| Summary: | Thesis (MScAgric)--University of Stellenbosch, 2002. === ENGLISH ABSTRACT: Lucerne is the most important pasture and fodder crop in the winter rainfall area of
South Africa. Various pests are known to cause damage to this crop. The use of the
sweep net for monitoring pests is a cheap, easy and quick technique. If the sweep net
is suitable for the lucerne pests in South Africa, potential pest status can be
determined easily and quickly and the necessary precautionary measures taken to
prevent crop losses. From a managerial point of view, it is also important to know
the composition of the insect community in order to follow practices in which the
number of beneficial insects can be increased and the injurious insects decreased.
Therefore a study was done to quantify the use of the sweep net as a survey
technique for monitoring pests on established lucerne stands. Insect diversity was
also determined to obtain information on the insect families and guilds on lucerne.
The redlegged earth mite, due to its importance as a pest, and the Anystis mite,
important as a predator, were also included.
The sweep net proved to be suitable for the sampling of the main lucerne pests. If a
29 cm diameter sweep net is swiped once per pace for six long paces, twelve
systematically chosen sampling units are recommended for the lucerne earth flea and
aphids. It is not necessary to differentiate amongst the three aphid species, or
between the winged and unwinged aphids. Actual counts should be used instead of
absence-presence data. Instead of counting all the insects in a sample, sub-samples
can be taken. Operational characteristic curves can be used to determine the risk
involved in the decision not to intervene, for example by spraying or grazing.
Recommendations for monitoring and the accuracy of control decisions for the
redlegged earth mite, Sitona weevil and lucerne butterfly can only be made after
threshold values have been determined.
The pea aphid, bluegreen aphid and lucerne earth flea showed peaks in their
population levels during spring. Peak numbers of the spotted alfalfa aphid occurred
during late summer and autumn. The Sitona weevil and lucerne butterfly numbers
reached peak levels during late spring and early summer. For all pests population levels were dramatically reduced after grazing or cutting of the plantings. Therefore,
these cultivation practices provided good control.
The herbivores made up more than 85% of the insect community in lucerne. The
largest herbivorous families, in terms of the number of individuals per family, were
the Aphididae and Sminthuridae. These two families contain the main lucerne pests,
the pea aphid, bluegreen aphid, spotted alfalfa aphid and the lucerne earth flea. The
largest predatory family was the Anystidae, represented by the Anystis mite, the
most important predator of the red legged earth mite and lucerne earth flea. Another
well represented predatory family was the Coccinellidae, containing natural enemies
of the aphids. The dryland plantings had a higher percentage of predators than the
irrigated lucerne.
The most important parasitaids were those in the superfamily Chalcidoidea and in
the family Braconidae. The main detritivores were springtails in the suborder
Arthropleona, insects in the families Mycetophilidae on irrigated lucerne, and
Mycetophagidae on dryland lucerne. The most abundant visitors were in the families
Chironomidae, Drosophilidae and Tephritidae. The dryland plantings had a lower
percentage of visitors than the irrigated plantings.
The number of insect families, as well as the number of individuals per family, was
lower at the dryland plantings than at the irrigated plantings. The vast majority of
insect families found on lucerne were collected during the one-year sampling period.
A lower diversity was found where grazing was more severe, and there was a
negative relationship between diversity and evenness. === AFRIKAANSE OPSOMMING: Lusern is die belangrikste wei- en voergewas 10 die winterreëngebied van Suid-
Afrika. Hierdie gewas word deur 'n verskeidenheid plae aangeval. Die gebruik van
die swaainet vir die monitering van plae is 'n goedkoop, maklike en vinnige tegniek.
