ENGLISH ABSTRACT: The banded fruit weevil, Phlyctinus callosus (Schönherr), is a key pest of apples, nectarines and
grapevines in the southern areas of the Western Cape. The control of P. callosus is not satisfactory
and the insecticides used to control this insect have not proved to be effective since the development
of tolerance to pyrethroids and acephate. A control method that can be used, despite it being very
labour-intensive, is that of tree trunk barriers. The use of such a method will prevent the weevils from
reaching the fruit, as they are unable to fly. Alternative control options, such as the use of
entomopathogenic nematodes, are urgently needed for the control of P. callosus.
Entomopathogenic nematodes belonging to the Steinernematidae and Heterorhabditidae are ideal
biocontrol agents for incorporation into an integrated pest management programme. In order to
develop control strategies for P. callosus, large numbers and a predictable quantity of different weevil
stages are needed. Especially large numbers of larvae are needed, as this is the stage that will be
targeted with nematodes. One of the aims of the current study was to assess various artificial diets for
rearing larvae of P. callosus. Though adult weevils were easily collected from orchards, it was very
difficult to obtain large numbers of larvae. Modified versions of an agar diet, as well as different carrot
based 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-grown
carrots in pots, kept at 25°C, resulting in the attainment of the highest percentage survival rate of
90%. A study was undertaken to assess how long, and at what temperature, P. callosus eggs could
be stored. A mean percentage hatch of 45.7% was obtained when eggs were stored at 4°C for 70
days. Eggs started hatching after 47 days and 10 days, when stored at temperatures of 11°C and
14°C, respectively. If the aim of the employment of such a method is only to delay egg hatching, the
two temperatures (11°C and 14°C) will be suitable.
For the following part of the study, several entomopathogenic nematode isolates were evaluated for
their potential use as biological control agents against P. callosus. The susceptibility of P. callosus
larvae and adults to nematode infection was assessed in the laboratory by screening for their
mortality, using different nematode isolates. Larvae were found to be more susceptible to nematode
infection than adults. Heterorhabditis isolates were found to cause higher levels of mortality than the
Steinernema isolates during screening, when a concentration of 400 infective juveniles (IJ) per insect
V
was used. Biological characteristics, such as the effect of different temperatures on nematode activity
and 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 four
days for the larvae, and the SF41 isolate of Heterohabditis zealandica was selected as the most
promising 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. callosus
larvae was also investigated in laboratory bioassays. After four days, the LD50 and LD90 values were
96 IJ/50 μl and 278 IJ/50 μl, respectively. Nematodes were found to be inactive at 11°C, with the
highest mortality rate of P. callosus resulting from nematode infection being recorded at 25°C. A
higher 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. callosus
larvae. The study showed that P. callosus larvae are suitable hosts for H. zealandica, and that the
control 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 in
the last part of the study. The experiment took place in a blueberry orchard, subject to a high rate of
infestation by P. callosus. Concentrations of 0, 20, 30 and 45 IJ/cm2 were topically applied, with
persistence being evaluated for days 1, 35 and 84. Percentage persistence for 30 IJ/cm2 was
calculated as 87.5% for days 35 and 84. The persistence of soil samples taken on day one, and kept
in plastic containers at room temperature, was again evaluated on day 128, with the finding that both
30 IJ/cm2 and 45 IJ/cm2 caused 100% mortality of Tenebrio molitor (L.). Results indicated good
persistence of H. zealandica after 84 days in field conditions, with a high maintenance of P. callosus
populations. The study indicated the potential use of H. zealandica for the control of P. callosus, with
the possibility of persistence for at least three months.
Future research into the control of P. callosus with nematodes should aim to investigate the technical
aspects of field application. The current study shows that entomopathogenic nematodes have
potential 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.
AFRIKAANSE OPSOMMING: Die gebande vrugtekalander, Phlyctinus callosus (Schönherr), is ʼn groot plaag in appel- en
nektarienboorde sowel as wingerde in die suidelike gebiede van die Wes-Kaap. Phlyctinus callosus
word nie voldoende beheer nie, en plaagdoders wat voorheen gebruik is om dié insek in toom te hou,
het doeltreffendheid ingeboet weens weerstandontwikkeling teen piretroϊede en asefaat. ʼn
Alternatiewe beheermetode is stamsperbande. Omdat die kalanders nie kan vlieg nie, moet hulle teen
stamme uitklim om die vrugte te bereik. Stamsperbande versper dus die insekte se toegang tot die
vrugte, maar is baie arbeidsintensief. Meer haalbare metodes vir die beheer van P. callosus is
daarom dringend nodig, en die gebruik van entomopatogeniese nematodes blyk ʼn besliste
moontlikheid te wees.
