Formation characteristics of long-term memory in Bactrocera dorsalis.

Studies on insects have contributed significantly to a better understanding of learning and memory, which is a necessary cognitive capability for all animals. Although the formation of memory has been studied in some model insects, more evidence is required to clarify the characteristics of memory formation, especially long-term memory (LTM), which is important for reliably storing information. Here, we explored this question by examining Bactrocera dorsalis, an agricultural pest with excellent learning abilities. Using the classical conditioning paradigm of the olfactory proboscis extension reflex (PER), we found that paired conditioning with multiple trials (>3) spaced with an intertrial interval (≥10 min) resulted in stable memory that lasted for at least 3 d. Furthermore, even a single conditioning trial was sufficient for the formation of a 2-d memory. With the injection of protein inhibitors, protein-synthesis-dependent memory was confirmed to start 4 h after training, and its dependence on translation and transcription differed. Moreover, the results revealed that the dependence of memory on protein translation exhibited a time-window effect (4-6 h). Our findings provide an integrated view of LTM in insects, suggesting common mechanisms in LTM formation that play a key role in the biological basis of memory.

Insecticidal and P450 mediate metabolism of fluralaner against red imported fire ant, Solenopsis invicta (Hymenoptera: Formicidae).

The red imported fire ant (Solenopsis invicta), a worldwide invasive and polyphagous pest, and often nests in residential areas. Finding an alternative pesticide that is both effective on S. invicta and environmentally friendly is urgent and crucial. Fluralaner, a novel isoxazoline insecticide, has been proven to possess selective toxicity for insects versus mammals and has been safe for mammals and non-target organisms, suggesting its potential in pest management. However, little toxicity information is available for the controlment of S. invicta. In this article, we studied the toxicity of fluralaner against S. invicta systematically, and the roles of metabolism-related enzymes in the metabolism process of fluralaner. The toxicity results showed that the topical application and feeding application were all effective for S. invicta. Moreover, fluralaner can be transmitted among workers by contacting and feeding which leads to a toxic reaction among nestmates. By exploring the biochemistry change, we found cytochrome P450 monooxygenase (P450) may be involved in the detoxification of fluralaner as well as carboxylesterase (CarE), but not glutathione S-transferase (GST). Synergism assays gave solid evidence in which piperonyl butoxide, an activity inhibitor of P450, increased the toxicity of fluralaner to S. invicta. Importantly, with the RNAi treatment, four of S.invicta P450 genes were significantly inhibited and showed more sensitivity to fluralaner at LC50 concentration. Our result indicated that fluralaner could be a potential alternative pesticide in S. invicta control. And CYP9AS16, CYP6AS161, CYP6SQ20, and CYP336A45 genes were closely associated with the metabolism process of fluralaner.

Effects of botanical pesticide itol A against the tobacco cutworm, Spodoptera litura (Fab.).

Itol A, an isoryanodane diterpene derived from Itoa orientalis Hemsl. (Flacourtiaceae), is a potential plant-based insecticide. However, the effect of itol A on the tobacco cutworm [Spodoptera litura (Fab.) (Lepidoptera: Noctuidae)], an important and widely distributed insect pest, remains unclear. In this study, the toxicity and inhibitory potency of itol A on S. litura were evaluated. The results indicated that itol A exhibited larvicidal activity against the third instar larvae in a concentration-dependent manner (LC50 875.48 mg/L at 96 h). Antifeedant activity also was observed, and the 24-h AFC50 values were 562.05 and 81.47 mg/L in the no-choice and choice experiments, respectively. The insect growth was inhibited after treatment of itol A, as reflected by long developmental periods, low-quality pupae, and various abnormalities. Itol A exerted ovicidal effect on S. litura, with an estimated LC50 of 759.30 mg/L. Itol A deterred oviposition in the choice experiment (ODI50 909.60 mg/L). Besides, the activities of α-amylase, general protease, superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) were inhibited after itol A treatment over time compared to controls, which may be a relevant mechanism underlying the toxicity of itol A toward S. litura. However, the activities of lipase, carboxylesterase (CarE), glutathione S-transferase (GST), and cytochrome P450 monooxygenase (P450) were increased. Taken together, these results suggest that itol A could be a good botanical pesticide to reduce the population of S. litura in integrated pest management programs.

