E. van der Meijden

The “Raison D'être” of pyrrolizidine alkaloids inCynoglossum officinale: Deterrent effects against generalist herbivores

In this study we tested whether pyrrolizidine alkaloids (PAs) ofCynoglossum officinale serve as antifeedants against herbivores. Total PA N-oxide extracts of the leaves significantly deterred feeding by generalist herbivores. Specialist herbivores did not discriminate between food with high and low PA levels. Three PAs fromC. officinale, heliosupine, echinatine, and 3′-acetylechinatine, equally deterred feeding by the polyphagous larvae ofSpodoptera exigua. Although the plants mainly contain PAs in their N-oxide form, reduced PAs deterred feeding byS. exigua more efficiently than PA N-oxides. On rosette plants, the monophagous weevilMogulones cruciger significantly consumed more of the youngest leaves, which had the highest PA level and the highest nitrogen percentage. Larvae ofEthmia bipunctella, which are oligophagous within the Boraginaceae, did not discriminate between leaves. All generalist herbivores tested significantly avoided the youngest leaves with the highest PA levels. In the field, the oldest leaves also were relatively more damaged by herbivores than the youngest leaves. It is hypothesized that the skewed distribution of PAs over the leaves of rosette plants reflects optimal defense distribution within the plant.

Production, dispersal and predation of seeds in the biennial Cirsium vulgare.

(1) Seed production of flowering plants of Cirsium vulgare without damage to the flowering stem varied strongly between years in the sand-dunes at Meijendel. The percentage of plants that were noticeably damaged by herbivores also varied between years, and caused a reduction of the seed production by 80% in 1982 and 15% in 1984. (2) Mean seed loss by predation in the flowerheads was 3% in 1982 and 17% in 1984. (3) Seed dispersal was limited: 50% of the seeds were dispersed within 1 m of the parent plant and only 11 % of the seeds were dispersed outside the local population. (4) On average 60% of the seeds were consumed by mice and voles after dispersal. Only 1% of the seeds produced were still viable after one year.

Herbivore exploitation of a fugitive plant species: Local survival and extinction of the Cinnabar Moth and Ragwort in a heterogeneous environment

Summary1.In the dune system under observation, Ragwort is distributed in small local populations that have only a restricted lifetime; (temporary) extinction has been observed frequently.2.Cinnabar Moth attack on these populations is of even shorter duration. Colonization and extinction of the Cinnabar Moth is related to the amount of food present.3.The presence of the predator Formica polyctena negatively influences the probability of local oviposition.4.There is a continual shift of the Cinnabar Moth over its food-plant populations. When populations in a favorable food situation in Formica-free habitats became scarce, there was a shift towards populations in Formica habitats.5.Negative effects of attack on the food-plant populations could be demonstrated, but they were small compared to fluctuations in these populations caused by other factors.6.After dispersal each year new plant populations providing a favorable food situation are colonized. The heterogeneity of the populations in time and space is the factor that ensures survival of the insect in the system of plant populations.

Glucosinolates and other metabolites in the leaves of Arabidopsis thaliana from natural populations and their effects on a generalist and a specialist herbivore

Abstract.Because many secondary metabolites in plants act as defense against herbivores it has been postulated that these compounds have evolved under selective pressure by insect herbivores. One explanation for the within-species variation in metabolite patterns in a particular species is that different populations are under selection by different herbivores. We tested this hypothesis, using Arabidopsis thaliana plants that originated from dune and inland areas. We analyzed Arabidopsis thaliana leaves using NMR spectroscopy and multivariate data analysis. Major differences in chemical composition were found in water-methanol fractions and were due to higher concentrations of sinigrin and fumaric acid in dune plants. Inland plants showed lower levels of glucose. Quantitative analysis of glucosinolates was performed with HPLC. Individual plants and populations demonstrated differences in glucosinolate composition and concentration. In growth chamber experiments, the generalist herbivore, Spodoptera exigua grew significantly better on the inland plants, while the specialist herbivore Plutella xylostella performed equally well on plants of both origins. Aliphatic glucosinolate as well as total glucosinolate concentrations negatively correlated with larval mass of Spodoptera exigua. No significant correlations, however, were found between larval mass of Plutella xylostella and glucosinolates in the leaves. A specialist and a generalist herbivore were responding differently to plant secondary chemistry, as was also found in several other studies. This is an important indication that differences in glucosinolate concentrations among populations may result from differential selection by different guilds of herbivores.