Entomopatogeniese nematodes, wat tot die Steinernematidae en Heterorhabditidae behoort, is
uitstekende biobeheermiddels vir insluiting by geϊntegreerde plaagbeheerprogramme. Om
doeltreffende beheerstrategieë vir P. callosus te bedink, is groot en voorspelbare hoeveelhede
kalanders nodig veral groot hoeveelhede larwes, aangesien nematodes op hierdie
ontwikkelingstadium gemik sal wees. Die eerste doel met die studie was dus om ʼn kunsmatige dieet
vir die teling van P. callosus larwes te ontwikkel. Volwasse kalanders kon maklik in vrugteboorde
ingesamel word, maar groot hoeveelhede larwes was moeiliker bekombaar. Aangepaste
weergawes van ʼn agardieet sowel as verskillende worteldiëte is by 21°C beproef. Die hoogste
persentasie larwale groei en -oorlewing op die agardieet was 50%, en 60% op een bepaalde soort
worteldieet. Die beste teelmetode blyk egter volgroeide wortels te wees wat in potte geplant is en by
25°C gehou word. Dié metode het ʼn oorlewingspersentasie van 90% opgelewer. ʼn Studie is
onderneem om te bepaal hoe lank en by watter temperature P. callosus eiers vir toekomstige
gebruik geberg kan word. ʼn Gemiddelde uitbroeipersentasie van 45.7% is verkry toe eiers vir 70 dae
by 4°C geberg is. Eiers wat onderskeidelik by 11°C en 14°C geberg is, het ná 47 en 10 dae
onderskeidelik begin uitbroei. Indien die doel is om die eiers slegs stadiger te laat uitbroei, sal
hierdie twee temperature dus geskik wees.
VII
Hierna is verskeie entomopatogeniese nematode-isolate vir moontlike gebruik as biologiese
beheermiddels vir P. callosus beoordeel. Phlyctinus callosus larwes en volwassenes se vatbaarheid
vir nematode infeksie is in die laboratorium bepaal deur dit met behulp van verskillende nematodeisolate
vir mortaliteit te toets. Dié toetse het getoon dat larwes meer vatbaar is vir nematode infeksie
as volwassenes. In die proefnemings het die Heterorhabditis-isolate hoër mortaliteit as die
Steinernema-isolate veroorsaak teen ʼn konsentrasie van 400 infektiewe larwes (IJ) per insek.
Biologiese eienskappe, soos die uitwerking van verskillende temperature op nematode aktiwiteit,
sowel as die minimum konsentrasie nematodes om aanvaarbare vlakke van beheer uit te oefen, is
ondersoek. Die persentasie mortaliteit vir die larwes het ná vier dae tussen 0% en 75% gewissel, en
die SF41-isolaat van Heterohabditis zealandica is as die belowendste isolaat vir die res van die
proefnemings gekies. Die vertikale beweging van nematodes in sand teenoor leemgrond, sowel as
die biologie van H. zealandica in P. callosus larwes, is ook bestudeer. Ná vier dae was die LD50- en
LD90-waardes onderskeidelik 96 en 278 IJ/50 μl. Wat temperatuur betref, is daar bevind dat
nematodes onaktief is by 15°C, terwyl die hoogste mortaliteit van P. callosus larwes as gevolg van
nematode infeksie by 25°C aangeteken is. Die mortaliteit was hoër in die leemgrond (95.2%) as in die
sandgrond (77.5%). Heterorhabditis zealandica kon sy lewensiklus suksesvol in 6de instar P. callosus
larwes voltooi. Die studie het derhalwe getoon dat P. callosus larwes geskikte gashere is vir H.
zealandica, en dat hierdie isolaat dus in die praktyk ʼn doeltreffende beheermiddel vir P. callosus kan
wees.
Die oorlewing van verskillende konsentrasies H. zealandica is ten slotte bestudeer. Die proefneming
is in ʼn bloubessieboord met ʼn groot populasie P. callosus uitgevoer. Konsentrasies van 0, 20, 30 en
45 IJ/cm2 is op die grond (uitwendig) toegedien, en oorlewing is op dag 1, 35 en 84 gemeet. Die
persentasie oorlewing vir die 30 IJ/ cm2 konsentrasie was 87.5% op sowel dag 35 as 84. Oorlewing in
grondmonsters wat op dag een ingesamel en by kamertemperatuur in plastiekhouers geberg is, is
weer op dag 128 beoordeel. Daar is bevind dat sowel die 30 IJ/cm2 as die 45 IJ/cm2 konsentrasie
100% mortaliteit by T. molitor veroorsaak het. Heterorhabditis zealandica blyk ʼn goeie oorlewing te hê
ná 84 dae in veld kondisies wat erg met P. callosus besmet is, en is dus ʼn moontlike beheermiddel vir
P. callosus, met potensiële oorlewing vir minstens drie maande.
