Søren Toft

Value of the aphid Rhopalosiphum padi as food for cereal spiders

1. Laboratory experiments were carried out to analyse various aspects of spider performance under different diets, focusing mainly on the value of the aphid Rhopalosiphum padi. Aphis fabae and a sciarid midge were considered in single experiments. Laboratory fruit flies were used as a standard prey for comparison. The spider species used were Erigone atra, Pardosa amentata and P. prativaga. 2. In preference experiments P. amentata ranked prey types in the order: fruit flies > sciarid > R. padi > A. fabae. 3. In P. prativaga consumption of fruit flies increased with the length of prior starvation period; consumption of aphids was much lower and independent of hunger. 4. First instar spiderlings of P. amentata and Erigone atra were unable to develop on a pure diet of R. padi; they died without moulting. 5. Egg production in wild caught E. atra females stopped after 2-4 egg-sacs on a pure diet of R. padi, whereas females given fruit flies or fruit flies + aphids produced an average of eight sacs. Egg numbers were high on a pure diet of fruit flies, but hatching success was best with fruit flies supplemented with aphids. The size of hatched young depended on both parent size and diet. 6. It was concluded that R. padi represented a low quality prey for cereal spiders which obtain very little energy but essential nutrients from the aphids. It remains to be established whether this has consequences for the prospects of managing spider communities with the view of improving their effectiveness against aphids. It is argued that with a low preference for aphids, improving availability of alternative prey in more diverse agrosystems may possibly reduce spider predation on aphids.

Optimal foraging for specific nutrients in predatory beetles

Evolutionary theory predicts that animals should forage to maximize their fitness, which in predators is traditionally assumed equivalent to maximizing energy intake rather than balancing the intake of specific nutrients. We restricted female predatory ground beetles (Anchomenus dorsalis) to one of a range of diets varying in lipid and protein content, and showed that total egg production peaked at a target intake of both nutrients. Other beetles given a choice to feed from two diets differing only in protein and lipid composition selectively ingested nutrient combinations at this target intake. When restricted to nutritionally imbalanced diets, beetles balanced the over- and under-ingestion of lipid and protein around a nutrient composition that maximized egg production under those constrained circumstances. Selective foraging for specific nutrients in this predator thus maximizes its reproductive performance. Our findings have implications for predator foraging behaviour and in the structuring of ecological communities.

Quantifying food limitation of arthropod predators in the field

Abstract A method for quantifying food limitation of arthropod predators in the field is presented and applied to species of ground beetles (Carabidae) and sheet-web spiders (Linyphiidae) from a cereal field. Food limitation is expressed quantitatively as accumulated hunger (=starvation) by transforming 24-h food consumption at 20°C of animals newly collected in the field into days of starvation at 20°C. This is done by means of a reference curve relating 24-h food intake at 20°C to starvation periods (days) at 20°C. Such a reference curve was obtained for the carabid beetle Agonum dorsale in the laboratory. For other species the reference curve was modified with species-specific data. The procedure makes it possible to compare the feeding conditions of different species populations that are part of the same community. Hunger levels in the field for females of A.␣dorsale were equivalent to c. 15 days of starvation in early spring, c. 5 days in June, increasing to c. 10 days in late summer. Two ground beetles occurring during summer also showed increasing hunger levels from June to July, probably the result of a dry summer. Two spider species experienced a hunger level in the range of 4–8 days of starvation.

Factors influencing cannibalism in the wolf spider Pardosa agrestis (Araneae, Lycosidae)

Abstract Cannibalistic tendencies are well known in spiders and may be a significant factor influencing population size. The wolf spider, Pardosa agrestis, is the dominant non-web-building spider in a wide range of central European agricultural habitats. Preliminary field observations indicated an extended reproductive period, which results in a very wide size distribution of juvenile instars. We hypothesised that if cannibalism is enhanced by differences in size, especially during periods when prey is scarce, these populations might be susceptible to cannibalism in an ecologically significant way. Laboratory studies were conducted on juvenile P. agrestis in arenas. We analysed the following specific aspects of cannibalism: (1) the effect of the weight ratio between the opponents; (2) the effect of weight per se, and (3) the role of hunger level in determining cannibalistic tendencies of spiders. The role of weight and hunger were analysed in separate experiments, in both cases by controlling for the other variable. The results showed that cannibalism was strongly positively correlated with both weight ratio and hunger, but absolute size/age of an individual could not predict the occurrence of a cannibalistic event. These experiments generated the plausible hypothesis that cannibalism might be an important phenomenon in the regulation of real populations, which should be tested specifically in future field experiments.

