1. Introduction
Corn (
Zea mays L.) is one of the most important cereal crops globally, yielding higher productivity per unit area compared to other cereals like wheat and barley [
1]. In the Mediterranean region, corn ranks as the second most significant agricultural product; however, its production is threatened by the Mediterranean Corn Borer (MCB),
Sesamia nonagrioides (Lefebvre) (Lepidoptera: Noctuidae) [
2,
3,
4,
5]. This pest exhibits a polyphagous nature, infesting a variety of host plants, including corn, sorghum, millet, rice, sugar cane, grasses, melon, asparagus, palms, and bananas [
6,
7]. Originating from the Mediterranean region, the MCB has spread extensively across Europe, North Africa, and the Middle East due to its adaptability to different climates and broad host range [
2,
8].
Studies have shown that MCB infestations can cause significant yield losses, particularly in late and second crop maize productions, with potential losses reaching up to 100% if not adequately controlled [
4]. Chemical insecticides are often recommended and applied multiple times throughout the growing season to manage this pest; however, even with intensive insecticide use, yield losses can still exceed 30% during severe outbreaks [
9]. Additionally, MCB infestations lead to secondary issues such as fungal infections at feeding sites, further compromising crop quality and safety [
10]. Despite the implementation of various control methods, including the frequent use of chemical insecticides, damage from MCB remains pervasive. However, chemical insecticides, while effective in reducing pest populations, pose significant threats to the environment and human health.
Entomopathogenic nematodes (EPNs) are soil-inhabiting organisms known for their capability to parasitize and eliminate insect pests [
11,
12,
13], making them valuable biocontrol agents against MCB. These nematodes, particularly those from the genera
Steinernema and
Heterorhabditis, display efficient host-seeking behavior through their infective juvenile stage, which actively locates and invades insect hosts [
14,
15]. Upon entering the host, EPNs release symbiotic bacteria (e.g.,
Xenorhabdus spp. for
Steinernema and
Photorhabdus spp. for
Heterorhabditis), which proliferate, produce toxins, and ultimately cause insect death by septicemia within 24–48 h [
16]. EPNs’ effectiveness in targeting soil-dwelling stages of pests, such as the larvae of the MCB, has been well documented [
17,
18,
19,
20]. Their use is particularly advantageous in integrated pest management (IPM) programs due to their minimal impact on non-target organisms, including beneficial arthropods and soil microbiota, and their compatibility with organic farming practices [
21,
22]. Additionally, EPNs’ resilience to various environmental conditions enhances their potential as a sustainable and reliable pest control method [
23].
Recent studies have also highlighted the synergistic potential of combining EPNs with other biocontrol agents or compatible chemical insecticides, which can lead to improved pest management outcomes while reducing the reliance on conventional pesticides [
24,
25]. Such integrated approaches not only improve the efficacy of pest control strategies but also align with the increasing demand for environmentally sustainable agricultural practices [
26,
27]. The integration of entomopathogenic nematodes (EPNs) with chemical insecticides offers a potentially synergistic approach to enhancing pest control efficacy while reducing reliance on conventional pesticides. Numerous studies have investigated the compatibility of EPNs with various chemical agents, including pesticides [
12,
13,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37,
38,
39], fertilizers, and microbial control agents.
The findings from these studies have been mixed, with some indicating negative effects of various pesticides on nematode infectivity and survival, while others have reported synergistic behavior that enhances pest control [
12,
40]. The susceptibility of infective juveniles (IJs) of EPNs to these chemical agents can vary widely depending on several factors, including the species and strain of the nematode, the application method and dose of the pesticide, the timing of application, and environmental conditions [
13]. For instance, it has been demonstrated that certain chemical insecticides could be combined with EPNs without compromising their effectiveness [
24], while some other studies have highlighted the adverse impacts of some pesticides on EPN survival and performance [
13,
28,
38]. These varying outcomes underscore the importance of understanding specific interactions between EPNs and pesticides to optimize their combined use.
Incorporating EPNs into integrated pest management (IPM) programs allows for more sustainable and environmentally friendly pest control strategies. This combined approach can reduce the application rates of chemical insecticides, thereby mitigating environmental contamination risks and minimizing the development of insecticide resistance [
12,
34]. Furthermore, the use of EPNs supports the principles of IPM by promoting biological control methods and reducing the ecological footprint of pest management practices [
26,
27]. By leveraging the synergistic potential of EPNs and chemical insecticides, farmers can achieve effective pest control while supporting sustainable agricultural practices.
This study aims to investigate the compatibility of two EPN species, Steinernema feltiae and Heterorhabditis bacteriophora, with four commonly used insecticides: deltamethrin, flubendiamide, spinetoram, and betacyfluthrin. By assessing the impact of these insecticides on EPN mortality, infectivity, and reproduction, this research seeks to identify combinations that maximize pest control efficacy while minimizing adverse effects on non-target organisms and the environment. The findings will contribute to the development of more effective and sustainable IPM strategies for managing MCB infestations in maize crops.
4. Discussion
This study provides valuable insights into the interactions between entomopathogenic nematodes (EPNs) and various chemical insecticides in managing the MCB. The findings suggest that EPNs can be as effective as chemical insecticides under optimal conditions [
43,
44], and their combined use in integrated pest management (IPM) programs can be efficient in terms of time, effort, and cost [
45]. The results indicate the potential for incorporating EPNs with insecticides in IPM strategies to optimize pest control and reduce environmental impacts.
