The problem of optimal clutch sizes is a central theme in life history theory. Optimal allocation of eggs is especially complicated for insects in tritrophic systems. In this study we analyze some of the processes determining clutch sizes of the thistle gallfly Urophora cardui, a monophagous tephritid fly associated with Cirsium arvense. U. cardui forms multilocular shoot galls, which vary broadly in their size and number of their gall cells. We investigate various fitness consequences of gall size. An analysis of the number of cells per gall (which is correlated with gall diameter and gall weight) showed that in U. cardui there is mutual facilitation rather than larval competition. Increasing numbers of larvae per gall led to a decreasing mortality and increasing larval weight. Larval weight in turn was positively correlated with the probability of survival to adulthood and with adult weight and fecundity. Thus, all fitness parameters measured favoured large galls. Clutch sizes in oviposition experiments were distinctly larger than the number of gall cells of field populations and in cage experiments, suggesting high mortality of eggs and/or early larval instars. There was a significant relationship between the internal structure (i.e., the size of the growing point) of the bud and clutch size, suggesting that U. cardui females are able to measure bud quality and adapt clutch sizes accordingly. Clutch size was positively correlated with the female's age at first oviposition and negatively with the number of previous ovipositions and previously laid eggs. Since the potential egg capacity per female is higher than the average number of larvae it is likely to produce during its short adult lifespan, U. cardui females tend to be time-limited rather than egglimited, which might favour large clutches once an appropriate oviposition site has been located. As the development of the gall and hence the fate of a clutch depends on a number of unpredictable factors, exclusive concentration of eggs in a few large clusters would involve risks which could be avoided by increasing the number of clutches. Therefore we interpret the high variation of clutch sizes in U. cardui as a mixed strategy of bet hedging and gambling.

Phytophagous insects which feed on the leaves of herbaceous host plants have to adapt their life histories to the fact that protein nitrogen is usually highest in growing tissues in spring. We monitored field populations of larvae and adults of three chrysomelid species (Galeruca tanaceti (L.) (main host Achillea millefolium (L.) Yarrow), Cassida rubiginosa (Mueller) (main host Cirsium arvense (L.) Scop.) and Oreina luctuosa (Suffrian) (host Centaurea scabiosa (L.)) together with the amount of protein nitrogen of their food resources and host plant biomass. As expected, the development of host quality, measured as concentration of protein nitrogen, and host plant biomass showed inverse trends during the season. The euryphagous G. tanaceti attacks Achillea early and profits from high nitrogen concentrations in the leaves. Occasional overexploitations of local populations of Achillea are compensated by the capacity to move to other host species. In C. rubiginosa, a species with a host range restricted to the Cardueae, the main larval feeding activity is postponed to a period when the nitrogen content of the host leaves had dropped to 50% of its initial value, but when host plant biomass had increased by 30%. In the monophagous O. luctuosa the larval development is synchronized with a still later phase of host phenology, at which the nitrogen content is below 50% but plant biomass has reached its maximum. There seem to be selection factors, which oppose the use of high quality food in spring and which force the latter two species to postpone their larval development to a later time in the year. This could be caused by numerous factors like, for example, mean daytime temperature. Later in the season the larvae have to cope with the low quality of their host plants. They have, however, the advantage of large quantities of food available.A laboratory study with adults and mature larvae of O. luctuosa shows that this species can overcome low levels of protein nitrogen either by selecting younger leaves with higher nitrogen concentrations or by increasing the daily food consumption rate (RCR) on leaves with a low level of nitrogen and by a prolongation of the feeding period. In this way the larvae compensate the effect of lower daily growth rates (RGR) and a lower food conversion index (ECI) on poor food quality: Regardless of the level of protein nitrogen there was no statistically significant difference in total gain of weight during the third-instar feeding period and in the weight at the end of the third larval stage. The three investigated chrysomelids show that there exists a broad spectrum of adaptations to overcome the dilemma of variable food quality.

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The weevil species Rhinocyllus conicus has been widely and successfully used for biocontrol of Carduus nutans and related thistles. An analysis of the taxonomic position of R. conicus shows that it belongs to a tribe where many taxa have developed close associations with Cardueae host plants. Within the genus Rhinocyllus, R. conicus, which predominates on C. nutans, is geographically the main species and the other Rhinocyllus spp. occupy restricted distributions on the southern fringe of the R. conicus range. The life history of R. conicus is summarized. An account of the natural control factors of the eggs, larvae, pupae and adults of R. conicus in Europe and North America and on the effect of R. conicus on C. nutans is given. Field records of a 20‐year survey show that the basic host range of R. conicus is restricted to the closely related Cardueae genera Carduus, Cirsium and Silybum. Within this overall range, C. nutans is the most important host, but many local or regionally distributed populations of R. conicus have developed preferences for species such as Carduus pycnocephalus, Silybum marianum, Cirsium vulgare or C. arvense. The results of various types of screening tests are summarized to demonstrate the specificity of R. conicus. It is shown that the host specificity in the field depends primarily on adult feeding on the host plant, as this determines oogenesis and the oviposition behaviour. Only with access to plants which elicit adult feeding is the female able to produce the protective cap which covers the egg. This is a prerequisite for the successful establishment of the young larva within the flower heads, although it does not necessarily result in establishment on non‐preferred hosts. Zusammenfassung Biologie und Wirtsspezifität von Rhinocyllus conicus (Froel.) (Col., Curculionidae), einem erfolgreich zur biologischen Bekämpfung von Carduus nutans L. eingesetzten Rüsselkäfer Der Rüsselkäfer Rhinocyllus conicus ist in Nordamerika und Neuseeland in vielen Gebieten erfolgreich zur biologischen Bekämpfung von Carduus nutans und verwandten Distelarten eingesetzt worden. Eine Analyse der systematischen Stellung von R. conicus zeigt, daß die Art zu einer Tribus gehört, in der viele Taxa eine enge Bindung an Cardueae‐Wirtspflanzen entwickelt haben. Innerhalb der Gattung Rhinocyllus ist R. conicus, eine primär mit Carduus nutans assoziierte Art, in geographischer Hinsicht eine „zentrale Art”, während die übrigen Rhinocyllus‐Arten kleine Teilareale am südlichen Rand des R. conicus‐Areals besetzen. Es wird ein Überblick über die Biologie von R. conicus und über die Mortalitätsfaktoren der in Europa und Nordamerika an C. nutans lebenden Populationen gegeben. Zwanzigjährige Felduntersuchungen in Europa zeigen, daß das Grundwirtspflanzenspektrum von R. conicus die nahe miteinander verwandten Cardueae‐Gattungen Carduus, Cirsium und Silybum umfaßt. Innerhalb dieses Grundspektrums ist C. nutans die wichtigste Wirtspflanze. Lokal oder regional hat R. conicus aber auch Präferenzen für Cirsium vulgare, C. a...