Adjuvants to improve aerial control of the citrus mealybug Planococcus citri (Hemiptera: Pseudococcidae) using entomopathogenic nematodes

Abstract

The citrus mealybug, Planococcus citri, is a highly destructive pest of citrus, occurring only in the aerial parts of plants. Humidity will be one of the key factors to consider when using entomopathogenic nematodes (EPN) as biological control agents. Different adjuvants can be added to suspensions of EPNs, to improve control as a foliar application. An aqueous suspension containing Heterorhabditis zealandica and 0.3% Zeba w significantly increased P. citri mortality by 22% at 80% relative humidity (RH) with a temperature cycle starting at 228C for 14 h and 118C for 11 h. The same polymer formulation was tested for Steinernema yirgalemense and mortality of P. citri increased by 21% at 60% RH and by 27% at 80% RH. The addition of Nu-Film-P w and Zeba w to H. zealandica suspensions did not significantly retard application run-off on citrus leaves. The combination of Nu-Film-P w and Zeba w , however, was able to significantly retard sedimentation, increasing the average number of nematodes deposited on 2-cm 2 leaf discs by 10 nematodes. In an aqueous suspension, nematodes settle rapidly to the bottom, resulting in an uneven distribution of nematodes. Xanthan gum, at a concentration of 0.2%, was highly effective at retarding sedimentation, with 72% of the initial nematode number still in suspension after 1 h. Zeba w , at a concentration of 0.3%, despite not being as effective as Xanthan gum, nevertheless still retarded sedimentation significantly. This is the first report of the potential of Nu-Film-P w and Zeba w to improve EPN performance against P. citri when used above ground in citrus orchards.

Potential of Local Entomopathogenic Nematodes for Control of the Vine Mealybug, Planococcus ficus

“…This was significantly higher than mortality at the lower percentage of RH. The finding concurs with those of previous studies on P. citri (Van Niekerk & Malan, 2013), which tested S. yirgalemense and H. zealandica against the citrus mealybug at different water activity (a w ) levels (roughly equivalent to %RH). Both the current study and the study by Van Niekerk and Malan (2013) clearly illustrate that S. yirgalemense performs best at maximum moisture levels.…”

“…It also showed that exposure to S. yirgalemense for longer than 3 h did not significantly improve mortality. This is comparable to the optimal time-to-mortality rate of S. yirgalemense against P. citri at 120 min (Van Niekerk & Malan, 2013). These results indicate that the method of applying S. yirgalemense to grapevine foliage for the control of P. ficus should aim to preserve optimum conditions for FIGURE 5 Mean percentage (95% confidence interval) mortality for female Planococcus ficus after exposure to Steinernema yirgalemense at different levels of relative humidity.…”

“…Steinernema yirgalemense IJs were found to cause the highest mortality in P. ficus at 25°C, followed by 30°C, with the mealybugs incubated at 15°C showing the lowest overall mortality. This finding is in contrast with those of Van Niekerk and Malan (2013), who observed no significant difference in H. zealandica efficacy when applied to P. citri and incubated at 15°C, 20°C and 25°C. Compared to H. zealandica, it can be inferred that S. yirgalemense is less tolerant of lower temperatures, showing much better performance against P. ficus at 25°C and 30°C than at 15°C.…”

“…It was indicated that the first two to four hours after application were critical for optimal infection of P. citri, with humidity being the main limiting factor preventing IJs from surviving long enough to infect their host. Follow-up research by Van Niekerk and Malan (2013) found that the addition of spreader and sticker adjuvants increased both the pathogenicity of H. zealandica and the deposition of nematodes on leaves. Stokwe and Malan (2016) tested EPNs for use against another South African mealybug pest, the obscure mealybug Pseudococcus viburni Signoret, which is a major pest of pome fruit.…”

Nematode Pathogenesis of Insects and Other Pests

“…The addition of adjuvants to suspensions of EPN, to improve control in a foliar application, was investigated (Van Niekerk & Malan, 2013). An aqueous suspension, containing H. zealandica and 0.3 % Zeba ® , significantly increased P. citri mortality at 80 % relative humidity (RH), with a temperature cycle starting at 22 ı C for 14 h, and continuing at 11 ı C for 11 h. The same polymer formulation was tested for S. yirgalemense, with the mortality of P. citri increasing by 21 % at 60 % RH, and by 27 % at 80 % RH.…”