Relative nutritional quality of C 3 and C 4 grasses for a graminivorous lepidopteran, Paratrytone melane (Hesperiidae)

We tested the hypothesis that C 4 grasses are inferior to C 3 grasses as host plants for herbivorous insects by measuring the relative performance of larvae of a graminivorous lepidopteran, Paratrytone melane (Hesperiidae), fed C 3 and C 4 grasses. Relative growth rates and final weights were higher in larvae fed a C 3 grass in Experiment I. However, in two additional experiments, relative growth rates and final weights were not significantly different in larvae fed C 3 and C 4 grasses. We examined two factors which are believed to cause C 4 grasses to be of lower nutritional value than C 3 grasses: foliar nutrient levels and nutrient digestibility. In general, foliar nutrient levels were higher in C 3 grasses. In Experiment I, protein and soluble carbohydrates were digested from a C 3 and a C 4 grass with equivalent efficiencies. Therefore, differences in larval performance are best explained by higher nutrient levels in the C 3 grass in this experiment. In Experiment II, soluble carbohydrates were digested with similar efficiencies from C 3 and C 4 grasses but protein was digested with greater efficiency from the C 3 grasses. We conclude (1) that the bundle sheath anatomy of C 4 grasses is not a barrier to soluble carbohydrate digestion and does not have a nutritionally significant effect on protein digestion and (2) that P. melane may consume C 4 grasses at compensatory rates.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47798/1/442_2004_Article_BF00317268.pd

The effect of dietary nicotine on the allocation of assimilated food to energy metabolism and growth in fourth-instar larvae of the southern armyworm, Spodoptera eridania (Lepidoptera: Noctuidae)

Dietary nicotine (0.5%), which is a substrate of the PSMO (polysubstrate monooxygenase) detoxification system in the southern armyworm Spodoptera eridania , has significant negative effects on the weight of food ingested, weight gained, relative growth rate (RGR), and efficiency of conversion of digested food (ECD) by fourthinstar S. eridania larvae on a nutrient-rich artificial diet. It has a significant positive effect on the weight of food respired by the larvae. Thus, the detoxification of nicotine by the PSMO system exacts a fitness cost and imposes a metabolic cost on S. eridania larvae. In contrast, dietary α-(+)-pinene, an inducer of the PSMO system, neither exacts a fitness cost nor imposes a metabolic cost on the larvae. We believe this to be the first study to demonstrate unequivocally that the negative effect of a dietary toxin on net growth efficiency (ECD) in an insect herbivore is due to an increase in the allocation of assimilated food to energy metabolism and not to a decrease in the amount of food assimilated. This study, therefore, supports the hypothesis that detoxification can impose a significant metabolic load on an insect herbivore. Implications of a corroboration of the metabolic load hypothesis are discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47797/1/442_2004_Article_BF00317425.pd

The digestion of protein and carbohydrate by the stream detritivore, Tipula abdominalis (Diptera, Tipulidae)

The digestive system of larvae of Tipula abdominalis (Diptera, Tipulidae), a stream detritivore, is poorly adapted for the digestion of the major polysaccharides in its diet, but well adapted for the digestion of protein. These crane fly larvae are unable to digest the major cell wall polysaccharides of higher plants, i.e., cellulose, hemicellulose and pectin. The only polysaccharides toward which the midguts of T. abdominalis exhibited any activity were α-amylose and laminarin, indicating that polysaccharide digestion is restricted to α-1,4-and β-1,3-glucans. The most concentrated source of these two classes of carbohydrates in submerged leaf litter would be associated fungal tissue. The midgut of T. abdominalis is strongly alkaline throughout, with a maximum pH near 11.5 in a narrow zone near the midpoint. Proteolytic activity in the midgut is extraordinarily high, and the pH optimum for midgut proteolytic activity is above 11. We conclude that the high alkalinity and high proteolytic activity observed in T. abdominalis larvae are manifestations of a highly efficient protein-digesting system, a system of crucial importance to a nitrogen-limited organism which must derive its nitrogen from a resource in which much of the limited nitrogen present is in a “bound” form in complexes of proteins with lignins and polyphenols.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47733/1/442_2004_Article_BF00346265.pd

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Nutritional indices in the gypsy moth ( Lymantria dispar (L.)) under field conditions and host switching situations

