Effect of trap type and height in monitoring the orange wheat blossom midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae) and its parasitoid, Macroglenes penetrans (Kirby) (Hymenoptera: Pteromalidae)
Introduction
The orange wheat blossom midge, Sitodiplosis mosellana (Géhin) (Diptera: Cecidomyiidae), is a common pest of wheat (Triticum aestivum L.) throughout the northern hemisphere. This univoltine species overwinters in the soil as cocooned larvae and, each spring, a proportion of these larvae pupate, after which the adults emerge and mate at the emergence site (Barnes, 1956; Pivnick and Labbé, 1992). The mated females fly in search of host plants at susceptible growth stages (i.e., from ear emergence until the end of flowering) in order to lay their eggs on the spikes (Ding and Lamb, 1999; Oakley et al., 1998). The eggs hatch a few days later and the larvae feed on the developing kernels, causing damage and shriveled grain (Reeher, 1945). After the feeding period, the larvae drop to the ground, burrow into the soil, spin a cocoon and enter into diapause (Barnes, 1956).
Populations of S. mosellana are commonly parasitized by an ovolarval endoparasitoid, Macroglenes penetrans (Kirby) (Hymenoptera: Pteromalidae), which is an important natural control agent of this pest (Affolter, 1990; Doane et al., 1989). Several studies have shown the beneficial action of M. penetrans to regulate S. mosellana populations (Affolter, 1990; Barnes, 1956). This parasitic wasp emerges at the same time as its host, or a few days later (Affolter, 1990; Chavalle et al., 2015a; Doane and Olfert, 2008; Elliott et al., 2011; Ellis et al., 2009). The female wasp lays an egg inside the egg of its host. Despite the presence of this parasitoid, the midge larva completes its development and overwinters in the soil. In spring, M. penetrans completes its development in the midge larva, consumes its host and emerges as an adult wasp (Affolter, 1990; Doane et al., 1989).
Attacks by S. mosellana can significantly reduce wheat yield (Chavalle et al., 2015b) and the quality of harvested grains (Dexter et al., 1987), but they also facilitate secondary fungal infections (Oakley, 1994). In Europe, important outbreaks causing significant damage have been reported in the United Kingdom (Oakley, 1994; Oakley et al., 2005), Germany (Gaafar et al., 2011), France (Rouillon et al., 2006) and Belgium (Chavalle et al., 2015b). Several studies have estimated the yield loss at about 100 kg/ha for a density of one larva per ear (Kurppa and Husberg, 1989; Oakley et al., 1998; Olfert et al., 1985; Rouillon et al., 2006). In the United Kingdom, crop losses exceeded £30 million in 1993 (Oakley, 1994) and £60 million in 2004 (Oakley et al., 2005). Damage due to this pest is not observed every year because it depends on the coincidence between flights and susceptible growth stages of wheat, but also on weather conditions conducive to adult midge flight and egg laying. The damage level is often underestimated because the adult midges are small and remain hidden in the crop canopy during the day and the larvae are covered by the wheat ear's envelopes (Lamb et al., 2002; Pivnick and Labbé, 1993). The difficulties in detecting S. mosellana complicate decisions on when to apply insecticide treatments against this pest. For farmers, it is necessary to use a reliable monitoring system for assessing the risk to their wheat crops in order to determine the timing of insecticide treatments and to avoid applying useless insecticide treatments (no coincidence between flights and susceptible growth stages of wheat, low infestation level). A reliable monitoring system would also help to preserve the parasitoids that act as biological control agents on the S. mosellana populations.
To detect and monitor S. mosellana flight patterns in order to evaluate the potential risk for wheat crops, a reliable monitoring system must use traps with highest sensitivity. Until the early 2000s, the traps used as warning system to detect S. mosellana and monitor its flights were unbaited traps, such as emergence traps (Affolter, 1990; Elliott et al., 2009; Ellis et al., 2009), yellow water traps (EPPO, 2007; Rouillon et al., 2006) and colored sticky traps (mainly yellow) (Affolter, 1990; Ellis et al., 2009; Knodel and Ganehiarachchi, 2008; Lamb et al., 2002; Oakley and Smart, 2002). In the early 2000s, the identification of the sex pheromone of S. mosellana as (2S, 7S)-nonadiyl dibutyrate (Gries et al., 2000) led to the development and commercialization of a pheromone trap (Bruce et al., 2007; Mircioiu, 2004). This pheromone-baited trap was a precise tool for flight monitoring and has been used in several studies on S. mosellana throughout the world (Ellis et al., 2009; Gaafar and Volkmar, 2010; Jacquemin et al., 2014; Knodel and Ganehiarachchi, 2008; Liatukas et al., 2009; Li et al., 2011). For monitoring M. penetrans, the main parasitoid of S. mosellana, only unbaited traps were available. In wheat fields, emergence traps (Affolter, 1990; Elliott et al., 2011; Ellis et al., 2009) and sticky traps (Affolter, 1990; Oakley and Smart, 2002) were used.
In our study, flights of S. mosellana and M. penetrans were monitored in a winter wheat field in order to compare (i) the capture efficiency of four trap types (yellow sticky, white sticky, yellow water and pheromone-baited traps) at 0.6 m above ground level, (ii) the capture efficiency of pheromone-baited traps and yellow water traps at three heights (0.2, 0.6 or 1 m above ground level) and (iii) the ability of the pheromone-baited trap and yellow water traps to capture males and females at three heights.
Section snippets
Field trapping experiments
The experiments were conducted in 2011 and 2013 at Gembloux (50°34′29″N, 4°44′29″E) in Belgium. Gembloux is in an important cereal-growing region with deep loamy soils. Each year, the traps were placed in a winter wheat field (2.2 ha) cropped with a susceptible variety: Popstart in 2011 and Sahara in 2013 (Jacquemin, 2014). This field was a source field and the infestation level by S. mosellana and the parasitism by M. penetrans were evaluated by extraction of larvae and insects reared from
Flight pattern and trap type
The flight patterns (Fig. 1) and the number of S. mosellana midges caught (Table 1) varied from year to year, depending on trap type and meteorological conditions. The flights occurred 6 weeks earlier in 2011 than in 2013: the first S. mosellana adults were caught on 23 April 2011 and on 5 June 2013 and the average difference of Julian day was 44.6 days (t = −525.46, df = 22241, p < 0.0001). This strong difference in phenology was due to a hotter spring in 2011 as confirmed by the
Discussion
Our study compared the relative efficiency of different trapping systems (trap types and trap heights) species by species and sex by sex for S. mosellana and its parasitoid, M. penetrans. In any one year, the flight patterns obtained with the four trapping systems differed, especially in the case of S. mosellana when pheromone-baited traps were used. As demonstrated by Ellis et al. (2009) and Jacquemin (2014), understanding and interpreting pheromone-baited trap catches depends on the field in
Acknowledgements
We thank the staff at the Walloon Agricultural Research Centre (CRA-W), especially Alain Mahieu and Jessica Denayer, for their technical assistance. We gratefully acknowledge the financial support from the Walloon Region (DGARNE). Censier F. was financially supported by a PhD grant from the Fonds pour la formation à la Recherche dans l’Industrie et l’Agriculture (FRIA, Belgium).
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