The entomopathogenic fungal infection of target hosts depends on pathogen fitness and host defence mechanisms. Pathogen-host interactions in red palm weevil, Rhynchophorus ferrugineus larvae are poorly understood. In order to explore this interaction, 4th, 8th, and 12th instar red palm weevil larvae were immersed in conidial suspensions of four isolates of Beauveria bassiana. Significant differences in the virulence of the tested isolates were revealed by LT 50 values. Conidia of B8463, a highly virulent isolate, showed 33.53 % higher relative hydrophobicity and twice as much Pr1 activity than conidia of B8465. Growth indices, calculated after 72 h of incubation, revealed significant differences in the food utilization efficiencies of all studied larval instars infected with different isolates. Conidial infection with B8463 caused 39-45 % reduction in efficacy of consumption of ingested food (ECI) and 55-61 % reduction in digested food (ECD). The least virulent isolate, B8465, caused the smallest reduction in ECI (2-4 %) and ECD (3-9 %). Furthermore, enhanced expression of target antioxidant genes (catalases and peroxidase) was observed in larvae infected with virulent isolates. Similarly, approximate digestibility showed an opposite trend, with the highest values being recorded from samples infected with the most virulent conidia (B8463) at each studied larval instar. In conclusion, isolate B8463 significantly affected the growth and development of red palm weevil larvae and has good potential for use in eco-friendly R. ferrugineus management.

This study aimed to explore the larvicidal and growth-inhibiting activities, and underlying detoxification mechanism of red palm weevil against phenylpropanoids, an important class of plant secondary metabolites. Toxicity of α-asarone, eugenol, isoeugenol, methyl eugenol, methyl isoeugenol, coumarin, coumarin 6, coniferyl aldehyde, diniconazole, ethyl cinnamate, and rosmarinic acid was evaluated by incorporation into the artificial diet. All of the phenylpropanoids exhibited dose- and time-dependent insecticidal activity. Among all the tested phenylpropanoids, coumarin exhibited the highest toxicity by revealing the least LD50 value (0.672 g/L). In addition, the most toxic compound (coumarin) observed in the current study, deteriorated the growth resulting tremendous reduction (78.39%) in efficacy of conversion of digested food (ECD), and (ECI) efficacy of conversion of ingested food (70.04%) of tenth-instar red palm weevil larvae. The energy-deficient red palm weevil larvae through their intrinsic abilities showed enhanced response to their digestibility resulting 27.78% increase in approximate digestibility (AD) compared to control larvae. The detoxification response of Rhynchophorus ferrugineus larvae determined by the quantitative expression of cytochrome P450, esterases, and glutathione S-transferase revealed enhanced expression among moderately toxic and ineffective compounds. These genes especially cytochrome P450 and GST detoxify the target compounds by enhancing their solubility that leads rapid excretion and degradation resulting low toxicity towards red palm weevil larvae. On the other hand, the most toxic (coumarin) silenced the genes involved in the red palm weevil detoxification mechanism. Based on the toxicity, growth retarding, and masking detoxification activities, coumarin could be a useful future natural red palm weevil-controlling agent.

Insects infected with entomopathogenic fungi, experience physiological changes that influence their growth and immune defence. The potential of nine isolates of entomopathogenic fungi was evaluated after determining percent germination and relative conidial hydrophobicity. However, nutritional indices were evaluated after immersing eighth-instar Rhynchophorus ferrugineus larvae into each isolate suspension (1 × 10 7 conidia/mL). The results showed that isolates B6884 and M9374 had 44.51% and 39.02% higher conidial hydrophobicity compared with isolate I03011 (least virulent). The results of nutritional index assays revealed a significant reduction in growth indices after infection with different isolates. Compared with control, B6884 and M9374 greatly decreased larval growth by reducing the efficacy of conversion of ingested food (36%-47%) and Efficacy of conversion of digested food (50%-63%). Furthermore, only isolate B6884 induced 100% mortality within 12 days. Compared with control, isolate I03011, possessing the lowest conidial hydrophobicity, only reduced 0.29% of the efficacy of conversion of ingested food (ECI) and 0.48% of the efficacy of conversion of digested food (ECD). Similarly, transcriptomic analysis of genes related to the Red palm weevil (RPW) immune response, including pathogen recognition receptors (C-type lectin and endo-beta-1,4-glucanse), signal modulator (Serine protease-like protein), signal transductors (Calmodulin-like protein and EF-hand domain containing protein) and effectors (C-type lysozyme, Cathepsin L., Defensin-like protein, Serine carboxypeptidase, and Thaumatin-like protein), was significantly increased in larval samples infected with B6884 and M9374. These results suggest that for an isolate to be virulent, conidial hydrophobicity and germination should also be considered during pathogen selection, as these factors could significantly impact host growth and immune defence mechanisms.

Crop damage and consequent loss in productivity associated with arthropod pests represent the most serious threats to the world economy. Different methods have been used in the past to control arthropod pests; however, the increase in incidences of resistance to pesticides, food safety concerns, and concerns related to environmental impact associated with the use of chemical pesticides have led to the recent expansion of biological control agents. Entomopathogenic fungi are important and promising bio-control agents for controlling arthropod pests. This review compiles recent information regarding the potential of entomopathogenic fungi used against arthropod pests, traits responsible for mycoinsecticide virulence, and possible ways to enhance the virulence of entomopathogenic fungi, in order to pave the way for the development of bio-rational and eco-friendly arthropod pest management strategies.

