habits of C. fasciata and Portevinia maculata determined the timing of the species' life cycles and, as a consequence, appeared to preclude the existence of a pupal parasitoid in Portevinia maculata. Further details of life history traits are demonstrated and discussed.     

Relationships between turnip root fly (Delia floralis) larval development and the sugar content of swede (Brassica napus ssp. rapifera) roots

Eight genotypes of swede (Brassica napus L. ssp. rapifera [Metz.] Sinsk.) at the 8–10 true leaf stage were inoculated with five, 10 or 20 eggs of the turnip root fly Delia floralis (Fall). The roots were sampled, with control roots, after 6 weeks of larval development. D. floralis root damage, as measured by reduction in root weight, was found to be linked to inoculation level. Neither D. floralis egg numbers nor swede genotype had a significant effect on the percentage of larvae developing to pupation. Mean pupal weight varied by a factor of ×1.4 and consistently decreased with increasing egg inoculation level. Changes in the root concentrations of glucose, sucrose and fructose were measured. All swede genotypes showed a similar response in their sugar concentrations after root damage. Glucose and fructose concentrations were reduced whilst sucrose concentration remained unaffected. The concentrations of glucose and fructose were highly correlated. Pupal weight, used as a measure of larval development, was significantly correlated with the concentrations of individual and total sugars in the roots. The implications of sugar responses to damage in brassicas, and the correlation between sugar concentrations in the roots and D. floralis pupal weights are discussed.     

Preference and performance linkage of a leaf-mining moth on different Salicaceae species

The relationship between oviposition prefer-ence and offspring performance of a leaf-mining moth (Paraleucoptera sinuella) on four Salicaceae species was investigated in 1997 and 1998. We observed the egg distribution pattern on different plant species in the field and carried out oviposition experiments in the laboratory to determine the preference of ovipositing females. We also examined larval survival, pupal mass, and developmental time to compare larval performance on each plant species. Egg density in the field differed significantly among plant species. However, egg density was not correlated exactly with demonstrated oviposition preference. No larvae could develop on two Salix species. This finding indicated that larval survival is the most critical index of larval performance. Larval performance on each plant species was correlated well with oviposition preference that was revealed by a no-choice experiment in the laboratory. However, this correlation was not found in the field. These results indicate that the preference–performance linkage that was observed under laboratory conditions, was not always maintained in the field. Received: September 25, 2000 / Accepted: April 27, 2001     

Effects of plant competition and herbivore density on the development of the turnip root fly (Delia floralis) in an intercropping system

In this study, interactive effects of plant competition and herbivory on plant quality and herbivore development were examined in a greenhouse experiment where cabbage plants [Brassica oleracea L. var. capitata (Brassicaceae)] were intercropped with red clover [Trifolium pratense L. (Fabaceae)]. Cabbages were grown with two red clover densities and attack rates by the root feeding herbivore the turnip root fly, Delia floralis Fall. (Diptera: Anthomyiidae). Above ground and below ground cabbage biomass was reduced through intercropping and larval damage. Intercropping also resulted in lower nitrogen and higher carbon root levels compared with levels in the roots of monocultured cabbage. Furthermore, both root nitrogen and carbon levels increased with herbivory. Root neutral detergent fibre (NDF) and lignin content increased in response to both increased plant competition and higher egg densities. For lignin, an interaction effect was observed in the form of elevated levels in intercropped plants subjected to larval damage, while levels in roots of monocultured cabbage remained unchanged. The quality changes brought about by clover competition affected D. floralis development negatively, which resulted in reduced pupal weight. In addition, increased egg density also decreased larval growth. The effects on the development of D. floralis in relation to host plant quality are discussed. Handling editor: Gimme Walter     