Role of granivory and insectivory in the life cycle of the carabid beetle Amara similata

1. The cereal field carabid beetle Amara similata was selected to elucidate the role of seeds and insects as food sources. Three experiments were performed to rank different weed seeds and insects in terms of food value: (i) fecundity in relation to adult diet, (ii) larval survival in relation to diet and (iii) larval survival in relation to parental diet.

Balancing of protein and lipid intake by a mammalian carnivore, the mink, Mustela vison.

Many herbivores and omnivores can balance their intake of macronutrients when faced with nutritionally variable environments. Carnivores, however, are widely believed to optimize their rates of prey capture and energy intake rather than balancing nutrients. We tested nutrient balancing in captive mink and found a pronounced ability to balance and regulate intake of protein and lipid. When faced with one of several different pairings of complementary foods varying in protein to lipid composition, mink apportioned intake between the two foods to defend a near constant ratio and amount (intake target) of the two macronutrients. When given only one food of fixed nutrient composition, mink balanced macronutrient intake relative to the intake target, without showing the excessive energy intake on diets with a low percentage of protein and energy deficit on diets with a high percentage of protein previously reported for herbivores and omnivores, including humans. This demonstration of nutrient balancing in a carnivorous mammal indicates that the capacity for nutrient balancing is a more general phenomenon across trophic levels than was hitherto believed to be the case.

Protein and carbohydrate composition of larval food affects tolerance to thermal stress and desiccation in adult Drosophila melanogaster

Larval nutrition may affect a range of different life history traits as well as responses to environmental stress in adult insects. Here we test whether raising larvae of fruit flies, Drosophila melanogaster, on two different nutritional regimes affects resistance to cold, heat and desiccation as well as egg production and egg-to-adult viability. We raised larvae on a carbohydrate-enriched and a protein-enriched growth medium. We found that flies developed on the high protein medium had increased heat and desiccation tolerance compared to flies developed on the carbohydrate-enriched medium. In contrast, flies developed on the carbohydrate-enriched growth medium recovered faster from chill coma stress compared to flies developed on a protein-enriched medium. We also found gender differences in stress tolerance, with female flies being more tolerant to chill coma, heat knockdown and desiccation stress compared to males. Egg production was highest in females that had developed on the protein-enriched medium. However, there was a sex-specific effect of nutrition on egg-to-adult viability, with higher viability for males developing on the sucrose-enriched medium, while female survival was highest when developing on the protein-enriched medium. Our study indicates that larval nutrition has a strong impact on the ability to cope with stress, and that the optimal nutrient composition varies with the type of stress.

The value of two Collembola species as food for a linyphiid spider

Egg production by the cereal spider Erigone atra was used as a fitness parameter for evaluating the food quality of two species of Collembola: Folsomia fimetaria and Isotoma anglicana. Drosophila melanogaster was used as reference prey. We tested the hypothesis that due to differences in food quality, the two Collembola species would affect the reproduction of the spider differently. The quality ranking of the prey types turned out as: I. anglicana > D. melanogaster > F. fimetaria. With F. fimetaria alone, spiders were unable to maintain reproduction. E. atra was more efficient in utilising I. anglicana and D. melanogaster. Thus, daily consumption rates of I. anglicana were lower in spite of higher egg laying rates by E. atra. A mixed diet of F. fimetaria and D. melanogaster resulted in a lower reproductive output than a pure diet of D. melanogaster, indicating a toxic element in F. fimetaria. In the mixed diet F. fimetaria had a negative influence on the consumption capacity of the spider towards D. melanogaster, while D. melanogaster had a positive influence on the consumption capacity towards F. fimetaria. It is concluded that a high abundance of I. anglicana may support a high reproductive output of E. atra, while the presence, of F. fimetaria in fields may reduce the spiders reproductive output.