To cite this paper: Al-Ayedh, H., A. Hussain, M. Rizwan-ul-Haq and A.M. Al-Jabr, 2016. Status of insecticide resistance in field-collected populations of Rhynchophorus ferrugineus (Olivier) (Coleoptera: Curculionidae AbstractThe first monitoring of the resistance to two organophosphates (methidathion and ethion) and one pyrethroid (cypermethrin) insecticides presented by three field collections of different Rhynchophorus ferrugineus (Olivier) populations was performed in this study. The LD50 doses of three insecticides against newly moulted eighth-instar R. ferrugineus larvae were estimated through toxicity bioassays, in which the larvae were fed on artificial diet supplemented with different individual doses of cypermethrin, methidathion or ethion. A population of R. ferrugineus collected from Wadi Ad-Dawasir was chosen as the reference strain, because of its susceptibility to each of the tested insecticide. Compared with the susceptible strain, populations sampled from Al-Ahsa and Al-Qatif showed 8.72 and 4.51-folds increase in resistance to cypermethrin and 3.77 and 2.85-fold increases in resistance to ethion, respectively. The LD50 values of cypermethrin (1.62 ppm), methidathion (3.15 ppm) and ethion (9.12 ppm) in the susceptible population (Wadi Ad-Dawasir) were used to investigate the feeding performances and physiological impacts on eighth-instar R. ferrugineus larvae. The larvae showed no resistance to the most potent insecticide (methidathion) resulting highest growth reduction. However, moderate resistance was observed from red palm weevil population collected from Al-Qatif and Al-Ahsa against cypermethrin resulting lowest reduction in the efficacy of conversion of ingested food (ECI) and digested food (ECD). In addition, glutathione-S-transferase (GST) activity assay further strengthen our findings. Results suggest that enhanced GST activity likely contributes to cypermethrin and ethion resistance exhibited by the R. ferrugineus populations sampled from Al-Qatif and Al-Ahsa. Toxicity bioassays, dietary utilization experiments and GST assays examined in this study support the use of methidathion against R. ferrugineus as the most potent insecticide against all the sampled populations. However, cypermethrin and ethion resistance appears to be evolving in the R. ferrugineus populations from Al-Qatif and Al-Ahsa, justifying the concern of farmers regarding the low efficacy of insecticides.

Current study reveals the likelihood to use pathogen and toxin mutually as an effective and eco-friendly strategy for Oligonychus afrasiaticus (McGregor) management, which could reduce toxicant dose and host killing time. Therefore, phytol and Beauveria bassiana in different proportions were evaluated to determine their effectiveness. Prior to ascertaining host mortality and defense mechanisms, we have recorded in vitro action of phytol using different concentrations (0.70, 1.40, 2.10, 2.80, and 3.50 mg/mL) against B. bassiana suspension. In vitro compatibility assays revealed that growth parameters (vegetative growth, sporulation, and viability) of B. bassiana were least affected by the action of phytol at all tested concentrations. Biological Index of B. bassiana exhibited compatibility with phytol allowed us to conduct Joint toxicity bioassays in which phytol and spores mixed in different proportions in order to attain maximum treatment effect in terms of high mortality at low concentration under short time. Results revealed that joint-application exhibited both synergistic (treatments with higher proportions of phytol), and antagonistic interaction (treatments with higher proportions of spores) interactions. Biochemical mechanisms involved in host antioxidant and detoxification response were explored by quantifying their respective enzymatic activities. Lethality of different treatments induced different patterns of detoxification enzymes including glutathione S-transferase (GST) and acetylcholinesterase (AchE). Overall, the least potent treatments (20% phytol:80% spores, and 40% phytol:60% spores) established in the current study induced relatively higher GST and AchE activities. On the other hand, the most potent treatment (80% phytol:20% spores) at its maximum concentration exhibited negligible relative GST and AchE activities. Antioxidant enzyme activities of CAT and SOD measured in the current study showed moderate to complex interaction might because of toxin-pathogen remarkable synergy. This study suggested that joint application of phytol with B. bassiana spores have shown tremendous acaricidal potential and found to be promising new strategy for controlling old world date mites.

Natural biopesticide development for invasive populations of red palm weevils is mainly responsible for the destruction of date palms and demands an extensive screening program of plant secondary metabolites. In the current study, the pesticidal potential of sesquiterpenes (C15 H24), an important class of plant secondary metabolites primarily composed of three isoprene units, was evaluated by laboratory toxicity, feeding performance bioassays, and host detoxification gene expression patterns. Dose-mortality response bioassays performed against mid-aged eighth-instar red palm weevil larvae revealed dose-dependent mortality. Only three sesquiterpenes, including Farnesol (LD50 = 6559 ppm) and Farnesyl acetate (LD50 = 7867 ppm), are considered to have significant toxicity, with Picrotoxin (LD50 = 317 ppm) being the most toxic. Furthermore, highly toxic sesquiterpene (Picrotoxin) established in the current study tremendously reduced the feeding performance indices, including the efficacy of conversion of digested food (ECD) (81.74%) and the efficacy of conversion of ingested food (ECI) (73.62%). The least toxic sesquiterpenes, including β-Caryophyllene, (+)-Cedrol, Nerolidol, (+)-Nootkatone, and Parthenolide, observed in the current study failed to impart significant reductions of ECI and ECD indices. Lethality of the least toxic sesquiterpenes was overcome by greatly inducing gene expressions of Glutathione S transferase (GST) and Cytochrome P450. These encouraging results enabled us to suggest Picrotoxin as a promising biopesticide for the control of red palm weevil infestations.

Catalase enzyme is found in most insect body tissues, and thus its dsRNA can cause broad-scale gene knockdown within the insect body, depending upon the application method. Significant larval mortality and growth inhibition after catalase knockdown in R. ferrugineus confirms its insecticidal potency and suggests a bright future for RNAi-based bioinsecticides in pest control. © 2016 Society of Chemical Industry.