栎黄枯叶蛾生长发育与几种寄主内含物的关系

【目的】探讨饲喂不同寄主植物对栎黄枯叶蛾Trabala vishnou gigantina Yang生长发育及繁殖的影响,分析寄主植物内含物与其生长发育及繁殖的关系,为研究不同寄主植物对栎黄枯叶蛾种群动态的影响提供理论依据。【方法】在室内条件下,利用沙棘Hippophae rhamnoides、山杏Armeniaca sibirica、山杨Populus davidiana、旱柳Salix matsudana饲养该虫,观察幼虫发育历期、蛹历期、蛹重及产卵量等指标,同时测定寄主叶片中可溶性糖、可溶性蛋白质、水分、单宁及黄酮的含量,并进行线性回归分析。【结果】不同寄主植物饲喂栎黄枯叶蛾幼虫的发育历期、蛹历期、蛹重及产卵量均存在显著差异。取食沙棘的幼虫历期最短,仅为(74.50?1.76)d,而取食旱柳历期最长,达(106.00?1.51)d。取食山杨的蛹历期最长,为(44.00?1.41)d,最短的是取食旱柳的,仅为(32.70?3.13)d。饲喂沙棘和山杏的雌、雄蛹重和成虫产卵量显著高于饲喂山杨和旱柳的。对不同寄主的营养物质、次生物质和水分含量测定结果表明:不同寄主间营养物质、次生物质与水分的含量差异显著。回归相关分析表明:幼虫发育历期与寄主植物可溶性蛋白质含量呈显著负相关;雌蛹重和产卵量与寄主植物可溶性糖含量呈显著负相关;雌、雄蛹重与寄主植物含水量呈显著正相关;蛹历期与单宁含量呈显著正相关,但黄酮含量的高低与栎黄枯叶蛾生长发育等指标无相关性。【结论】沙棘是栎黄枯叶蛾生长发育及繁殖的最适寄主植物;并且寄主植物中可溶性糖、单宁的含量低、含水量和可溶性蛋白质含量高有利于栎黄枯叶蛾的生长发育和繁殖。     

Effects of the mineral composition and water content of intact plants on the fitness of the African armyworm

The effects of organic nitrogen, nitrate, phosphorus, potassium and water content of leaves of intact maize plants, grown in a gravel culture system, on the fitness of the African armyworm, Spodoptera exempta (Walker)(Lepidoptera: Noctuidae) were studied. Organic nitrogen concentrations ranged from 1.3% to 3.7% over four treatments differing only in nitrate supply to the plants. Water content and other mineral levels were all positively correlated with the organic nitrogen level. Feeding damage by the caterpillars was most severe on the lowest nitrate treatments, where it could be least well compensated for by new leaf growth. Larval and pupal fitness variables were not affected by treatment, except for larval development on the lowest nitrate treatment which was delayed by just 1 day. The large compensatory capacity of the larvae was underlined by a similar mineral composition of the pupae in all treatments. Adult fitness variables hardly differed between the upper three nitrate treatments, but revealed a trend over all treatments: the higher the organic nitrogen content of the leaves, the shorter the pre-oviposition period and the higher the fecundity. This trend, however, might have been due to differences in available food quantity rather than in food quality. It is concluded that fitness of the African armyworm is only slightly affected over a wide range of nitrogen concentrations in its food. Though effects might be larger under field conditions, the large differences in outbreak development between years seem not to be attributable to observed differences in nitrogen levels in host plants between years in primary outbreak areas. Other environmental factors appear to be of greater importance.     

Soil nutrients and water availability interact to influence willow growth and chemistry but not leaf beetle performance

We investigated the effects of soil nutrient and water availability on the growth and chemistry of the silky willow (Salix sericea Marshall), and on the performance of the imported willow leaf beetle (Plagiodera versicolora Laichartig). Our major aims were to determine whether there are nutrient–water interactions on plant traits and whether this leads to parallel interactions for herbivore performance. We used a 2 × 3 fully factorial design, which consisted of high and low nutrient treatments crossed with dry, field capacity, and flooded water treatments. We found that nutrient additions increased plant growth, but only in field capacity and flooded conditions (nutrient–water interaction). Leaf nitrogen content also depended on the interaction between soil nutrients and water: nutrient addition resulted in a larger increase in foliar nitrogen in the field capacity treatment than in the flooded and dry treatments. Of the two phenolic glycosides measured, salicortin and 2′‐cinnamoylsalicortin, only one was affected by the treatments. 2′‐cinnamoylsalicortin concentration was lower in the high nutrient–dry treatment compared with the other treatments. In contrast to plant responses, there were no interactions found for larval or pupal weight or development time. Nutrient addition led to an increase in female pupal weight, and foliar N was positively correlated with female pupal weight and negatively correlated with female development time. In addition, leaf water was positively correlated with female development time. The lack of interactions for insect performance may stem from the small absolute differences in foliar nitrogen content associated with the interaction between the nutrients and water. Taken together, our results suggest that nutrient–water interactions influence plant traits that are potentially important for insect performance (leaf nitrogen and water), but these interactions do not produce parallel interactions in beetle performance.     

