The present study was undertaken to assess the efficacy of currently registered dilute dust protectants against different Zimbabwean populations of S. zeamais and P. truncatus.

Location

The experiments were conducted in the laboratory at the Department of Biological Sciences, Faculty of Science, University of Zimbabwe.

Source and physical characteristics of maize grains

Clean, healthy dry grains of 30G19 PHB PIONEER HI-BRED maize variety bought from a local farmer in Bindura were used for both rearing of insects and the studies. The maize grains were checked visually for damage and then deep-frozen for two weeks to kill hidden infestations. The maize grains were then kept at an ambient temperature of 27±2°C and 65±5% relative humidity in the laboratory for moisture equilibration for three weeks before use in the experiments. 

Test insects

The initial stock cultures of P. truncatus were originally obtained from naturally-infested maize sampled from farm granaries in Bindura, Murehwa, and Headlands. For S. zeamais starter cultures were from insects sampled from infested maize in Murehwa, Headlands, Zvimba and Masvingo. The laboratory cultures for both P. truncatus and S. zeamais were provided courtesy of Biological Sciences Department, University of Zimbabwe. These strains have been maintained for more than five years on whole maize grain and were used as the standard/susceptible populations in the experiments. Insects from these laboratory strains had not been subjected to any insecticide selection pressure during this period of rearing. All insect cultures were reared in a constant temperature and humidity (CTH) room. Culture jars were clearly labeled with date of culture, insect species and area of origin of the insect species/strain.

Insecticides and grain treatments

The insecticides used for the study were bought from a hardware shop in Harare. The application rates were as per the manufacturers’ recommendations (Table 1). The amounts of grain and insecticide were weighed out using an electronic balance. The insecticides were added to grain in glass jars and thorough admixture was achieved by shaking and tumbling the jar for one minute. The control had untreated grain. 

Determination of the relative susceptibilities of different populations of S. zeamais and P. truncatus to currently registered grain protectants.

Two hundred grammes of maize grains were separately admixed with various insecticides in 350 ml glass jars. The treatments included three replicates for each of the insect population and each of the following treatments: Untreated control, Hurudza^®, Shumba Super Dust^®, Actellic Super Chirindamatura Dust^®, Chikwapuro^®, Ngwena Yedura^® and Actellic Gold Chirindamatura Dust^®. These treatments were left for eight weeks before being infested. Thirty unsexed adult insects were introduced into each jar. Mortality counts were taken at day 7 post-infestation. An insect was considered dead if it did not respond to three probings of a blunt needle. Percentage mortality data were corrected for untreated control mortalities using Abbott’s (1925) formula: 

In the case of P. truncatus, data were analyzed by ANOVA as a factorial design comprising four geographic populations and seven grain protectants. For S. zeamais, data were analyzed as a factorial design of five geographic populations and seven grain protectants. Prior to analysis, % data were transformed by arcsine.

100% mortality was recorded across all the five S. zeamais populations and no significant differences (P> 0.05) in mortality were noted among these different populations. Significant effects of population (F_3,72 = 18.68, P < 0.05), treatments (F_5,72=165.04, P< 0.05)  and population  treatment interaction (F_15,72 = 4.08, P < 0.05) (Table 2) was observed in the case of P. truncatus. Mortality due to Chikwapuro^®, Actellic Gold Chirindamatura Dust^® and Ngwena Yedura^® was highest across all the geographic strains and there were no significant differences among the three products. Actellic Super Chirindamatura Dust^®, Shumba Super Dust^® and Hurudza Grain Dust^® achieved the least mortalities against the Bindura, Headlands and Murehwa LGB populations. However, these three protectants significantly controlled the susceptible laboratory strain achieving above 70% mortality.

These results reveal that S. zeamais was more susceptible to the commercially registered insecticides than P. truncatus. Sitophilus species are known to be significantly more susceptible to organophosphates compounds like pirimiphos-methyl (Kljajić et al., 2014). These insecticides ‘cocktails’ were able to control the maize weevils because of the presence of organophosphates compounds.

Contrary to the above scenario, the beetle P. truncatus are very susceptible to pyrethroids, while less to organophosphates, particularly pirimiphos-methyl (Kljajić et al., 2014; Golob and Hanks, 1990; Richter et al., 1998). Cocktail products which combine an organophosphate and a pyrethroid such as Actellic Super Chirindamatura Dust^® were therefore developed to control both pests (Kljajić et al., 2014; Richter et al., 1998). The present study on susceptibility status of S. zeamais and P. truncatus clearly indicated that the later has become tolerant to some of the registered ‘cocktail’ insecticides in Zimbabwe.

