Gall inducers are favoured as biocontrol agents of weeds because they tend to have a narrow host range. Six insect and one nematode gall inducer used in Canada are described in terms of their biology, gall morphology, gall physiology, and effectiveness in weed control. The species differ in plant organ attacked, requirement for moisture, whether the galls are induced by secretions or by severing xylem, and effectiveness, which in part relates to the ability of the gall to import nutrients. The most powerful galls divert assimilates from other sinks via a gall’s vascular system joined to that of their host. One of our examples also has mechanisms to compensate for reduction of turgor during drought. Two of the gall inducers enhance their nutrient supply by severing xylem in a plant nutrient sink. One, in the short-term sink of a thistle capitulum, obtains about a quarter of its assimilates at the expense of other capitula. The other, in the long-term sink of a rosette root, approximately halves seed production. Hypotheses are presented to explain various aspects of gall development and function.

Effectiveness of gall inducers in weed biological control

The analysis of metapopulation systems is to a great extent based on presence/absence patterns. It is possible to extract the rates of extinction and colonization from these patterns making several assumptions : (i) the time-scale of within-patch dynamics is considerably shorter than that of between-patch dynamics, (ii) the probability of extinction depends on patch area, whereby patch area is used as a surrogate of population size, (iii) the probability of colonization depends on isolation of patches. Field data on the metapopulation dynamics of a tephritid species together with habitat parameters were used to test the validity of the assumptions above. U. cardui is a monophagous stem gall former on the creeping thistle Cirsium arvense, forming a spatially structured population system due to the patchy distribution of the host plant with frequent extinctions of local populations and recolonization of patches. Gall numbers of U. cardui proved to be proportional to patch area. Thus, patch area is a reliable predictor for population size. The size of newly founded populations was 50% below the population size expected from patch area. The extinction probability of local populations was predominantly a function of population size and therefore patch area. The colonization probability of empty host plant patches was greater during years with low larval mortality and increased with patch size and neighbourhood density. Within the studied population system isolation was of minor importance for the colonization of empty patches because the distance between patches is shorter than the mean dispersal distance of the tephritid species. The colonization rate depends mainly on the number of dispersing individuals and hence on the size of existing populations. Between- and within-patch dynamics operate on similar, short time-scales. Nevertheless, metapopulation dynamics can occur as long as there are local breeding populations and a high risk of extinction of local populations. Presupposumption is that the species-specific dispersal ability by far exceeds the distance of patches.

Metapopulation Dynamics of the Tephritid Fly Urophora cardui: An Evaluation of Incidence-Function Model Assumptions with Field Data

Urophora cardui is a phytophagous tephritid species that induces conspicuous stem galls on Cirsium arvense. Using these galls it is simple to map the distribution of U. cardui. U. cardui has distributional gaps in Europe. Previous authors ascribed these gaps to an unfinished re-immigration since the Pleistocene. We investigated the ecological and genetic spatial pattern in an area in north-eastern Bavaria, where this fly species is known to have invaded part of such a gap. The mean annual rate of range expansion of U. cardui into this distributional gap is about 2 km per year, which is much higher than postulated by some previous studies and consistent with results of others. Between 50 and 70% of host-plant patches are occupied by U. cardui within the colonized part of the study area

Ecological and genetic spatial patterns of Urophora cardui (Diptera: Tephritidae) as evidence for population structure and biogeographical processes

Random amplified polymorphic DNA polymerase chain reaction (RAPDPCR) was used to assess genetic diversity in four subpopulations (86 individuals) of wild barley, Hordeum spontaneum, sampled from Tabigha microsite near the Sea of Galilee, Israel. The microsite consists of two 100 m transects that are topographically separated by 100 m, each equally subdivided into 50 m of basalt and terra rossa soil types. Despite the same macroclimate characterizing the area around the Sea of Galilee, the microsite offers two edaphically different microhabitats, with basalt being a more ecologically heterogeneous and broader-niche than the relatively drier but more homogeneous and narrow-niche terra rossa. Analysis of 118 putative loci revealed significant (P<0.05) genetic differentiation in polymorphism (P0.05) between the two soils across the transects with P being higher in the more heterogeneous basalt (mean P0.05 = 0.902), than in terra rossa (mean P0.05 = 0.820). Gene diversity (He) was higher in basalt (mean He=0.371), than in terra rossa (mean He=0.259). Furthermore, unique alleles were confined to one soil type, either in one or both transects. Rare alleles were observed more frequently in terra rossa than basalt, and in transect II only. Gametic phase disequilibria showed a larger multilocus association of alleles in basalt than terra rossa, and in transect I than II. Spearman rank correlation (rs) revealed a strong association between specific loci and soil types, and transects. Also, analysis of multilocus organization revealed soil-specific multilocus-genotypes. Therefore, our results suggest an edaphically differentiated genetic structure, which corroborates the niche width-variation hypothesis, and can be explained, in part, by natural selection. This pattern of RAPD diversity is in agreement with allozyme and hordein protein diversities in the same subpopulations studied previously.

RAPD divergence caused by microsite edaphic selection in wild barley

Fruit flies of the family Tephritidae have been well researched for several decades because they include pests of many cultivated plants. Examples are the Mediterranean fruit fly Ceratitis capitata Wied., which feeds on a broad range of fruits, the olive fruit fly Dacus oleae Gmel., the European cherry fruit fly Rhagoletis cerasi L., and the apple maggot fly Rhagoletis pomonella Walsh. Other Tephritids which feed on weeds are host specific and therefore potential agents in biological control. Because of their economic importance detailed research has been carried out on Tephritid physiology, ecology, genetics, and evolution.

Genetic Polymorphism and its Ecological Background in Tephritid Populations (Diptera: Tephritidae)

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Genetic Polymorphism and its Ecological Background in Tephritid Populations (Diptera: Tephritidae)