High-density populations of diamondback moth have broader host-plant diets.

Although ecological specialists exploit a relatively limited resource base, it is unclear whether specialization limits local population density. Here, we focus on the relationship between diet specialization and local population density of a phytophagous insect, the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae). We compared key life history traits and food plant use among five pairs of high- and low-density populations from widely separate geographical areas. Moths from populations that persist at high densities were more generalized in food plant use than moths from low-density populations. In particular, the oviposition preference and larval performance of moths from some high-density populations were less variable across a suite of food plants, suggesting that moths from high- density populations had a broader diet. In addition, low- density populations were less similar to each other, exhibiting opposing preferences for particular plant species. Hence diet breadth may explain some of the persistent differences in the population density of diamondback moths in the field, consistent with the idea that ecological specialization may be generally associated with population density.

Contrary choices: possible exploitation of enemy-free space by herbivorous insects in cultivated vs. wild crucifers.

The pressure to escape natural enemies may shape how herbivorous insects use their plant resources. On wild crucifers, ovipositional preferences of the diamondback moth (Plutella xylostella; DBM) were similar to searching preferences of its main parasitoid, an ichneumonid wasp (Diadegma insulare). But on cultivated crucifers, these species had opposite preferences. In addition, DBM ovipositional preferences did not correlate with growth or reproduction on several foodplants. We interpret these patterns as evidence of evolution for use of enemy-free space in agricultural systems.

Effects of experimental design and nitrogen on cabbage butterfly oviposition.

To test the prediction that P. rapae egg densities increase with N fertilizer in large-scale systems as they do in model systems with potted plants, we used field experiments with Brassica oleracea var. acephala L. (collards and kale) planted in pots or large field plots, and treated with different levels of nitrogen fertilizer. In small-scale field experiments with potted kale and collards, egg densities were significantly higher on plants with high N than those with low N. But in larger scale experiments with field-grown collards, average seasonal P. rapae egg densities were not significantly correlated with leaf N content. These differences among experiments did not depend on the magnitude of the difference in foliage N levels.

The ecological importance of insect frass: allelopathy in eucalypts.

Our previously published experiments on allelopathic effects of insect frass in Eucalyptus communities (Silander et al. 1983) have been criticized on the grounds that our estimates of annual frass production were exscessive (Ohmart 1985). However, we spanned the entire array of estimates of frass fall available from eucalypt communities, and we demonstrated allelopathic effects at even the lowest levels suggested by Ohmart. We suggest that average values of frass fall per hectare are irrelevant because they ignore both large scale variation among sites in time and space, and small scale variation in patchiness of frass accumulation. At the ecological scales relevant to germinating seeds, frass concentrations in local pockets may be considerably higher than the averages calculated for the entire ground surface.

Chemical interference among plants mediated by grazing insects.

The experiments on Eucalyptus trees reported here demonstrate that allelopathic effects can be mediated by insects grazing on foliage. We show that the allelochemical nature of insect frass suppresses germination, growth and survival of herb layer species, that plant species vary in their tolerance of this material, and that as a result, the structure and composition of associated, herbaceous understory plant communities are markedly affected by frass fall.

Specialization: species property or local phenomenon?

Many herbivorous insects have generalized diets over the species' entire geographical ranges but they function as specialists with restricted diets in local communities. Local feeding specialization can be produced by biochemical, behavioral, ecological, and evolutionary processes. Much evidence is incompatible with the widely held assumptions that diet breadth is a species characteristic and that specialization among herbivorous insects implies greater efficiency and less niche overlap.

The loss of phytochrome photoreversibility in vitro : II. Properties of killer and its reaction with phytochrome.

"Killer", a substance extracted from stem tissue of etiolated pea seedlings (Pisum sativum L. v. Alaska), interacts specifically with the far-red-absorbing form of phytochrome (Pfr) in vitro in a temperature-independent, rapid, stoichiometric fashion to cause a loss of phytochrome photoreversibility. The chromatographic, solubility, and spectral properties of partially purified fractions indicate that Killer is a cyclic, unsaturated molecule containing ionizible hydroxyl groups; its molecular weight is unknown, although probably low. Possible mechanisms by which the Killer-phytochrome interaction results in the loss of photoreversibility are discussed.

Loss of phytochrome photoreversibility in vitro: I. Extraction and partial purification of killer.

Crude pea (Pisum sativum L. var. Alaska) phytochrome extracts contain a substance, "Killer," which interacts with the far red-absorbing form of phytochrome causing a net loss of spectrophotometrically detectable phytochrome in vitro. Killer is absent from crude extracts of Avena phytochrome, is separable from pea phytochrome by gel filtration, and is alcohol-extractable from etiolated pea seedlings. Killer activity in alcohol extracts behaved, during partial purification, in a manner identical to that derived from pea phytochrome preparations. The mass extraction and partial purification of Killer are described.

Differences in Photoresponse and Phytochrome Spectrophotometry Between Etiolated and De-etiolated Pea Stem Tissue.

Morphologically similar pea plants having a 4-fold difference in spectrophoto-metrically detectable phytochrome can be produced by pretreatment of etiolated plants with red light (R) or with red and far-red light combined (RF). A search for response differences which could be ascribed to differences in phytochrome content has resulted only in the establishment of differences due to de-etiolation. Segments of etiolated plants differ from those of plants de-etiolated by R and RF pretreatments in 2 ways. Segments from etiolated plants appear to respond rapidly to the far-red absorbing form of phytochrome (P(FR)), while segments from de-etiolated plants do not respond rapidly to P(FR). This statement is based upon 2 observations: (i) the red light induced growth inhibition in segments from etiolated plants rapidly escapes reversibility by far-red light, while with segments from R or RF pretreated plants, the red light effect is fully reversed by subsequent far-red light for up to 2 hr; and (ii) segments from etiolated plants were inhibited to a greater degree than were segments from RF pretreated plants when various photostationary state levels of P(FR) were maintained for 30 or 90 min and then removed by photoconversion to P(R). The in vivo nonphotochemical transformation curves of the phytochrome of etiolated and RF pretreated plants appear to differ in 2 related respects: (i) the amount of phytochrome destroyed in de-etiolated tissue is greater than that in etiolated tissue, perhaps as a result of the fact that (ii) the rate and extent of apparent reversion of P(FR) to P(R) in etiolated tissue is about twice that in de-etiolated tissue.

Response of tissue with different phytochrome contents to various initial photostationary States.

Pretreatment of etiolated pea plants with red light and with red combined with far-red light produced morphologically similar plants having 4-fold differences in spectrophotometrically detectable phytochrome. Stem segments from the variously pretreated plants respond in the same way to different percentage conversions of phytochrome to P(FR). These results suggest that the P(FR)./P(R) ratio, rather than the concentration of P(FR), governs pea stem segment elongation. However, the ratio hypothesis does not explain contradictions between spectrophotometric and physiological assays previously obtained with this tissue, nor does it explain similar contradictions obtained in other systems. The only hypothesis consistent with the data to date is that of the existence of bulk and active phytochrome fractions, with the latter present in insufficient quantities to be spectrophotometrically detectable.