Influence of dietary nicotine and colony source ofManduca sexta [Lepidoptera: Sphingidae] on its suitability as a host ofCotesia congregata [Hymenoptera: Braconidae]

Larval tobacco hornworms,Manduca sexta (L.), of 2 different colonies were exposed to parasitism by the gregarious endoparasitoid,Cotesia congregata (Say). A comparison was made of parasitoid larval, pre-pupal, and pupal mortality, female and male dry weight and larval development time. In general, “Maryland” hornworms were more suitable hosts than “North Carolina” hornworms. Although the presence of dietary nicotine increased parasitoid mortality in individuals reared from hornworms of both colonies, the effect was more severe among individuals parasitizing the North Carolina hornworms. Scientific contribution No. 8125, article No. A-5066 of the Maryland Agricultural Experiment Station, Department of Entomology.     

Performance of cabbage stem flea beetle larvae (Psylliodes chrysocephala) in brassicaceous plants and the effect of glucosinolate profiles

The cabbage stem flea beetle (CSFB), Psylliodes chrysocephala L. (Coleoptera: Chrysomelidae), is one of the most important pests in European winter oilseed rape production. Adult beetles feed on young leaves whereas larvae mine within the petioles and stems. Larval infestation can cause significant crop damage. In this study, the host quality for CSFB of four oilseed rape (Brassica napus L.) cultivars and seven other brassicaceous species with different glucosinolate (GSL) profiles was assessed under controlled conditions. Larval instar weights and mortality were measured after 14 and 21 days of feeding in the petioles of test plants. To study the impact of GSL on the performance of larvae, the GSL contents in petioles from non-infested and infested plants were analysed before, and 21 days after, the start of larval infestation. Larval performance was not significantly different between the four cultivars of oilseed rape, but differed considerably among the other brassicaceous species tested. In comparison to the weight of larvae in the standard B. napus cv. Robust, the larval weight was higher in turnip rape (Brassica rapa L. var. silvestris) and significantly reduced in white mustard (Sinapis alba L.), oil radish (Raphanus sativa L. var. oleiformis), and cabbage (Brassica oleracea L. convar. capitata var. alba). The duration of larval development increased in white mustard and oilseed radish. The GSL profiles of the petioles showed little difference between non-infested and infested plants of oilseed rape whereas the content of aliphatic GSL increased in the infested turnip rape plants. In contrast, the aliphatic and benzenic GSL decreased in infested Indian rape (B. rapa subsp. dichotoma Roxb.). Larval weight was not correlated with the total GSL content of plants, neither before infestation nor 21 days after. Larval weight was positively correlated with progoitrin and 4-hydroxyglucobrassicin. White mustard, which provides inferior host quality for larval development, has the potential to introduce insect resistance into high-yielding oilseed rape cultivars in breeding programmes.     

Effects of elevated CO2 on development and larval food-plant preference in the butterfly Coenonympha pamphilus (Lepidoptera, Satyridae)

The objective of this study was to determine how increasing atmospheric CO₂ change plant tissue quality in four native grassland grass species (Agrostis stolonifera, Anthoxanthum odoratum, Festuca rubra, Poa pratensis) which are all larval food‐plants of Coenonympha pamphilus (Lepidoptera, Satyridae). We assessed the effect of these changes on the performance and larval food‐plant preference of C. pamphilus in a greenhouse experiment. Furthermore, we tested the interactive effects of elevated CO₂ and soil nutritional availability in F. rubra and its effect an larval development of C. pamphilus. In general, elevated CO₂ decreased leaf water concentration, nitrogen concentration and specific leaf area (SLA), while leaf starch concentration was increased in all grass species. A species‐specific reaction to elevated CO₂ was only found for foliar starch concentration. P. pratensis did not increase its starch concentration under elevated CO₂ conditions, whereas the other three species did. Fertilisation, investigated only for F. rubra, increased leaf nitrogen concentration and amplified the CO₂‐induced decrease in leaf nitrogen. Development time of C. pamphilus was on the average prolonged by two days under elevated CO₂ and the prolongation differed from 0.7 to 5.3 days among food‐plant species. Pupal fresh weight differed marginally between CO₂ treatments. Fertilisation of the larval food‐plant F. rubra shortened development time by one day and significantly increased pupal and adult fresh weights. C. pamphilus larvae showed a clear food‐plant preference among grass species at the age of 36 h or older. Additionally, a change of food‐plant preference under elevated CO₂ was found. Larvae at ambient CO₂ preferred Agrostis stolonifera and F. rubra, while under elevated CO₂Anthoxanthum odoratum and P. pratensis were preferred. The present study demonstrates that larval development of C. pamphilus is affected by food‐plant species and CO₂ induced changes in foliar chemistry. Although we found some species‐specific reactions to elevated CO₂ for foliar chemistry, no such CO₂ by species interaction was found for insect development. The change in food‐plant preference of larvae under elevated CO₂ implies potential changes in selection pressure for grass species and might therefore affect evolutionary processes.     