A large proportion of gypsy moths ( Lymantria dispar (L.)) are likely to experience multiple species diets in the field due to natural wandering and host switching which occurs with these insects. Nutritional indices in fourth and fifth instar gypsy moth larvae were studied in the field for insects that were switched to a second host species when they were fourth instars. The tree species used as hosts were northern pin oak ( Quercus ellipsoidalis E. J. Hill), white oak ( Q. alba L.), big-tooth aspen ( Populus grandidentata Michx.), and trembling aspen ( P. tremuloides Michx.). Conclusions of this study include: 1) Insects which fed before the host switch on northern pin oak performed better after the host switch than did insects with other types of early dietary experience. While the northern pin oak-started insects had very low relative food consumption rates on their second host species immediately after the switch, one instar later they had the highest ranked consumption rates. During both instars they had the second highest efficiencies of converting ingested and digested food to body mass. High food consumption rates and relatively high efficiency of food conversion helped these insects to obtain the highest ranked mean relative growth rates in the fifth instar compared to the relative growth rates obtained by insects from any of the other first host species. 2) Among the four host species examined, a second host of trembling aspen was most advantageous for the insects. Feeding on this species after the switch led to higher larval weights and higher relative growth rates for insects than did any of the other second host species. The insects on trembling aspen attained excellent growth despite only mediocre to low food conversion efficiencies. The low efficiencies were offset by high relative food consumption rates. 3) Low food consumption rates often tend to be paired with high efficiency of conversion and vice versa. 4) There is no discernable tendency for the first plant species eaten to cause long-term inductions which affect the ability of gypsy moths to utilize subsequent host plants. Insects did not tend to consume more, grow faster, or be more efficient if their second host plant was either the same as their rearing plant or congeneric to it. Methods are delineated which allow values of nutritional indices to be obtained for insects on intact host plants under field conditions. These methods are useful for the purpose of answering questions about the relative effects that different diet treatments have on insect response.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47803/1/442_2004_Article_BF00323145.pd

Effects of host switching on gypsy moth ( Lymantria dispar (L.)) under field conditions

Effects of various single and two species diets on the performance of gypsy moth ( Lymantria dispar (L.)) were studied when this insect was reared from hatch to population on intact host trees in the field. The tree species used for this study were red oak ( Quercus rubra L.), white oak (Q. alba L.), bigtooth aspen ( Populus grandidentata Michaux), and trembling aspen ( P. tremuloides Michaux). These are commonly available host trees in the Lake States region. The study spanned two years and was performed at two different field sites in central Michigan. Conclusions drawn from this study include: (1) Large differences in gypsy moth growth and survival can occur even among diet sequences composed of favorable host species. (2) Larvae that spent their first two weeks feeding on red oak performed better during this time period than larvae on all other host species in terms of mean weight, mean relative growth rate (RGR), and mean level of larval development, while larvae on a first host of bigtooth aspen were ranked lowest in terms of mean weight, RGR, and level of larval development. (3) Combination diets do not seem to be inherently better or worse than diets composed of only a single species; rather, insect performance was affected by the types of host species eaten and the time during larval development that these host species were consumed instead of whether larvae ate single species diets or mixed species diets. (4) In diets composed of two host species, measures of gypsy moth performance are affected to different extents in the latter part of the season by the two different hosts; larval weights and development rates show continued effects of the first host fed upon while RGRs, mortality, and pupal weights are affected strongly by the second host type eaten. (5) Of the diets investigated in this study, early feeding on red oak followed by later feeding on an aspen, particularly trembling aspen, is most beneficial to insects in terms of attaining high levels of performance throughout their lives.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/47802/1/442_2004_Article_BF00323144.pd

Investigating dietary preferences in two competing dipterans, <i>Coelopa frigida</i> and <i>Coelopa pillipes</i>, using stable isotope ratios of carbon and nitrogen

Two species of seaweed fly, Coelopa frigida (Fabricius) and Coelopa pilipes (Halliday) (both Diptera: Coelopidae: Coelopini), compete for resources within deposits of marine algae washed ashore on British beaches. Previous studies report that adult flies exhibit algal-specific behaviour that may influence interspecific interactions. It is predicted that coelopid larvae may also demonstrate algal-specific dietary preferences. Larval dietary preferences are investigated by comparing the ratios of 13C/12C and 15N/14N in both wild flies and macroalgae to those of laboratory-reared flies. Results showed only a small difference between the stable isotope ratios of the most abundant algae, Laminaria spp. (Laminariaceae) and Fucus spp. (Fucaceae), although there were significant differences between wild adult coelopids. This result illustrates different metabolic processes in two closely related species. The stable isotope ratios of wild-caught coelopids were found to differ significantly from laboratory-reared coelopids. This is either the result of red algae in the diet of natural populations or a difference in bacterial communities. We suggest that experiments with laboratory-reared flies/specimens can greatly increase the utility of stable isotope analysis in the investigation of animal food webs, even where potential diets are isotopically similar. However, this approach is dependent on re-creations that accurately mimic natural conditions