lndien die swaainet geskik is vir die betrokke plae in Suid-Afrika, kan potensiële
plaagstatus van die plae dus maklik en vinnig bepaal word en die nodige
voorsorgmaatreëls getref word om verliese te voorkom. Vanuit 'n bestuursoogpunt
is dit ook belangrik om te weet wat die samestelling van die insekgemeenskap is
sodat praktyke gevolg kan word waardeur die getal voordelige insekte verhoog en
nadelige insekte verlaag word. Gevolglik is 'n studie uitgevoer om die gebruik van
die swaainet te kwantifiseer as 'n monsternemingsmetode vir die monitering van plae
op gevestigde lusernstande. Insekdiversiteit is ook bepaal ten einde inligting te
bekom oor die insekfamilies en -gildes op lusern. Die lusernerdvlooi en
swartsandmyt, vanweë hul belang as plae, en die Anystis-roofmyt, vanweë sy belang
as predator, is ook ingesluit.
Die swaainet blyk geskik te wees vir die monitering van die. vernaamste lusernplae.
Wanneer 'n 29 cm deursnee swaainet vir ses lang treë een keer per tree geswaai
word, word 12 sistematies gekose steekproefnemingseenhede vir die lusernerdvlooi
en plantluise aanbeveel. Daar hoef nie onderskeid tussen die plantluisspesies en
tussen gevleuelde en ongevleuelde plantluise getref te word nie. Daar moet gebruik
gemaak word van werklike insektellings en nie van aanwesigheid-afwesigheid data
nie. In plaas van om al die insekte in 'n monster te tel, kan submonsters geneem
word. Operasionele karakteristieke kurwes kan gebruik word om die risiko verbonde
aan die besluit om nie op te tree, deur byvoorbeeld te spuit of bewei nie, te bepaal.
Vir die swartsandmyt, Sitona-snuitkewer en lusernskoenlapper moet drempelwaardes
eers vasgestel word voordat aanbevelings vir monitering en die akkuraatheid van
besluite rakende beheer, gegee kan word.
Vir die ertjieluis, blougroenluis en lusernerdvlooi het die bevolkingsvlakke 'n piek in
die lente bereik. Die gevlekte lusernluis se piekgetalle was hoofsaaklik in die laat
somer en herfs. Die Sitona-snuitkewer en lusernskoenlapper het piekgetalle gehad in die laat lente en vroeë somer. Vir al die plae het bevolkingspieke drasties afgeneem
nadat die aanplantings bewei of gesny is. Hierdie verbouingspraktyke blyk dus goeie
beheer te verskaf.
Die herbivore op lusern het meer as 85% van die insekgemeenskap beslaan. Die
grootste herbivoorfamilies, in terme van aantal individue per familie, was die
Aphididae en Sminthuridae. Hierdie twee families bevat die vernaamste lusernplae,
naamlik die ertjieluis, blougroenluis, gevlekte lusernluis en lusernerdvlooi. Die
grootste predatoriese familie was die Anystidae, wat verteenwoordig is deur die
Anystis-roofmyt. 'n belangrike predator van die swartsandmyt en lusernerdvlooi.
Nog 'n predatoriese familie wat goed verteenwoordig was, was die Coccinellidae,
natuurlike vyande van plantluise. Die droëland aanplantings het 'n hoër persentasie
predatore gehad as die besproeide lusern.
Die belangrikste parasitoïede aanwesig was dié in die superfamilie Chalcidoidea en
familie Braconidae. Die vernaamste detritivore was erdvlooie in die suborde
Arthropleona, insekte in die families Mycetophilidae by besproeide lusern, en
Mycetophagidae by droëland lusern. Die volopste besoekers was lede van die
families Chironomidae, Drosophilidae en Tephritidae. Die droëland aanplantings het
'n laer persentasie besoekers gehad as die besproeide lusern.
Die aantal insekfamilies, asook die aantal individue per familie, was laer by die
droëland aanplantings as by die besproeide aanplantings. Die oorgrote meerderheid
insekfamilies wat op lusern voorkom, is gedurende die een jaar opnameperiode
waargeneem. 'n Laer insekdiversiteit is gevind waar beweiding strawwer was, en
daar was 'n negatiewe verband tussen diversiteit en gelykmatigheid.
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