SPATIAL STRATIFICATION IN LITTER DEPTH BY FOREST-FLOOR SPIDERS

Abstract Two novel sampling techniques were used to survey the spider community of the leaf litter in a deciduous forest in Kentucky, USA. Using modified pitfall traps and litter-grab techniques, we sampled separately the top, middle and bottom litter layers from April–October. Our sampling program captured over 3,000 spiders encompassing 18 different families. Both techniques revealed that the web-spinning families were more abundant in the lower litter layers. In contrast, the non-web building cursorial spiders, which actively pursue their prey, were more abundant in the top litter layer. Cursorial spiders, on average, were larger than the web-building spiders found in the leaf litter. Web-building spiders from the top litter layer were also larger than the web-building spiders caught in the middle and the bottom litter layers. Comparison between the two sampling techniques revealed that the spider community profile is greatly influenced by the sampling method employed. The stratified litter-grab technique revealed the numerical dominance of Dictynidae (38% of the spiders captured) and Linyphiidae (32%), families that are predominately minute web-building spiders. In contrast, the pitfall-trap technique suggested Lycosidae (24%), a family of active foragers, to be numerically dominant, with Dictynidae representing only 1% of the spiders captured. The results indicate that major groups of spiders differ in their vertical distribution within deciduous leaf litter, and that sampling method can dramatically affect inferences about spider community structure.

The influence of three cereal aphid species and mixed diet on larval survival, development and adult weight of Coccinella septempunctata

The developmental time, weight and fecundity of the aphid specialist predator the seven spotted ladybird (Coccinella septempunctata L.) vary considerably according to the species of aphid they are raised on (Blackman, 1967a; Ghanim et al., 1984; Malcolm, 1992). The results of these studies showed that prey of low food quality slowed larval development and produced smaller adult animals. The experiments were carried out with single-species diets. So far there seems to be no studies on the possible benefits of diet mixing in coccinellid beetles (Hodek & Hon ěk 1996). Hodek (1956) and Blackman (1967b) found evidence that C. septempunctata is unselective in its choice of prey; it cannot recognise and avoid unsuitable or toxic species of aphids. ̧ Sengonca & Liu (1994) noticed that the larvae ofC. septempunctata revealed no difference in responding to the kairomones from four different species of aphid prey. It is therefore likely that the larvae ofC. septempunctata just eat the aphids that they meet. This may be an optimal foraging strategy, when the prey is clumped and unpredictable and acceptance of toxic prey is no worse than starvation. Even if coccinellids are unselective in a choice situation, a mixture of prey species may provide a higher nutritional diversity and thus be beneficial in terms of improved performance i.e. a lower mortality, a faster development and a higher adult body weight. We used three cereal aphid species Sitobion avenae(F.), Rhopalosiphum padi (L.) and Metopolophium dirhodum(Walker) and an equal number mixture of these three species as prey and compared the performance of C. septempunctata larvae. The three cereal aphid species all have grasses as their host plants, thus the coccinellid larvae may encounter all three species on the same plant. It is therefore ecologically relevant to examine if there is an advantage of a mixed diet of these three aphid species. Our hypothesis is that an aphid specialist predator as C.septempunctata , may gain advantage from developing on a diversified diet, compared to a singlespecies aphid diet, because different species of aphids may have a different composition of nutrients, thus satisfying better a varied nutrient demand. This should be revealed by better performance of the larvae, indicating a higher food value of the mixed diet. Such relationships are known for a wide range of generalist consumers (Reichert & Harp, 1987; Waldbauer & Friedman, 1991; Wallin et al., 1992; Bernays et al., 1994).