This study showed that the type of insecticide used significantly influences EPN mortality rates. Among the tested insecticides, deltamethrin showed high compatibility with both EPN species, maintaining high infectivity and reproductive capacity. This observation is in line with recent studies that highlight the lower toxicity of pyrethroids, such as deltamethrin, on EPNs due to their specific action on insect nervous systems [
30,
34,
46]. Deltamethrin, a pyrethroid, acts on sodium channels in nerve cells, leading to pest paralysis and death. Several authors have reported the non-toxic nature of various pyrethroids to EPNs [
24,
30,
47,
48]. Although Head et al. (2000) [
49] noted that pyrethroids strongly influence EPN infectivity but not viability, Mráček (2010) [
50] found minimal impact of pyrethroids on both mortality and infectivity of
Steinernema feltiae,
S. arenarium, and
S. kraussei. Specifically, deltamethrin did not reduce EPN survival [
34,
46]. Conversely, flubendiamide and spinetoram exhibited higher toxicity, particularly after 48 h and at higher doses. The delayed toxicity effect of flubendiamide may be related to EPN resilience to immediate calcium disruption caused by ryanodine receptor modulators, but adverse effects were noted over prolonged exposure. El Roby et al. (2023) [
51] evaluated the compatibility of EPNs
Heterorhabditis bacteriophora (HP88) and
Steinernema carpocapsae (AT4) with lambda-cyhalothrin and flubendiamide against
Spodoptera frugiperda larvae, finding synergistic effects. Spinetoram, a spinosyn targeting nicotinic acetylcholine receptors, causing continuous activation and eventual paralysis, exhibited significantly higher mortality rates at 48 h, corroborating findings by De Nardo and Grewal (2003) [
40] on the adverse impacts of certain insecticides on EPN performance. Betacyfluthrin, another pyrethroid acting on sodium channels, showed variability in toxicity, suggesting different EPN species have varying resistance levels. De Nardo and Grewal (2003) [
40] and Özdemir et al. (2020) [
13] highlighted the synergistic use of imidacloprid with EPNs to enhance pest control efficacy without significantly affecting nematode viability.
The infectivity assays demonstrated that deltamethrin had a minimal impact on nematode infectivity, preserving their ability to infect the host. This aligns with recent studies suggesting pyrethroids do not significantly hinder EPN infectivity [
13,
47,
48,
52]. Flubendiamide and spinetoram significantly reduced infectivity, likely due to their action on nematode physiology and behavior. Conversely, some chlorantraniliprole formulations were reported to have no adverse effects on EPN survival or infectivity [
53]. In [
54], synergistic or additive interactions were found when combining
H. bacteriophora with chlorantraniliprole against
Holotrichia oblita (Faldermann) (Coleoptera: Scarabaeidae) larvae, leading to faster larval mortality than EPNs or insecticides alone. It was observed that there were additive effects when combining flubendiamide with various EPNs for controlling
Helicoverpa armigera [
55]. Özdemir et al. (2021) [
38] reported no adverse effects of chlorantraniliprole on the survival and infectivity of
S. feltiae KV6 Turkish isolate used against
Leptinotarsa decemlineata larvae. However, it was noted that there was high larval mortality when combining flubendiamide with
H. indica [
56]. Incompatibility of some EPNs with certain insecticides was also reported in various studies, suggesting careful selection is needed [
55,
56,
57,
58].
The progeny production capacity varied significantly between
H. bacteriophora and
S. Feltiae, depending on insecticide exposure.
H. bacteriophora showed higher progeny production in the presence of deltamethrin, flubendiamide, and the control group, whereas
S. feltiae exhibited higher progeny production when exposed to spinetoram and flubendiamide. This species-specific response underscores the importance of selecting appropriate EPN species for use with particular insecticides to maximize biocontrol efficacy. Rovesti and Deseö (1990) [
32] and Özdemir et al. (2020) [
13] reported varying effects of insecticides on EPN reproductive capacity, consistent with our results. It was reported that long-term exposure to sub-lethal doses of insecticides could adversely affect EPN reproductive capabilities, aligning with our findings that betacyfluthrin and spinetoram significantly influenced progeny production [
29]. A study showed that sublethal effects on EPN reproduction vary widely depending on nematode species and the specific chemicals used, supporting the need for tailored pest management strategies [
13].
The integration of EPNs with chemical insecticides offers a promising approach to reducing reliance on chemical controls alone, mitigating environmental contamination, and minimizing insecticide resistance development. Previous studies have highlighted the compatibility of EPNs with various agricultural practices and their minimal impact on non-target organisms. The findings of this study support the potential for EPNs to enhance the efficacy of chemical insecticides to control MCB, particularly when used strategically.
The varying responses of EPNs to different insecticides emphasize the need for careful selection and optimization of pest control strategies. For instance, the lower toxicity of deltamethrin and flubendiamide to EPNs suggests they could be effectively combined with EPNs without significantly impacting their survival and reproductive capabilities. Conversely, the higher toxicity of betacyfluthrin necessitates more cautious application to avoid detrimental effects on EPN populations.