Synergistic Effects and Toxic Mechanism of Phosphine with Ethyl Formate against Citrus Mealybug (Planococcus citri)

Methyl bromide (MB) has been used in a wide range of applications, but since it was determined to be an ozone-depleting compound, it has only been used for pre-shipment and quarantine purposes in trade. Phosphine (PH3) is currently the ideal fumigant as an MB alternative worldwide. However, the development of PH3 resistance in the target insect pest and longer PH3 fumigation treatment times raise questions about the continued use of PH3. This study attempted to shorten treatment time via combination treatment with ethyl formate (EF). Planococcus citri was used as the main quarantine pest in Korea, and the acute toxicity of EF, PH3, and EF + PH3 was determined at every developmental stage. EF treatment at 4 h showed LCT99 values of 45.85~65.43 mg∙h/L, and PH3 treatment at 20 h showed that of 0.13~0.83 mg∙h/L depending on the developmental stage. The efficacy of PH3 decreased after reducing the treatment time, but synergistic effects were observed at all stages of development of P. citri when both fumigants were used simultaneously for 4 h. After combined treatment, dihydrolipoyl dehydrogenase expression and the production of two phospholipids, PI(O-16:0) and PC(18:2), were significantly reduced in treated P. citri adults compared with the control. Therefore, combined treatments might be key to reducing the treatment time and resistance of PH3 in the field.

Planococcus citri (citrus mealybug).

Planococcus citri is a highly polyphagous, adaptable mealybug that can feed on many host plants in a variety of conditions, and can reproduce rapidly. It has been reported on over 200 host-plant species belonging to 191 genera and 82 families, and can seriously damage many crops, particularly citrus and glasshouse tomatoes. It is known to transmit some plant virus diseases like Cacao swollen shoot virus. The mealybug is of Old World origin, but its polyphagy has facilitated its spread about the world by human transport of infested plants over many years, and it is now established in in all the temperate and tropical zoogeographic regions, and lives under glass in higher latitudes. Its small size and cryptic habits makes it difficult to detect and identify at plant quarantine inspection. The increase in international trade in fresh plant material in recent years is facilitating its continued spread.

Evaluation of RNA Interference for Control of the Grape Mealybug Pseudococcus maritimus (Hemiptera: Pseudococcidae)

The grape mealybug Pseudococcus maritimus (Ehrhorn, 1900) (Hemiptera: Pseudococcidae) is a significant pest of grapevines (Vitis spp.) and a vector of disease-causing grape viruses, linked to its feeding on phloem sap. The management of this pest is constrained by the lack of naturally occurring resistance traits in Vitis. Here, we obtained proof of concept that RNA interference (RNAi) using double-stranded RNA (dsRNA) molecules against essential genes for phloem sap feeding can depress insect survival. The genes of interest code for an aquaporin (AQP) and a sucrase (SUC) that are required for osmoregulation in related phloem sap-feeding hemipteran insects (aphids and whiteflies). In parallel, we investigated the grape mealybug genes coding non-specific nucleases (NUC), which reduce RNAi efficacy by degrading administered dsRNA. Homologs of AQP and SUC with experimentally validated function in aphids, together with NUC, were identified in the published transcriptome of the citrus mealybug Planococcus citri by phylogenetic analysis, and sequences of the candidate genes were obtained for Ps. maritimus by PCR with degenerate primers. Using this first sequence information for Ps. maritimus, dsRNA was prepared and administered to the insects via an artificial diet. The treatment comprising dsRNA against AQP, SUC and NUC significantly increased insect mortality over three days, relative to dsRNA-free controls. The dsRNA constructs for AQP and NUC were predicted, from sequence analysis to have some activity against other mealybugs, but none of the three dsRNA constructs have predicted activity against aphids. This study provides the basis to develop in planta RNAi strategies against Ps. maritimus and other mealybug pests of grapevines.

Toxicity of green silver nanoparticles of plant extracts against citrus mealybug Planococcus citri

Planococcus citri is an economically important polyphagous insect pest and is a serious threat to horticultural crops especially mango and citrus. It is mainly controlled by synthetic insecticides which have deleterious effects on humans, livestock and environment. To dispense with the detrimental consequences of insecticides, plant extracts and their silver nanoparticles can be potential alternatives. The present study reports the effectiveness of four plant extracts and their silver based nanoparticles against P. citri. All the plant extracts and their silver nanoparticles caused mortalities of 2nd and 3rd instars of P. citri at all concentrations and exposure durations. The ginger extracts and their silver nanoparticles caused maximum mortality of 2nd and 3rd instars of P. citri (94 and 90% respectively) at the highest concentration after 72 hours followed by neem extracts and their silver nanoparticles. Similarly, the mortalities caused by garlic silver nanoparticles followed the similar pattern. Eucalyptus plant extracts and their silver nanoparticles were found to be the lease effective. It was also observed that with an increase in concentration and exposure period, the mortality also increased showing a direct relationship. Ginger plant extract and their silver nanoparticles gave the best results and lowest LC50 values by a large margin. Neem extracts ranked second followed by garlic and eucalyptus and gave significantly poor results as compared to the first two plants. The required concentration for killing 50% of the nymphal population was observed to decrease with the passage of time.