The current results correspond to what Golob and Hanks (1990) observed when they sprayed cobs with insecticides such as permethrin or permethrin in a mixture with pirimiphos-methyl. However, earlier studies in Kenya and Tanzania (Tefera, 2012; Dales and Golob, 1997) saw Actellic Super (a cocktail of  1.6%  pirimiphos-methyl and 0.3% permethrin) being promoted as a chemical which is effective against LGB. Also, at the time pyrethroid-organophosphate “cocktails” were introduced for stored-grain protection, a cocktail containing the same active ingredients for permethrin and pirimiphos-methyl was then able to provide effective control of postharvest insect pests. Nonetheless, synthetic pyrethroids were already known to select for resistance in insect populations very readily (Denholm et al., 1983). In the present study, Actellic Super Chirindamatura Dust®, a cocktail containing the same active ingredients for permethrin and pirimiphos-methyl, could not manage P. truncatus across the populations, except against the ‘susceptible’ laboratory strain. Therefore, development of resistance by P. truncatus is the most probable reason of the observed loss of effectiveness of Actellic Super Chirindamatura Dust® in the current studies.

Studies on loose grain in Togo revealed that the combination of pirimiphos-methyl and deltamethrin in concentrations of 7.5 and 0.25 ppm, respectively, as well as 5 and 0.5 ppm resulted in best protection of bagged grain against P. truncatus for nine months of storage (Richter et al., 1998). In another study carried out by von Berg and Biliwa (2008), it was revealed that deltamethrin (2 ppm) and the mixture of fenvalerate (7.5 ppm) and fenitrothion (37.5 ppm) satisfactorily controlled a wide range of beetles (including P. truncatus) for more than ten months. This is consistent with the results of the present study, where Chikwapuro^® which is  comprised of pirimiphos-methyl (2.5% m/m) and deltamethrin (0.1% m/m) and Ngwena Yedura^® which has pirimiphos-methyl in the same concentration and 0.2% m/m deltamethrin, both managed to contain all populations of S. zeamais and P. truncatus. However, considering that the treatments were just eight weeks old at the time insects were introduced, the current results are not indicative of absence of resistance to Chikwapuro^® and Ngwena Yedura^® by LGB as farmers all over Zimbabwe are currently reporting poor control of the bostrichid by the two protectants as well as Shumba Super^® (Chinwada, personal communication).

Results of the current study may also  indicate  that  P. truncatus is now resistant to Actellic Super Chirindamatura Dust^®. This is supported by very low LGB mortalities in Murehwa, Headlands and Bindura and a relatively high mortality against the susceptible laboratory strain. Actellic Gold Chirindamatura Dust^®, which Syngenta recently registered, was quite effective as it gave 100% mortality across all the populations of S. zeamais and P. truncatus. This level of control was due to the replacement of permethrin by thiamethoxam. This result probably indicates a need to increase application rates of the active ingredients in the organophosphate-pyrethroid cocktails, as suggested by FAO and WHO (1980) to raise the application rates of permethrin above the present 1 mg/kg grain level, though probably not above 2 mg.

From the results of the study, it is imperative that farmers should divert their attention from synthetic chemical insecticides to other control measures, particularly use of local botanicals. In fact, local studies with as many plant species as possible is necessary so as to determine their usefulness in stored-product primary insect pest management, especially at the smallholder level.

Responsible departments and/or ministries, in this case, The Ministry of Agriculture through the Pesticide Registration Office, must be proactive and always conduct regular insecticide susceptibility tests on the major primary and secondary pests associated with stored grain. The information gleaned from such work will make it possible for the Pesticide Registration Office to know in advance which grain protectants needs to be de-registered when companies renew registrations for such products. The current scenario whereby many of the currently registered products available on the local market are clearly no longer efficacious against LGB is worrying as this threatens national food security.

The author has not declared any conflict of interests.

The author is grateful to Dr. P. Chinwada who guided this research with all his expertise, patience and constructive comments. Thanks also go to the Biological Sciences Department staff (especially Mr G. Ashley, Mr S. Ndoma and Ms. B. Chikati) for their assistance during collection of the data.