Impact of insect growth regulators on non-target species, with an emphasis on Coccinellids on citrus, in Swaziland.
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Date
1998
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Abstract
This study investigated effects of insect growth regulators (IGRs), recommended for use on
citrus in southern Africa, on non-target organisms, particularly species of Coccinellidae in and
around three citrus estates in the lowveld region of Swaziland, i.e. Tambuti, Tambankulu and
Tunzini estates. Some of the species are important predators of citrus pest insects. The
distribution of coccinellids within an agricultural land mosaic and factors affecting this
distribution pattern were also investigated. Results showed that coccinellid densities and
species diversity were lowest in the natural veld surrounding the orchards. In contrast, citrus
orchards had the highest coccinellid densities and diversity. River borders, along the Great
Usuthu river, had intermediate densities and diversity. While temperature, relative humidity and
ground insolation had no significant effect on coccinellid population densities, the presence /
absence of prey (host plant), on the other hand, was an important factor. This was the case
both for phytophagous and predatory coccinellid species. These showed close patterns of
distribution with their respective host plants or prey species. Economically important species,
such as Rodolia spp., were restricted to the orchards, while other species, especially those
whose economic role is uncertain, such as Cheilomenes lunata and Scymnus spp., were found
in most habitat types.
Coccinellid population densities were assessed during four treatment programmes: 1) an
integrated pest management (IPM) programme where no IGRs were used, 2) a programme
where any of the recommended IGRs were to be applied, 3) conventional pesticide
programme, and 4) an orchard which did not receive any chemically treatment over the last
four years (at Tambankulu estate only). Significant differences between treatment programmes
were observed at all three estates, when only economically important coccinellid species were
assessed. Orchards under an IPM programme (with no IGRs) were found to have significantly
higher population densities compared to those in IGR-treated or under a conventional pesticide
regime. The untreated orchard at Tambankulu, however, had significantly higher coccinellid
densities of economically important species and higher overall population densities than any
of the managed orchards. Of the three estates assessed, Tunzini had significantly higher
population densities of all beneficial coccinellids, (excluding the untreated orchard at Tambankulu). Natural vegetation around Tunzini and Tambuti contributed to the higher
coccinellid densities compared to Tambankulu, which was surrounded by other agricultural
land.
Laboratory and field experiments on two non-target species, the ladybird Chilocorus nigritus
(Fabricius) (all stages) and the moth Bombyx mori (Linnaeus) (larval stage) assessed specific
IGR effects, if any. Three IGRs, buprofezin, teflubenzuron (both chitin synthesis inhibitors) and
pyriproxyfen (a juvenile hormone analogue) were used. All three pesticides are currently
recommended for use on citrus in southern Africa and were tested at the recommended
dosages. Laboratory experiments showed that all three IGRs have a negative impact on both
non-target species. When B. mori received direct applications, there was larval mortality as a
result of the failing to complete moulting or dying immediately after moulting. In addition, no
larvae were able to pupate after having fed on contaminated leaves. In the case of C. nigritus,
larvae fed IGR-treated scale or sprayed with buprofezin suffered significantly higher mortality
than controls, while IGR effects on those sprayed with, or fed scale-treated with, pyriproxyfen
or teflubenzuron were not significant. No adults however, emerged from any pupae in any of
the treatment groups. All three IGRs had ovicidal activity on C.nigritus eggs. Adult fecundity
in both field and laboratory experiments was not affected significantly after exposure to any
of the three IGRs. In field experiments, the proportion of larvae of the moth and ladybird that
developed up to the reproductive adult stage, after exposure to buprofezin, was not
significantly different from the control. This was not the case for pyriproxyfen and
teflubenzuron. Buprofezin was therefore found to be the least detrimental of the three IGRs
tested.
Minimal drift was observed when a knapsack sprayer was used. However, there was spray drift
up to 32m (the furthest distance assessed here) where commercial sprayers were used. This
suggests that pesticide drift from orchards to adjacent areas would have serious implications
for biological diversity in the river borders and rivers adjacent to the estates. The sensitivity
of the non-target species to the IGRs tested needs serious consideration, as it indicates a
broader spectrum of activity for the compounds than what is promoted. Additionally, natural
control may be affected. This is because the timing of IGR applications and increasing coccinellid populations coincide, resulting in a reduced pool of coccinellid predators.
This study emphasised the importance of an appropriately heterogenous landscape to maximise
habitat availability for the coccinellids. Although the economic role of the multihabitat
coccinellid species recorded here is unknown, they nevertheless clearly contribute to citrus pest
control. Such a mosaic landscape, in conjunction with IPM, with no IGR use, promotes
ecological diversity and controls pests with minimum disruption to biodiversity. The use of
IGRs in citrus thus needs carefiil reconsideration in light of the non-target effects observed on
the two species, especially the valuable predatory species, C. nigritus.
Description
Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1998.
Keywords
Citrus--Diseases and pests--Biological control., Ladybirds., Insect pests--Biological control., Aphids--Biological control., Theses--Entomology.