Environmental risks for non-target species
Since 2012, France has had a ministerial order governing the import of exotic macro-organisms for biocontrol purposes. As Cotesia typhae is an exotic species, the application form for authorisation to introduce it into the environment must include, among other information, an environmental risk assessment. Three types of risk must be assessed: the risk of parasitism of other, non-target species; the risk of establishment and the risk of dispersal.
Risk for non-target species:
A list of non-target species has been drawn up on the basis of their biological, genetic and ecological proximity to S. nonagrioides.
This list includes 14 species of lepidopteran stem borers of cattail (Typha), common reed (Phragmites) and other plants in the grass family (Poales) (for example, Figure 1). Of these species, 11 are closely related to S. nonagrioides (Noctuidae) and 3 are more distantly related (Crambidae and Glyphipterigidae). Most of these species are common in France, and can be found in untended drainage ditches and small ponds. None of these moth species are listed as threatened (CITES, https://cites.org/eng/disc/species.php).
Figure 1: Four species of moth on the list of non-target species and their distribution in France. The larvae of Lenisa geminipuncta and Archanara neurica feed mainly on the stems of common reed, while the larvae of Globia sparganii and Nonagria typhae feed on cattails.
Field collections in the fields (2017-2022) made it possible to collect enough larvae of 8 different species to carry out the appropriate experiments. These species cover the ecological diversity of the 14 species listed and therefore constitute a representative sample.
The impact of C. typhae was initially determined in the laboratory using a sequential analysis. The attractiveness of the larvae, their ability to be accepted as a host by C. typhae and their viability for the development of the parasitoid were tested (Figure 2). Secondly, an in planta choice test was carried out with species for which the risk exceeded 5%, as stem borer larvae are protected inside the plant so the real risk may be lower.
Figure 2: Sequential method for testing the acceptability of C. typhae and the suitability of non-target species for its development, and for estimating the risks incurred by non-target species in the presence of the potential biological control agent (adapted from van Lenteren 2003).
Figure 3: Type of test carried out in the laboratory to measure the capacity for acceptance by the parasite, the attractiveness and the suitability of the larvae for the development of the parasite: A) Tests without choice during the direct exposure of non-targeted larvae to C. typhae ; B) Choice tests in an olfactometer, where two odour sources are presented to C. typhae and its olfactory preference is measured; C) Choice tests in planta.
Only four non-target species allowed parasites to develop in laboratory tests: Nonagria typhae, Archanara neurica, Lenisa geminipuncta and Chilo phragmitella. However, the risk of C. typhae developing there (determined by multiplying the rates of acceptance, development and olfactory preference) is on average low (1.7%) compared with that for the target species, S. nonagrioides (48,8%).
Le risque que les chenilles non ciblées meurent du parasitisme, que le parasitoïde se soit développé ou pas, peut être déterminé de la même façon. Ce risque est en moyenne de 5,4% pour les espèces non ciblées, contre 48,8% pour les espèces cibles. The four species (N. typhae, A. Neurica, L. geminipuncta and G. sparganii) with a risk greater than 5% were exposed to C. typhae in planta, and their average risk of mortality was re-estimated at 3.5%.
Nevertheless, we found that during parasitism, some of the genetic material of C. typhaecould be incorporated into the genetic material of S. nonagrioides larvae and certain non-target species (Muller et al. 2022). However, the parasitoid's genetic material was not transmitted to the progeny of the S. nonagrioides individuals that resisted parasitism. However, the risk of horizontal transfer of C. typhae DNA to the offspring of these species appears to be negligible, since the average risk of non-target larvae being parasitised and surviving to form pupae was estimated at 0.17%.
All these results have been published (Fortuna et al., 2024). The risk to non-target species will be further mitigated by the fact that the vast majority of them spend the winter in the egg stage, which C. typhae And the parasitoid has little resistance to winter temperatures, as documented on the next page.
Environmental risks: establishment of risk assessment
The environmental risk posed by the introduction of an exotic species will depend on its ability to establish itself in the long term. As Cotesia typhae is a species described only in East Africa (Kaiser et al. 2015), it is not accustomed to the cold temperatures that can be encountered in winter in France. Consequently, its activity, development and survival at low temperatures could represent the main barrier to its long-term establishment in France. This knowledge was acquired by exposing the different stages of the insect to different ranges of winter temperatures over different periods of time.
