Non‐target habitat exploitation by Trichogramma brassicae (Hym. Trichogrammatidae): what are the risks for endemic butterflies?

Abstract

1 Trichogramma brassicae Bezdenko is inundatively released against the European corn borer in Switzerland. Because parasitoids dispersing from the release fields might pose a threat to native butterflies, the searching efficiency of T. brassicae was investigated in nontarget habitats. 2 In field studies, T. brassicae was released at rates of 120 000 females/ha. Parasitism of sentinel Ephestia kuehniella egg clusters was 1.6-3.6% in meadows and 2.0-4.0% in flower strips. The respective figures were 57.6-66.7% and 19.2-46.9% in maize, significantly higher than the parasitism rates in the nontarget habitats. Experiments carried out in small field cages confirmed these results: Again, significantly higher parasitism rates were found in maize compared to meadows and flower strips, and also compared to hedgerows (in sleeve cages). 3 To elucidate potential factors underlying the low searching efficiency in nontarget habitats, the behaviour of individual T. brassicae females was investigated on four meadow plants comparatively to maize and a filter paper control. Mean (AESE) walking speed on maize was 2.2 AE 0.2 mm/s, similar to three of the plants tested and filter paper but significantly higher than on Trifolium pratense (0.85 mm/s). A higher turning rate was found on T. pratense, Viola wittrockiana and Plantago lanceolata, in contrast to the longer leaved maize and Alopecurus pratensis. The number of wasps leaving the plant within the observation period differed significantly between plant species, and was twice as high for T. pratense (and the filter paper control) compared to the other plant species. 4 In a choice experiment carried out in a climate cabinet with all five host plant species in cages, we obtained the highest parasitism rates on maize and the lowest parasitism on T. pratense, thus confirming the behavioural observations. 5 In conclusion, there is evidence for a decreased searching efficiency on plants in nontarget habitats compared to maize. However, the data explain only part of the differences found between parasitism in maize compared to nontarget habitats. Other factors, such as the structural complexity of a habitat, may also play a role. We conclude that the risk for butterfly populations in the tested nontarget habitat due to mass released T. brassicae is low.

Methods Used to Assess Non-target Effects of Invertebrate Biological Control Agents of Arthropod Pests

“…In a field test, Babendreier et al (2003b) exposed sentinel eggs of E. kuehniella and two non-target butterflies (including one rare species) at distances of 2 and 20 m from a maize field where T. brassicae had been released in densities of 200,000 adults/ha. In another experiment, the searching efficiency of T. brassicae in non-target habitats was investigated by placing 72 sentinel egg masses of E. kuehniella in meadows, hedgerows, flower strips and maize (Babendreier et al, 2003e). To rule out dispersal from experimental plots, the same habitats were studied with the aid of cages.…”

Effects of mass releases of Trichogramma brassicae on predatory insects in maize

“…All the predators tested in the present study are also likely to be present during this period and could therefore be potential non-target hosts for T. brassicae. As T. brassicae is searching efficiently in maize (Babendreier et al, 2003c), we hypothesised that the inundative releases of T. brassicae into maize fields may negatively affect populations of other beneficial insects present in maize, which could in turn lead to increasing pest populations and an increasing use of pesticides. Evidence for such adverse effects on pest control due to intraguild competitors is accumulating (e.g., Dinter, 1998;Yasuda & Kimura, 2001), and Rosenheim et al (1993) have also shown that the experimental addition of top predators such as Zelus renardii (Kolenati) (Hemiptera: Reduviidae) or Nabis spp.…”

“…In addition, laboratory and field studies revealed that the detrimental effects on butterfly populations outside of maize may be low due to a low searching efficiency in non-target habitats (Babendreier et al, 2003b,c). However, inside maize fields the T. brassicae females have been shown to search efficiently (Suverkropp, 1997;Babendreier et al, 2003c). Mass releases may therefore have important consequences for populations of other beneficial insects present in maize.…”

Alien Insects and Insect Conservation

“…This principle is perhaps universal, and can be coupled with the reality that parasitoids and predators of insects 'clearly have characteristics that could allow them to impact nontarget organisms' but 'whether those characteristics actually result in nontarget impacts and the relative level and risk posed by those impacts is a question that can only be answered by empirical trials' (Orr et al 2000 , writing on augmentative release of egg parasitoid Trichogramma wasps used extensively to combat pest Lepidoptera). Non-target effects of mass-released parasitoids, such as Trichogramma brassicae , on endemic hosts in habitats adjacent to cropping areas, may occur (Switzerland; Babendreier et al 2003 ). Trichogramma also exemplifi es a category of biological control agents that engender concern because they are reintroduced at intervals -such as every year or every anticipated pest incidence.…”