of single plot netting indicating that the second method could be the most .... an adaptation process and the second coming from outside the net with a ...
Integrated protection of fruit crops IOBC-WPRS Bulletin Vol. 91, 2013 pp. 127-131
Adaptation to exclusion netting of the codling moth (Cydia pomonella L.) in apple orchards Myriam Siegwart, Mylène Pierrot, Jean-François Toubon, Sandrine Maugin, Claire Lavigne INRA, UMR 1115 Plantes et Systèmes de culture Horticoles, Agroparc, 84914 Avignon Cedex 9, France
Abstract: Due to widespread insecticide resistance to granuloviruses, increasing economic losses due to the codling moth (Cydia pomonella L., Tortricidae, Olethreutinae) have been recorded in organic orchards in France. Exclusion netting of the tree canopies, using a net named Alt’Carpo, is used as an alternative to insecticides in southern France. Laboratory and field behavioural tests and observations in a network of commercial orchards have shown that the moths were able to lay eggs through the nets and to escape. It was thus supposed that the efficacy of the nets results from altering the reproduction of the pest mainly by preventing it from flying over the canopy for reproductive encounters (Sauphanor, 2012). Despite the good efficacy of this method in commercial orchards, some cases of failures raised the question of the evolution of the pest towards a modified behaviour enabling reproduction under nets. We investigated this hypothesis by comparing the fecundities and fertilities in confined space of four codling moth populations two being collected in two orchards, covered either by single row or single plot nets, and two being control populations from orchards without nets. Further their fecundities and fertilities were compared to a reference lab strain. All populations were collected as diapausing larvae, and analyzed in the following spring at adult emergence. To assess fertility and fecundity in confined space, individual pairs were confined in small boxes in the laboratory. Populations differed significantly from controls in terms of fecundity in the case of single plot netting, but not in single row netting. This may be the sign of adaptation in the case of single plot netting indicating that the second method could be the most sustainable. Key words: Cydia pomonella, Exclusion netting, adaptation
Introduction Chemicals insecticides have been widely used to prevent pest attack in agriculture since more than 60 years. Even if secondary effects were pointed out since 1962, their popularity has been little affected. This could be explained by the easiness of use and the high efficiency of these practices. However, this long term use of chemicals had detrimental effects on the environment, human health and ecosystems. In France, fruit tree orchards are perfect examples of intensively treated areas. However, alternative control methods with reduced environmental impacts compared to insecticide treatments have been recently developed. In apple tree orchard, one of the key pests is the codling moth (Cydia pomonella L.). To control these insect populations, the two widely implemented alternative methods are microbiological insecticides (Arthurs et al., 2007) and mating disruption (Witzgall et al., 2010). Unfortunately, codling moth was the first pest in which resistance to an entomopathogenic virus lead to a complete loss of effectiveness of commercial virus formulations in the field. Moreover, insecticides are still needed in complement of mating disruption to maintain this pest at low densities.
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Codling moth attacks can also be prevented by using physical barriers such as netting. Exclusion netting efficiency was tested in southern France in 2007 (Severac, 2007). The first trials using these nets called Alt’Carpo demonstrated their strong potential for preventing codling moth attacks (Severac and Romet, 2008; Sevrac, 2010). Due to the lack of nonchemical solutions and spectacular results obtained with this method, excluding netting is used in 1500 to 2000 ha of commercial orchards in 2012 in France (Severac, personal com.) after only 5 years of technical development. Laboratory and field behavioural tests and observations in a network of commercial orchards have shown that the moths were able to lay eggs through the nets and to escape. It was thus supposed that the efficacy of the nets results from altering the reproduction of the pest mainly by preventing it from flying over the canopy for reproductive encounters (Sauphanor, 2012). Despite the good efficacy of this method in commercial orchards, some cases of failures raised the question of the evolution of the pest towards a modified behaviour enabling reproduction under nets. To maintain the sustainability of this method, we aimed to test the adaptive potential of this pest to excluding netting. This study focused on codling moth population’s ability to reproduce in confined space which could lead to an adaptation to the nets.
