Myriam Siegwart

Resistance to bio-insecticides or how to enhance their sustainability: a review

After more than 70 years of chemical pesticide use, modern agriculture is increasingly using biological control products. Resistances to conventional insecticides are wide spread, while those to bio-insecticides have raised less attention, and resistance management is frequently neglected. However, a good knowledge of the limitations of a new technique often provides greater sustainability. In this review, we compile cases of resistance to widely used bio-insecticides and describe the associated resistance mechanisms. This overview shows that all widely used bio-insecticides ultimately select resistant individuals. For example, at least 27 species of insects have been described as resistant to Bacillus thuringiensis toxins. The resistance mechanisms are at least as diverse as those that are involved in resistance to chemical insecticides, some of them being common to bio-insecticides and chemical insecticides. This analysis highlights the specific properties of bio-insecticides that the scientific community should use to provide a better sustainability of these products.

Is the efficacy of biological control against plant diseases likely to be more durable than that of chemical pesticides

The durability of a control method for plant protection is defined as the persistence of its efficacy in space and time. It depends on (i) the selection pressure exerted by it on populations of plant pathogens and (ii) on the capacity of these pathogens to adapt to the control method. Erosion of effectiveness of conventional plant protection methods has been widely studied in the past. For example, apparition of resistance to chemical pesticides in plant pathogens or pests has been extensively documented. The durability of biological control has often been assumed to be higher than that of chemical control. Results concerning pest management in agricultural systems have shown that this assumption may not always be justified. Resistance of various pests to one or several toxins of Bacillus thuringiensis and apparition of resistance of the codling moth Cydia pomonella to the C. pomonella granulovirus have, for example, been described. In contrast with the situation for pests, the durability of biological control of plant diseases has hardly been studied and no scientific reports proving the loss of efficiency of biological control agents against plant pathogens in practice has been published so far. Knowledge concerning the possible erosion of effectiveness of biological control is essential to ensure a durable efficacy of biological control agents on target plant pathogens. This knowledge will result in identifying risk factors that can foster the selection of strains of plant pathogens resistant to biological control agents. It will also result in identifying types of biological control agents with lower risk of efficacy loss, i.e., modes of action of biological control agents that does not favor the selection of resistant isolates in natural populations of plant pathogens. An analysis of the scientific literature was then conducted to assess the potential for plant pathogens to become resistant to biological control agents.

Multiple origins of the sodium channel kdr mutations in codling moth populations.

Resistance to insecticides is one interesting example of a rapid current evolutionary change. DNA variability in the voltage-gated sodium channel gene (trans-membrane segments 5 and 6 in domain II) was investigated in order to estimate resistance evolution to pyrethroid in codling moth populations at the World level. DNA variation among 38 sequences revealed a unique kdr mutation (L1014F) involved in pyrethroid resistance in this gene region, which likely resulted from several convergent substitutions. The analysis of codling moth samples from 52 apple orchards in 19 countries using a simple PCR-RFLP confirmed that this kdr mutation is almost worldwide distributed. The proportions of kdr mutation were negatively correlated with the annual temperatures in the sampled regions. Homozygous kdr genotypes in the French apple orchards showed lower P450 cytochrome oxidase activities than other genotypes. The most plausible interpretation of the geographic distribution of kdr in codling moth populations is that it has both multiple independent origins and a spreading limited by low temperature and negative interaction with the presence of alternative resistance mechanisms to pyrethroid in the populations.

Progressive Adaptation of a CpGV Isolate to Codling Moth Populations Resistant to CpGV-M

The NPP-R1 isolate of CpGV is able to replicate on CpGV-M-resistant codling moths. However, its efficacy is not sufficient to provide acceptable levels of control in natural (orchard) conditions. A laboratory colony derived from resistant codling moths was established, which exhibited a homogeneous genetic background and a resistance level more than 7000 fold. By successive cycles of replication of NPP-R1 in this colony, we observed a progressive increase in efficacy. After 16 cycles (isolate 2016-r16), the efficacy of the virus isolate was equivalent to that of CpGV-M on susceptible insects. This isolate was able to control both CpGV-M-susceptible and CpGV-M-resistant insects with similar efficacy. No reduction in the levels of occlusion body production in susceptible larvae was observed for 2016-r16 compared to CpGV-M.