Interaction of fipronil and the red imported fire ant (Solenopsis invicta): Toxicity differences and detoxification responses.

Fipronil (FIP), a phenyl-pryazole pesticide, has been widely used for crop protection due to its broad insecticidal spectrum, especially for urban insect management. FIP also serves as the active ingredient of major baits used for the control of the red imported fire ant (RIFA; Solenopsis invicta). Although a vast majority of laboratory-based research has been performed using worker ants as a model, limited information is available regarding the toxicity of FIP in individuals from different castes and developmental stages. In this study, we investigated the interaction between FIP and this important pest, including FIP toxicity and transformation, RIFA enzyme activity and responses to FIP exposure. The topical and feeding toxicity of FIP in five adult castes, worker larvae and worker pupae were determined and compared. Topical toxicity assays showed that there were significant differences in FIP toxicity among adult workers (LD50 = 1.17 µg/g), larvae (LD50 = 1891.00 µg/g) and pupae (LD50 = 23981.00 µg/g). Although, no obvious differences in topical toxicity were observed among the adult castes, the differences in feeding toxicity were significant. For example, the LC50 value for the workers was 3.96-fold lower than that for soldiers at 24 h, and the LC50 value was slightly lower for male alates than for female alates at day 3 and day 4, respectively. The activities of detoxification enzymes in individuals of different castes and developmental stages were investigated with or without FIP treatment. Cytochrome P450 activity was approximately 24-fold higher in larvae than in workers, and adult workers exhibited 4-fold higher FIP-induced cytochrome P450 activity than individuals from other adult castes. In addition, in vitro experiments demonstrated that FIP was transformed into FIP-sulfone, and this process may be primarily mediated by RIFA P450(s).

Toxicity and biochemical effects of itol A on the brown planthopper, Nilaparvata lugens (Stål) (Hemiptera: Delphacidae).

Itol A, a novel isoryanodane diterpene derived from Itoa orientalis Hemsl., has potent activities against insect pests. This study was conducted to determine the contact toxicity and biochemical effects of itol A on the Nilaparvata lugens. After macropterous females of N. lugens were exposed to itol A from 0.5 to 24 h, the mortality and poisoning symptoms were measured. Effects of itol A on the major enzymes activity and oxidative stress level were assessed in dose-response (with LD10-LD70 at 24 h) and time-course (with LD50 at 0.5-24 h) experiments for the potential toxicity mechanisms. Based on the results, the mortality of N. lugens showed significant dose- and time-dependent effects, with the 24-h LD50 value was 0.58 µg/insect. The symptoms of excitation, convulsion and paralysis were also observed. However, acetylcholinesterases (AChE) activity was not altered after itol A treatment compared to control. Na+/K+-ATPases, Ca2+-ATPases, Ca2+/Mg2+-ATPases, glutathione S-transferases (GSTs), cytochrome P450 monooxygenases (P450s), superoxide dismutases (SOD) and catalases (CAT) activities were significantly reduced in dose-response and time-course experiments. While acid phosphatases (ACP) and glutathione peroxidases (GPX) activities were significantly increased. We further revealed that itol A exposure resulted in the decrease of GSH/GSSG (reduced to oxidized glutathione) ratio and the increase of hydrogen peroxide (H2O2) and malondialdehyde (MDA) levels in both experiments. The results indicated that the inhibition of Na+/K+-ATPases, Ca2+-ATPases, Ca2+/Mg2+-ATPases, GSTs, P450s, SOD and CAT activities and the induction of oxidative stress was one of the potential biochemical mechanisms of itol A against N. lugens.