Conclusions

Our review of the evidence on R. conicus and its relevance for resolving the controversy over the data available was required to assess the risk of deliberate introductions for the biological control of weeds, such as true thistles, and it leads to three conclusions.

First, in retrospect, the pre-release plus the early post-release data did suggest that R conicus would be likely to feed, and could develop, on multiple Cirsium species, including important clues that North American species would be among these species. However, several factors influencing the programme at the time led the investigators to discount this evidence. These factors were: (i) a widespread acceptance of the host race paradigm; (ii) a focus on preventing damage to economic plants, which entailed selecting test plants for the screening tests which were almost exclusively cultivated plants; and (iii) a strong desire to move quickly to eliminate the weed problem. These factors contributed to a de-emphasis on the data that showed significant feeding, oviposition and development on some European and a couple of North American Cirsium species. A strong emphasis was placed, instead, on why the evidence was not sufficient to predict a major effect by R conicus on native plant species. This latter emphasis precluded further studies, to examine conflicting data and to determine the ecological factors influencing host selection in the field.

Second, again in retrospect, the studies needed to quantify the likely magnitude of feeding and development by R. conicus on Cirsium species, and thus the ecological consequences of that feeding, were not done in this case, nor in any other up to that time as far as we are aware. Thus, a clear prediction of the intensity of direct effects, and of the type and magnitude of indirect ecological effects resulting from the feeding and potential development, was not possible. However, the early data did provide reasons to hypothesize that ecological effects might occur if R conicus did include North American Cirsium species in its list of acceptable, phenologically exploitable host plants. For example, starvation feeding tests showed 'regular and spontaneous feeding' by R conicus on at least two European Cirsium spp. (C. arvense, C. vulgare), and preference feeding tests showed higher acceptance of European Cirsium acaule than of Carduus nutans. Furthermore, larval development was also completed on European Cirsium arvense and C. palustre.

Third, the findings imply the potential for a direct, negative effect by R conicus on fitness and seed production of Cirsium species when exploited. Interactions with other insects in the floral herbivore guild were noted, implying the potential for indirect interactions with insects in the North American inflorescence guild. However, these clues in the observations and data from pre-release and early post-release studies were not pursued. Some of the information needed could have been developed through quantitative field studies of relative egg loads on alternative hosts, under a range of environmental conditions within the geographical range of R. conicus in Europe. Such data would quantify the potential variation in magnitude of use of less preferred, but acceptable, host plant species. Thus, we conclude that more information could have been acquired to assess the ecological consequences and potential control effectiveness, and these data are now required under contemporary conditions.

In summary, we conclude that there were enough data suggesting that Cirsium species were acceptable host species to have aroused more suspicion over the potential consequences of introducing and redistributing R. conicus in North America without further testing. However, accurate prediction of the observed quantitative ecological effects would have required more field and laboratory experiments. Contemporary concerns now mandate such additional, pre-release testing.

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