Material and methods Wild individuals have difficulties to reproduce in confined space. But after few generations they can adapt to laboratory conditions and strains with high fecundity can be maintained in continuous rearing all the year. Here, we compare the fertility and fecundity of wild populations sampled in orchards with or without netting and of a reference laboratory strain. Insects sampling Insects were sampled in orchards protected following two methods i) caging of the whole canopies of trees: rows are separately wrapped up with an Alt’Carpo net (2.2 x 5.4 mm mesh) and ii) caging of the whole plot: rows are wrapped up together. The first method is called single-row and the second single-plot netting. In 2011 a protection failure occurred in a commercial orchard with single-row netting and we collected a codling moth sample of this population. As negative control, we chose a close commercial orchard without netting, but with the same apple variety and similar practices. To test single-plot netting we worked on an experimental orchard composed of 6 rows: 3 rows were wrapped up together and the 3 other were free. We sampled under the net and on the rows without net (negative control for singleplot netting). These four populations were collected as diapausing larvae, and analysed in the following spring at adult emergency. As positive control we used a reference strain well adapted to confined rearing conditions. Method At adult emergence individual pairs were created and placed in small boxes (8 cm diameter and 5 cm height) for their reproduction in controlled conditions (23 °C, 60% humidity, 16:8 D:N). Eggs and larvae were counted weekly, during two weeks.
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Results and discussion There were significant differences in fertility and fecundity between the laboratory strain and all wild populations (df = 1, F = 60.98, p < 0.001 and df = 1, F = 99.73, p < 0.001 respectively) (Figure 1). It confirms our intuitive prediction that long time reared individuals have abilities to reproduce in confined space as compared to wild insects.
Figure 1. Box plot of fecundity and fertility of codling moth populations sampled in orchards wrapped up with two types of exclusion netting (Net) or not (Free) and reference strain (Lab) in confined space. Letters represent different statistically significant groups with Tukey test p < 0.05.
Single-plot netting. Moreover, individuals collected under single-plot netting had a significantly higher fecundity than those collected in the nearby control orchard (Table1) (df = 1, F = 4.57, p = 0.01). Fertility followed the same pattern, but differences between wild populations caught under the net or not were less significant (df = 1, F = 4.56, p = 0.04).
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Single-row netting. Contrary to single-plot netting, individuals captured under the net and in a free area aren’t different neither in fecundity nor in fertility.
Table1. Fecundity and fertility means of codling moth individuals pairs under confined area. a n pairs = number of pairs b Fecundity is expressed in number of eggs/female c Fertility is expressed in number of larvae per female. Insects n pairs a Fecundity mean b Laboratory strain 26 213 Under net 21 131 Single-plot netting negative control 21 66 Under net 29 66 Single-row netting negative control 22 67
Fertility mean c 157 65 27 29 34
These first results on codling moth behavioural adaptation to excluding netting have shown some interesting tendency. In single-plot netting, individuals collected under the net had two types of profiles (bimodal distribution of the population, result not shown). The first profile was equivalent to the lab reference strain (in terms of fecundity and fertility), and the second was close to that of individuals collected outside nets. The nets are not completely impermeable to fly entrance which could explain these two profiles: the first corresponding to an adaptation process and the second coming from outside the net with a completely wild profile. A mix of these two sub-populations could explain the non-significant difference of fertility. Moreover, the high egg mortality reduced drastically the number of observations. On the other hand, it could be interesting to determine, in a next test, the proportion of unfertilized eggs to verify if, under the net, females have developed the capacity to lay more eggs but the fecundation is still disrupted, or if the sexual behaviour has changed after few generations in a confined space. To conclude, our results suggest that an adaptation to single-plot netting can be possible in the future and that single-plot netting seems to be more sustainable.
Acknowledgements We thank ONEMA for the financial support in the framework of the call for proposals of research 2009 for the program "Risks evaluation and reduction of pesticides use", in support of the axis 3 of the Ecophyto plan 2018.
References Arthurs, S., Lacey, L., Miliczky, E. 2007: Evaluation of the codling moth granulovirus and spinosad for codling moth control and impact on non-target species in pear orchards. Biological Control 41: 99-109. Sauphanor, B., Severac, G., Maugin, S., Toubon, J. F., Capowiez, Y. 2012: Exclusion netting may alter reproduction of the codling moth (Cydia pomonella) and prevent associated fruit damage to apple orchards. Entomologia Experimentalis et Applicata. In press.
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Severac, G. 2010: Pomme, poire, etc. et toile, Alt'Carpo se développe: Alt'Carpo a fait ses preuves en vergers contre le carpocapse, intéresse contre d'autres insectes et étend ses filets sur la toile internet. Phytoma 632: 18-20. Severac, G., Romet, L. 2007: Des vergers enveloppés de filets contre le carpocapse. Réussir fruits et légumes 266: 60-61. Severac, G., Romet, L. 2008: Alt'Carpo, une alternative efficace. Phytoma 612: 16-20. Witzgall, P., Kirsch, P., Cork, A. 2010: Sex Pheromones and Their Impact on Pest Management. Journal of Chemical Ecology 36: 80-100.
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