Helmut Zwölfer

The problem of optimal clutch size in a tritrophic system: the oviposition strategy of the thistle gallfly Urophora cardui (Diptera, Tephritidae)

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 females 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.

Niches and size relationships in Coleoptera associated with Cardueae host plants: adaptations to resource gradients

SummaryThe ecological meaning of size relationships within guilds is still a matter of debate. We analyzed the niches and size relationships in Coleoptera associated with Cardueae host plants. Species were grouped into guilds using distributional data, host records and feeding strategies: a) The species of the genus Larinus are inhabitants of flower heads within the Cynaroideae. Two types of Larinus species were distinguished: one type attacks immature flower heads, the other exploits the floer heads in a more advanced stage. The females of the first group have elongated rostres adapted to piercing through the bracts of closed flower heads, the other group possesses blunt, short rostres. For an oligophagous group of four Larinus species we are able to show that the distribution of average female rostre length is non-random. b) In southern France four stem boring species of the genera Agapanthia and Lixus coexist within the same hosts. The frequency distributions of body length from these species are clearly overdispersed. c) Coexisting species of the folivorous genus Cassida show no differences in body sizes. We conclude that morphometric differences within the investigated guild of endophytic species (Larinus and Agapanthia/Lixus) evolved in response to size of the used plant structures and the size of potentially competing species, a pattern not evident in ectophytic species. We suggest that these differences are part of a general pattern as the evolution of herbivorous guild may strongly depend on the way how the host resource is exploited (endophagy vs ectophagy). So current differences in statements on the organization of herbivore communities could perhaps be reconciled.

Plant diversity has contrasting effects on herbivore and parasitoid abundance in Centaurea jacea flower heads

Abstract High biodiversity is known to increase many ecosystem functions, but studies investigating biodiversity effects have more rarely looked at multi‐trophic interactions. We studied a tri‐trophic system composed of Centaurea jacea (brown knapweed), its flower head‐infesting tephritid fruit flies and their hymenopteran parasitoids, in a grassland biodiversity experiment. We aimed to disentangle the importance of direct effects of plant diversity (through changes in apparency and resource availability) from indirect effects (mediated by host plant quality and performance). To do this, we compared insect communities in C. jacea transplants, whose growth was influenced by the surrounding plant communities (and where direct and indirect effects can occur), with potted C. jacea plants, which do not compete with the surrounding plant community (and where only direct effects are possible). Tephritid infestation rate and insect load, mainly of the dominant species Chaetorellia jaceae, decreased with increasing plant species and functional group richness. These effects were not seen in the potted plants and are therefore likely to be mediated by changes in host plant performance and quality. Parasitism rates, mainly of the abundant chalcid wasps Eurytoma compressa and Pteromalus albipennis, increased with plant species or functional group richness in both transplants and potted plants, suggesting that direct effects of plant diversity are most important. The differential effects in transplants and potted plants emphasize the importance of plant‐mediated direct and indirect effects for trophic interactions at the community level. The findings also show how plant–plant interactions critically affect results obtained using transplants. More generally, our results indicate that plant biodiversity affects the abundance of higher trophic levels through a variety of different mechanisms.

Biologische, chemische und biotechnische Schädlingsbekämpfung

Die Vielfalt der Verfahren, die fur die Bekampfung von Insektenarten, die als Krankheitserreger, Krankheitsubertrager oder Pflanzenschadlinge auftreten, zur Verfugung stehen, hat im Verlauf des 20. Jahrhunderts stetig zugenommen. Gleichzeitig ist aber mit der wachsenden Weltbevolkerung und der Intensivierung des Landbaus auch die Bedeutung vieler Schadinsektenarten gestiegen und viele Bekampfungsverfahren haben an Wirksamkeit verloren, bzw. neue Probleme geschaffen. Daher ist die Entwicklung geeigneter Schutz- und Bekampfungsmethoden nach wie vor eine grose Herausforderung fur die angewandte Entomologie. Eine Voraussetzung dafur ist neben taxono-misch-systematischen Grundlagen eine umfassende Kenntnis der Biologie, der Physiologie, des Verhaltens und der Okologie der betreffenden Arten, da sich hier vielfaltige Ansatzpunkte fur die Entwicklung neuer Bekampfungsverfahren ergeben.

Insekten und Pflanzen

Mehr als drei Viertel aller mitteleuropaischen Insektenarten zeigen zumindest in bestimmten Lebensabschnitten Beziehungen zu Pflanzen. Solche Arten wurden lange Zeit einseitig entweder als „Pflanzenfeinde“, oder, soweit sie Blutenpflanzen bestauben, als „Nutzlinge“ betrachtet. Erst seit etwa 25 Jahren ist das Thema „Insekt und Pflanze“ auch von der Physiologie, Evolutionsbiologie und Okologie intensiv aufgegriffen worden. Denn die zwischen rund 500000 beschriebenen Insektenarten und den uber 300000 Gefaspflanzen sich abspielenden Interaktionen bilden einen betrachtlichen Teil der biologischen Vielfalt und bieten unerschopfliche Forschungsmoglichkeiten. Als Beispiele seien hier die Bucher von Ahmad (1983), Bernays und Chapman (1994), Howe und Westley (1993), Price et al. (1991) oder Williams (1994) sowie die lange Reihe der Verhandlungsbande der Internationalen Symposien on Insect-Plant Relationships erwahnt.

Regulation der Populationsdichte

Da die Populationen aller Organismen einerseits ein unbegrenztes Wachstumspotential besitzen, andererseits aber in einer raumlich begrenzten Umwelt leben, ist die Frage, welche Mechanismen das jeweilige Populationswachstum regeln und welches Dichteniveau eingehalten wird, ein zentrales Thema der Populationsokologie. Die ersten Untersuchungen, die dieses Thema aufgriffen, kamen fast ausschlieslich aus dem Bereich der angewandten Entomologie: Plotzlich auftretende Massenvermehrungen von Landwirtschafts- und Forstschadlingen verlangten nicht nur Bekampfungsmasnahmen, sondern gaben auch den Anstos, die einem solchen „Massenwechsel“ von Insektenpopulationen zugrunde liegenden Faktoren zu analysieren. Seit dem Beginn dieses Jahrhunderts trugen auch die im Rahmen von biologischen Schadlingsbekampfungs-Projekten gemachten Erfahrungen zu einer intensiven und oft kontroversen Diskussion der Mechanismen der Dichteregulation bei.