In addition, exposure to low temperatures has non-lethal effects on parasitoid fertility. (Bressac et al. 2023)
All the results point to a low probability of winter survival, since temperatures below 15°C result in high mortality at all stages of the insect. Considering that two weeks spent at 10°C or less on average would prevent any survival, it has been possible to calculate the percentage of non-permissive winters by department, between 2000 and 2020:
Regions where the sesamid is a maize pest are shown in dark grey. In some of these départements, the winter survival rate of C. typhae may not be zero and will be estimated following the field releases planned as part of the Biocosma project.
Parasitic efficiency (in greenhouse conditions)
Four greenhouse experiments have been conducted to test the parasitoid efficiency. The first two trials aimed at estimating immediate post-release efficiency depending on the temperature and the parasitoid density. Results showed that the parasitoid was active between 15 and 25°C and the parasitism rate varied with the density of parasitoids released. A third and a forth trial was conducted to evaluate long term efficiency. The third trial showed that one release of the parasitoid at mixed developmental stages allowed a stable parasitism rate of 58% in average along 3,5 months, during which one generation of S. nonagrioides has developed. An improved protocol used for the fourth trial resulted in an almost full suppression of the pest by C. typhae over the same duration (1), and significant improve of plant health (2). Long lasting activity of C. typhae following a single release is very encouraging (Fortuna et al. Phloème 2022).
Methodology of greenhouse assays
Parasitoids production
The purpose is to develop methods to mass rear the potential biological control agent (and its host) with a perspective of industrial production and commercialization. The available laboratory-scale techniques for continuous rearing of S. nonagrioides on artificial diet and its parasitoid C. typhae (Giacometti, 1995; Poitout et al., 1972; Overholt et al., 1994) was used as a starting point. Because the demand will be seasonal in France, the aim is to be able to shift from a baseline of C. typhae production of about 1000 females/week to 50 times more. In fact, 50,000 females would allow to treat fifty hectares of maize based on Cotesia flavipes large scale use on sugarcane in Brazil (Dinardo-Miranda et al., 2014), at a maximum cost of 10-15 €/Ha to meet recommendation after a market analysis done by Bioline AgroScience (France).
Improvements of environmental conditions (optimal temperature and relative humidity) and of parasitism yield as well as better pathogens (pathogens-free rearing colonies) and inbreeding (regular infusion of wild/field specimen) management are under progress at icipe (International Center of Insect Physiology and Ecology) to attain this objective. In addition, studies are also undertaken to develop a protocol for parasitoid's storage, so the production would be sufficient during the peak of orders.
References:
- Dinardo-Miranda et al. 2014 : https://www.scielo.br/j/brag/a/TQmDyGGvz94FQdyHSV6MMvj/?lang=en
- Giacometti R 1995. Rearing of Sesamia nonagrioides Lefebvre on a meridic diet (Lepidopdera Noctuidae). Redia LXXVIII (1): 19-27.
- Overholt WA, Ochieng JO, Lammers, P, Ogedah K 1994. Rearing and field release methods for Cotesia flavipes Cameron (Hymenoptera: Braconidae), a parasitoid of tropical Gramineous stem borers. International Journal of Tropical Insect Science 15: 253–259.
- Poitou HS, Le Rumeur C, Buès R 1972. Élevage sur milieu artificiel simple de deux Noctuelles parasites du Coton, Earias Insulana et Spodoptera littoralis. Entomologia Experimentalis et Applicata 15: 341-350.
Market analysis
Sesamia nonagrioides is present all around the Mediterranean see, particularly in France, Spain, Portugal, Italy and Greece, where the area affected by this borer is estimated at around 450,000 ha. The most affected country is France, with 300,000 ha concerned (mainly grain maize), mainly in the South West, the Atlantic coast and the Rhone Valley. However, its range is gradually extending towards more northern regions due to climate change.
The pest causes direct loss of yield, which can reach on average 11% by strong pest pressure, as well as loss of quality of the harvest due to attacks at the end of the maize cycle, increasing the risk of development of Fusarium spp., which produce mycotoxins.
Currently, no effective biocontrol solution is authorized to control the 2nd generation of this pest, which is the most damaging. Crops are mainly protected with chemical insecticides based on pyrethroids or anthranilamide. The current cost of this protection varies between 30 to 45 €/ha depending on the specialties used (product plus application). It represents, at the current price of maize, an expenditure equivalent to approximately 2 quintals of grain. Sesamia biocontrol can therefore be economically viable for medium to high damage levels.
In addition, the beneficial agent C. typhae could be used in conjunction with the release of Trichogramma (micro Hymenoptera) against the European corn borer Ostrinia nubilalis because this pest is often present with the Sesamia. Both species are often called respectively European and Mediterranean corn borer.
