Rearing of the banded fruit weevil, Phlyctinus callosus (Schonherr) (Coleoptera: Curculionidae) and control with entomopathogenic nematodes

[摘要] ENGLISH ABSTRACT: The banded fruit weevil, Phlyctinus callosus (Schönherr), is a key pest of apples, nectarines andgrapevines in the southern areas of the Western Cape. The control of P. callosus is not satisfactoryand the insecticides used to control this insect have not proved to be effective since the developmentof tolerance to pyrethroids and acephate. A control method that can be used, despite it being verylabour-intensive, is that of tree trunk barriers. The use of such a method will prevent the weevils fromreaching the fruit, as they are unable to fly. Alternative control options, such as the use ofentomopathogenic nematodes, are urgently needed for the control of P. callosus.Entomopathogenic nematodes belonging to the Steinernematidae and Heterorhabditidae are idealbiocontrol agents for incorporation into an integrated pest management programme. In order todevelop control strategies for P. callosus, large numbers and a predictable quantity of different weevilstages are needed. Especially large numbers of larvae are needed, as this is the stage that will betargeted with nematodes. One of the aims of the current study was to assess various artificial diets forrearing larvae of P. callosus. Though adult weevils were easily collected from orchards, it was verydifficult to obtain large numbers of larvae. Modified versions of an agar diet, as well as different carrotbased diets, were tested at 21°C. The highest percentage survival obtained for the agar diet was 50%and 60% for one type of carrot diet. A better rearing method proved to be that of planting full-growncarrots in pots, kept at 25°C, resulting in the attainment of the highest percentage survival rate of90%. A study was undertaken to assess how long, and at what temperature, P. callosus eggs couldbe stored. A mean percentage hatch of 45.7% was obtained when eggs were stored at 4°C for 70days. Eggs started hatching after 47 days and 10 days, when stored at temperatures of 11°C and14°C, respectively. If the aim of the employment of such a method is only to delay egg hatching, thetwo temperatures (11°C and 14°C) will be suitable.For the following part of the study, several entomopathogenic nematode isolates were evaluated fortheir potential use as biological control agents against P. callosus. The susceptibility of P. callosuslarvae and adults to nematode infection was assessed in the laboratory by screening for theirmortality, using different nematode isolates. Larvae were found to be more susceptible to nematodeinfection than adults. Heterorhabditis isolates were found to cause higher levels of mortality than theSteinernema isolates during screening, when a concentration of 400 infective juveniles (IJ) per insectVwas used. Biological characteristics, such as the effect of different temperatures on nematode activityand the minimum concentration of nematodes needed to obtain acceptable levels of control for P.callosus, were also investigated. The percentage mortality ranged from no infection to 75% after fourdays for the larvae, and the SF41 isolate of Heterohabditis zealandica was selected as the mostpromising isolate for further laboratory experiments. The vertical movement of nematodes in sand,compared with such movement in sandy loam soil, and the biology of H. zealandica in P. callosuslarvae was also investigated in laboratory bioassays. After four days, the LD50 and LD90 values were96 IJ/50 μl and 278 IJ/50 μl, respectively. Nematodes were found to be inactive at 11°C, with thehighest mortality rate of P. callosus resulting from nematode infection being recorded at 25°C. Ahigher percentage mortality rate was obtained with the sandy loam soil (95.2%) than with the sand(77.5%). Heterorhabditis zealandica could successfully complete its life cycle in 6th instar P. callosuslarvae. The study showed that P. callosus larvae are suitable hosts for H. zealandica, and that thecontrol of P. callosus in the field by the selected isolate holds promise.The persistence of the SF41 isolate of H. zealandica at different concentrations was investigated inthe last part of the study. The experiment took place in a blueberry orchard, subject to a high rate ofinfestation by P. callosus. Concentrations of 0, 20, 30 and 45 IJ/cm2 were topically applied, withpersistence being evaluated for days 1, 35 and 84. Percentage persistence for 30 IJ/cm2 wascalculated as 87.5% for days 35 and 84. The persistence of soil samples taken on day one, and keptin plastic containers at room temperature, was again evaluated on day 128, with the finding that both30 IJ/cm2 and 45 IJ/cm2 caused 100% mortality of Tenebrio molitor (L.). Results indicated goodpersistence of H. zealandica after 84 days in field conditions, with a high maintenance of P. callosuspopulations. The study indicated the potential use of H. zealandica for the control of P. callosus, withthe possibility of persistence for at least three months.Future research into the control of P. callosus with nematodes should aim to investigate the technicalaspects of field application. The current study shows that entomopathogenic nematodes havepotential for controlling the soil stages of P. callosus. The capacity to rear large numbers of P.callosus larvae in the laboratory, for later use in laboratory and field trials, is of key importance.

[发布机构] Stellenbosch University