Wspólnotowy Serwis Informacyjny Badan i Rozwoju - CORDIS

Impact of natural enemies on WCR (Western Corn Rootworm) populations and sustainable management options

About 33% additional contribution in mortality of WCR (Western Corn Rootworm) populations would have been needed to suppress the maximum realised net reproductive rate (Rm) of WCR in this study to a level of no population growth (R m <1). In the worse case of performing its maximum potential progeny, the potential capacity of the net reproductive rate (R p) could have been suppressed by additional mortality of 68% in the life - table.

Two sustainable control options could be considered, i.e.
- The enhancing of indigenous natural enemies in Europe (Conservation biocontrol) and

- The inundative or inoculative release of exotic biological control agents (Classical biological control).

The first option has to be neglected since this study showed a low effect of indigenous natural enemies on WCR. This is mainly due to:
- The exotic status of WCR in Europe,

- The exotic nature of maize in Europe, and

- To the reduced complexity and diversity in the managed crop system.

None of the life stages of WCR were attacked by effective natural enemies. Very few numbers of adult beetles were infested by Beauveria bassiana or Metharizium anisopliae having an intensity of mortality (kx) close to zero. No predators of WCR became obvious in this study, which could significantly reduce populations. In general predation has to be considered as a less frequent key factor in reducing exotic herbivores than parasitism. Even parasitism by a single parasitoid species has been frequently identified as key factor in life - tables on exotic herbivores, whereas predation by a set of generalist predator species has been more frequently identified as a key factor in native herbivore populations and in natural habitats. Thus, the second sustainable control option by using biological control agents, such as European entomopathogenic nematodes or exotic parasitoids in inundative or inoculative biological control is highly addressed.

Generally, it is difficult to make theoretical generalisations about which stage of WCR might be the most suitable as a target for biological control actions because knowledge about interactions between mortality factors is limited. Two scenarios of action of biological control agents can be simulated. First, the agents would parasites WCR, contemporaneously with other natural mortality factors in each age interval. Secondly, the agents would parasite WCR and cause mortalities independently and in addition to natural mortalities in each age interval. The second case is most desirable, and hypothised biological control agents would have highest impact in reducing progeny when occurring as additional morality to high natural mortality, such as in first instar larvae.

However, since it is not likely that all mortality factors operate independently and additionally, the first scenario is more realistic. Hereby, it s is assumed that control agents would have the same chance in each of the life stage of WCR to cause an additional significant change in the survival and in reducing progeny. For example, a marginal attack rate of 10% parasitism in any age interval would have the potential to additionally reduce the progeny by 10%. Thus, entomopathogenic nematodes as inundative biological control agents would have the potential to reduce the progeny in WCR. In third instar larvae, an average parasitism of 63% by Steinernema arenarium, as shown in semi-natural trials, can potentially reduce 63% of progeny, when contemporaneously calculated in the life - table. It could cause a loss of up to 98 % when acting independently and in addition to natural mortalities. In the same two ways a parasitism of 40.7% third instar larvae by Steinernema feltiae can potentially reduce progeny by 26% and up to 63.6%.

Furthermore, parasitoids of adult WCR are known from its area of origin in Central America, such as the tachinid fly Celatoria compressa. Adult WCR can be parasitised by 27% of C. compressa as an overall mean at different densities, and would cause a direct loss in progeny of up to 27% when acting before the maturation of adults. The potential impact of such a parasitoid on WCR populations is estimated to be high since they can directly reduce the realisation of fecundity in the population, which is the major factor behind population increase in WCR. Since WCR is syn-ovigenic, the parasitoids can even reduce progeny over the whole reproductive period of WCR from July to September. Thus, WCR, such as many herbivorous insects in natural or cultivated habitats seems to be a good subject to different sources of top down control. When considering the above-mentioned need of additional control to prevent population growth, parasitoids may be an important source of population regulation for the WCR, and studies on host parasitoid density dependence are required.

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