Effect of food plant switching by a herbivore on its parasitoid: Cotesia melanoscela development in Lymantria dispar exposed to reciprocal dietary crosses

1. Host plant switching by dispersing early instar lepidopterans could have implications for parasitoid performance, but this possibility has not been evaluated thoroughly. 2. The relative growth rates of Lymantria dispar parasitized by Cotesia melanoscela, and the weight of larvae at the time of parasitoid emergence, were affected most by the second larval food plant consumed. 3. The relative growth rates, pupal weights, weight of larva at the time of parasitoid emergence, and development times of L. dispar were affected significantly by the second larval food plant consumed. 4. Development time and size of Cotesia melanoscela were affected most by the second larval food plant consumed. 5. Parasitoid performance was affected most by the larval host’s relative growth rate and the final weight of the host larva at the time of parasitoid emergence. 6. Host plant switching affected the weight of L. dispar larvae at the time of parasitoid emergence, but the effect of switching per se was not a significant factor in C. melanoscela size or development. 7. Lymantria dispar larvae that fed on Populus as their second host outperformed larvae that fed ultimately on Acer. 8. Parasitoids yielded from L. dispar larvae that fed ultimately on Populus outperformed parasitoids yielded from larvae that fed ultimately on Acer. 9. Per cent mortality of L. dispar due to parasitism and percentage adult C. melanoscela emergence were highest in parasitized larvae fed Populus, poor in hosts fed Acer, and intermediate in switching larvae.     

Influences of plant species on life history traits of Cotesia rubecula (Hymenoptera: Braconidae) and its host Pieris rapae (Lepidotera: Pieridae)

The relative suitability of four plants was studied for larvae of Pieris rapae L. and its parasitoid Cotesia rubecula (Marshall). For unparasitized P. rapae, pupal dry weight and egg-pupa growth rate were higher on cabbage, radish and nasturtium than on Indian hedge mustard. Larval developmental rate and size were greatest for C. rubecula when its host was feeding on nasturtium. Wasp survival was not affected by the host insect/plant combination in which the parasitoid developed. These results indicate that the plant on which host larvae feed is an important factor in development of the parasitoid.     

Do multitrophic interactions override N fertilization effects on <Emphasis Type="Italic">Operophtera</Emphasis> larvae?

We examined how performance of Operophtera brumata (Lepidoptera) larvae was affected by nitrogen (N) fertilization of boreal forest understorey vegetation. We monitored larval densities on Vaccinium myrtillus plants for a period of 7 years in a field experiment. Preliminary results indicated that the N effect on larval densities was weak. To examine if this was due to indirect interactions with a plant pathogen, Valdensia heterodoxa, that share the same host plant, or due to top-down effects of predation, we performed both a laboratory feeding experiment (individual level) and a bird exclusion experiment (population level) in the field. At the individual level, altered food plant quality (changes in plant concentration of carbon, N, phenolics, or condensed tannins) due to repeated infection by the pathogen had no effect on larval performance, but both survival to the adult stage and adult weight were positively affected by N fertilization. Exclusion of insectivorous birds increased the frequency of larval damage on V. myrtillus shoots, indicating higher larval densities. This effect was stronger in fertilized than in unfertilized plots, indicating higher bird predation in fertilized plots. Predation may thus explain the lack of fertilization effect on larval densities in the field experiment. Our results suggest that top-down effects are more important for larval densities than bottom-up effects, and that bird predation may play an important role in population regulation of O. brumata in boreal forests.Electronic Supplementary Material Supplementary material is available for this article at     

Onion maggot feeding and development on heterogeneous sections of the onion bulb

When given a choice of feeding sites in laboratory arenas, Delia antiqua (Meigen) larvae exhibited a 3: 1 bias for internal vs outer sections of onion bulbs and distributed themselves non-randomly among identical preferred onion sections. Larval clustering did not appear to be correlated with increased fitness in the laboratory; larval development was identical across the range of densities from 1 to 50 larvae per onion. Larvae feeding on preferred internal sections developed 14% faster, were 38% heavier upon pupation, and were two times more likely to survive to the adult stage than larvae feeding on outer sections. D. antiqua larval food preference was therefore positively correlated with increased fitness.