Environmental risk assessment
Since 2012, there is a ministerial order in France to regulate the entry of exotic macro-organisms useful for biocontrol. As Cotesia typhae is an exotic species, the form to request the authorisation of introducing this species should include, among other informations, an environmental risk assessment. Three types of risks should be documented: the risk of parasitizing non target species; the risk of establishment and the risk of dispersion.
Risk for non target species:
A list of non target species was established based on their biological, genetic and ecological proximity to S. nonagrioides.
This list includes 14 lepidopteran stemborers species of cattail (Typha), common reed (Phragmites) and other Poales plants (e.g. figure 1). From these species, 11 are closely related to S. nonagrioides (Noctuidae) and 3 are more distantly related (Crambidae and Glyphipterigidae). Most species are common in France, and they can be found in unlined drainage ditches and small ponds.
None of these moth species are threatened (CITES, https://cites.org/eng/disc/species.php).
Figure 1: Four moth species included in the list of non target species and their distribution in France. Larvae of Lenisa geminipuncta and Archanara neurica feed mainly on stems of the common reed, while larvae of Globia sparganii and Nonagria typhae feed on cattail plants.
Field collections (2017-2022) allowed to collect larvae of 8 species in sufficient number to be properly tested. These species were well representing the 14 listed. The impact of C. typhae was determined in the lab first by a sequential analysis that included tests of acceptance and attractiveness of the larvae and their suitability for the parasitoid development (figure 2); second, a choice test in planta was conducted for species for which the risk exceeded 5%, because stemborer larvae are protected inside their plants and the actual risk may be lower.
Figure 2: Sequencial method to test the acceptance and suitability of non target species to the development of C. typhae and to estimate the non target risks of the potential biological control agent (adapted from van Lenteren 2003).
Figure 3: Type of tests conducted in the lab to measure the acceptance and attractiveness of the larvae and their suitability for the parasitoid development : A) Non choice tests during direct exposure of the non target larvae to C. typhae; B) Choice tests in olfactometer, where two odour sources are presented to C. typhae and its olfactory preference is measured; C) Choice tests in planta.
The risk of development of C. typhae on the non target species (determined by the rate of acceptation *development* olfactory preference) was on average quite low (1,7%) compared to that on the target species, S. nonagrioides (48,8%). Only four non target species allowed parasitoid development in the lab: Nonagria typhae, Archanara neurica, Lenisa geminipuncta and Chilo phragmitella.
The risk that non target species die from parasitism by C. typhae, with or without development of the parasitoid, was estimated using the same method. This risk was on average 5,4% for the non target species, versus 48,8% for the target species. The four species (N. typhae, A. neurica, L. geminipuncta and G. sparganii) having a risk higher than 5% were exposed to C. typhae in planta, and their mean risk of mortality was re-estimated at 3,5%.
Nonetheless, we found that upon parasitism, some C. typhae genetic material could be integrated into the chromosomes of the larvae of S. nonagrioides and of some non target species (Muller et al. 2022). The wasp genetic materiel was not, however, transmitted to the offspring of S. nonagrioides that resisted to parasitism. Though it has not been studied yet, the risk of horizontal transfer of DNA from C. typhae to the offspring of non target species seems to be negligible, since the mean risk that non-target larvae may be parasitized and survive to form pupae was estimated to 0,17% .
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.