Population Dynamics of the Cinnabar Moth (Tyria Jacobaeae): Oscillations Due To Food Limitation and Local Extinction Risks

We report on a 17-year study on the population dynamics of the cinnabar moth (Tyria jacobaeae). As in other long-term studies of this species scramble competition for food leads to periodic crashes in abundance. This results in delayed density-dependent oscillations in the rate of increase of this insect with strikingly similar amplitudes. Recovery after a crash is delayed for a second year, although food is then not limiting. Delayed density-dependent factors such as parasitism and food quality reduction are assumed to be responsible. During a crash the cinnabar moth goes extinct in most local patches of ragwort. Mechanisms that may prevent extinction over large areas containing many such local populations (metapopulations) are heterogeneity in environmental conditions that affect oviposition and predation, and the distribution of egg production in time during the season.

The role of primary and secondary metabolites in chrysanthemum resistance toFrankliniella occidentalis

The presence of deleterious secondary metabolites to western flower thrips (WFT) (Frankliniella occidentalis Pergande) in several chrysanthemum (Dendranthema grandiflora Tzvelev) cultivars was tested. WFT performance on leaf sap mixed with artificial diet was compared to WFT performance on a control of artificial diet mixed with water. Leaf sap of six of 10 chrysanthemum cultivars tested had a significant negative effect on WFT performance and therefore contained deleterious secondary metabolites. Leaf sap of a resistant and a susceptible chrysanthemum cultivar were fractionated by gel to study the role of secondary metabolites in greater detail. None of the 20 individual fractions obtained had a negative effect on WFT performance, whereas after mixing of these fractions activity was found again. Moreover, WFT performance on the fraction mixture of the resistant cultivar was significantly lower than on the susceptible fraction mixture. The results suggest that WFT resistance in the cultivars studied was caused by several secondary metabolites that had an additional or synergistic negative effect on WFT performance. The role of a deficiency of primary metabolites in WFT resistance of chrysanthemum was also investigated. WFT performance on leaf sap with and without extra primary metabolites (artificial diet) was compared. The variance in WFT performance was significantly smaller when extra primary metabolites were added; WFT performance on leaf sap with and without extra primary metabolites was not correlated. The results suggest that resistance was partly caused by a deficiency of primary metabolites.

Changes in the Distribution Pattern of Tyriajacobaeae During the Larval Period

1. Egg-batch distribution in the Cinnabar Moth is related to plant size. The number of eggs per batch, however, is independent of size and other characteristics of the plant. 2. The adult behaviour of laying eggs in larger batches and the behaviour of young larvae to live gregariously leads to a higher survival. 3. The dispersal behaviour of larvae may have a positive influence on larval and pupal survival, and on adult fecundity, since the negative influences of high density, starvation and sub-optimal food are avoided. 4. Suggestions are given about the mechanism that underlies dispersal behaviour. 5. Dispersal is discussed in relation tot the aposomatic characteristics of the larvae.

Can Hosts Escape From Their Parasitoids

Apanteles popularis, a parasitoid of Tyria jacobaeae, does not regulate its host numbers, though the percentage attack may be very high. The effects of food shortage of the host on its parasite are discussed and suggestions are made why Apanteles does not regulate its host. This lack of a regulating influence is also mentioned in several other parasite-host studies in naturally evolved systems. A hypothesis on co-selection is formulated that might explain the difference in effect of parasites in naturally evolved systems and man-made systems, where biological control through imported parasites is often successful.

The influence of rabbit scrapes on dune vegetation

Abstract The rabbit, the only grazer of importance in the coastal dunes of northwestern Europe, is thought to stabilize the species composition of the typical dune grasslands. In addition to selectively grazing, it produces shallow scrapes. The function of this scrape-digging behaviour for the rabbit, and the effect on the distribution and abundance of dune grassland species, were analysed. It is concluded that root consumption is one of the main purposes for digging scrapes. The main effect on the vegetation is a comparative increase in the number of individuals of annual plant species, especially where percentage vegetation cover is not very high. The effect on annuals only becomes manifest after the next seed production of these species. Where vegetation cover is high, the effect of scrape digging is less obvious because perennial species have already filled the scrapes before seed production of the annuals. In a sand-dune system rabbit scrapes do not provide unique environments for specific plant species.

Experiments On Dispersal, Late-Larval Predation, and Pupation in the Cinnabar Moth (Tyria Jacobaeae L.) With a Radioactive Label (192Ir)

A number of experiments is described from which it is concluded that a predating ant, Lasius alienus Forst, locally has a very important influence on the population dynamics of the Cinnabar Moth. Moreover, Tyria appears to be absent in some of the dry dune vegetations and it is suggested that this is due to the presence of Lasius in high densities in those areas. The distribution pattern of pupae in the field could be explained by preference for certain pupation sites. The applied technique of labelling, with an external radioactive mark, proved satisfactory.