A single sex-linked dominant gene does not fully explain the codling moth's resistance to granulovirus

BACKGROUND In 2004, resistance to a commercial formulation of the Cydia pomonella granulovirus (CpGV) was identified in a field population of Cydia pomonella from an organic orchard in southern France. The genetic inheritance of this resistance was analysed in the resistant laboratory strain RGV. This strain was obtained using successive crosses between the resistant field population and a susceptible laboratory strain, SV, with selection for CpGV resistance at each generation. RESULTS After eight generations of introgression of the resistant trait into SV, the RGV-8 strain exhibited 7000-fold higher resistance than SV. Mass-crossing experiments showed that resistance to CpGV is strongly dominant, sex dependent and under the control of a single major gene. However, the contribution of other genes is required to explain all of the data obtained in this study. These additional genes do not follow the laws of classical Mendelian transmission. CONCLUSION Transmission of granulovirus resistance in the RGV-8 strain of C. pomonella cannot be fully explained by the effect of a locus located on the Z chromosome. The action of other factors needs to be considered.

Biological Characteristics of Experimental Genotype Mixtures of Cydia Pomonella Granulovirus (CpGV): Ability to Control Susceptible and Resistant Pest Populations

The detection of resistance in codling moth (Cydia pomonella) populations against the Mexican isolate of its granulovirus (CpGV-M), raised questions on the sustainability of the use of this biological insecticide. In resistant host cells, CpGV-M is not able to complete its replication cycle because replication is blocked at an early step. Virus isolates able to overcome this resistance have been characterized—among them, the CpGV-R5 isolate. In mixed infections on resistant insects, both CpGV-M and CpGV-R5 viruses replicate, while CpGV-M alone does not induce mortality. Genetically heterogeneous virus populations, containing 50% of each CpGV-M and CpGV-R5 appear to control resistant host populations as well as CpGV-R5 alone at the same final concentration, even if the concentration of CpGV-R5 is only half in the former. The use of mixed genotype virus preparations instead of genotypically homogeneous populations may constitute a better approach than traditional methods for the development of baculovirus-based biological insecticides.

Biochemical and Molecular Mechanisms Associated With the Resistance of the European Corn Borer (Lepidoptera: Crambidae) to Lambda-Cyhalothrin and First Monitoring Tool

Abstract The European corn borer (Ostrinia nubilalis (Hübner)) is one of the most serious corn pest in Europe where it is controlled with pesticides, in particular, pyrethroids. First control failures with this chemical family occurred on the field in 2008 in the center of France, and the first resistance case was described in 2012. In the present study, we investigate resistance mechanisms involved in seven French populations of O. nubilalis collected in the field. Resistances to deltamethrin and lambda-cyhalothrin were confirmed, with a higher resistance ratio for lambda-cyhalothrin (63.79 compared to 7.67). Resistance to the two active compounds was correlated except for one population, indicating a high probability of cross-resistance. Analyses of the activity of three major families of detoxification enzymes in resistant individuals showed a significant increase of the average MFO activity in males of four populations (activity ratios of 2.76–5.73) and higher GST activity in females of two other populations (activity ratios 4.48 and 5.21). Molecular investigation of the sodium channel gene sequence showed the presence of the kdr mutation in a highly resistant individual. We designed a PCR-RFLP screening tool to search for this mutation in the field, and we found it in five populations but not in the susceptible one. The resistance of O. nubilalis to pyrethroids in France seems to result from a combination of resistance mechanisms, possibly as a consequence of a selection pressure with an exceptional duration (almost 40 yr old).

Differentiating Oriental Fruit Moth and Codling Moth (Lepidoptera: Tortricidae) Larvae Using Near-Infrared Spectroscopy

ABSTRACT Cydia pomonella (L.) and Cydia molesta (Busck) (Lepidoptera: Tortricidae) are two important lepidopteran pests that may co-occur in apple orchards and are difficult to differentiate in the larval stage. We investigate the possibility of using near-infrared spectroscopy (NIRS) coupled with partial least squares analysis to distinguish the larvae of the two species. We further assess whether wild individuals can be differentiated using laboratory strains of the two species for model calibration. The NIRS spectra of C. molesta and C. pomonella differed most in the wavelengths between 1,142 and 1,338 nm. Using these wavelengths, partial least squares analysis allowed the differentiation of C. molesta and C. pomonella at the larval stage with very low error, but only as long as both the calibration and prediction sets for individuals had the same origin (either both from the laboratory or both from the field). Errors that appeared when using laboratory individuals for calibration were owing to the divergence of the C. pomonella laboratory strain, most likely following evolution during rearing. Thus, NIRS appears to be a promising tool for the easy and rapid identification of individuals in the field, provided that it is calibrated based on a subset of field individuals.