Mortality of the leaf-mining moth, Aristaea thalassias (Gaertn.) F. Muell (Lepidoptera: Myrtaceae), a biological control agent of Leptospermum laevigatum
Thembelihle Mlokoti*, Fiona Impson, John Hoffmann and Candice Lyons
Fynbos Forum, Swellendam, 2017 Introduction
Leptospermum laevigatum originates from Australia
It was introduced into South Africa in the 1800’s to stabilise dunes
It poses threat to the fynbos biome in the Western Cape and Eastern Cape
Two biological control agents Dasineura strobila and Aristae thalassias were introduced to supress the weed in the 1980s Aristae thalassias
The leaf – mining moth lays eggs on the adaxial and abaxial surfaces of newly formed leaves at the tip of stems of L. laevigatum
The early instar larvae feed on parenchyma within the leaf, forming serpentine mines which become broader as the larvae grow
Eventually all of the parenchyma is consumed at which stage the hollow leaves become air-filled and puffy and soon after they fall from the plant
Aristae thalassias
Three life stages:
―Mined leaves‖ – mining stage characterized by visible mines
―Puffy leaves‖ - pre-pupal stage
―Rolled leaves‖ - pupal stage Questions
What are the causes of mortality in the different immature life stages of the moth?
Which life stages are most prone to mortality and what are the levels of mortality?
How does mortality vary seasonally? Methodology
Study site- Walshacres farm, near Stanford and Agricultural Research Council, Plant Protection Research Institute (ARC- PPRI)in Stellenbosch
The investigation took place between July 2015 and Feb 2017
Sample collection- mined, puffy and rolled leaves (30 each type) were collected on a weekly basis in the field.
Dead and live pre-pupae and pupae were recorded via means of dissection
For possible causes of mortality, intraspecific competition, presence and absence of parasitoids and mites was inspected
• Percentage mortality of larvae and Results pupae was high, but highest in rolled
100 leaves ± 90
80 70 • There was no difference in mortality Mined(L) 60 discernible between seasons Puffy(L) 50
Pre-pupae(L) S.E.) 40 Pupae • Mortality of larvae in mined leaves 30 showed a slight increase in winter, 20 10 while mortality in rolled leaves was % mortality of ( and pupae of larvae mortality % 0 highest in summer and autumn Summer Autumn Winter Spring
Fig. 1: Mortality of A. thalassias larvae and pupae at different • Mortality attributed to parasitism was seasons and percentage mortality due to parasitism. low: Autumn= 20% and Spring = 10%
100
S.E.) 90 ± 80
70
60 Mined(L) 50 Puffy(L) 40 30 Rolled(L) 20 Pupae 10 0 Summer Autumn Winter Spring
% mortality % of larvae andpupae (
Results 100 100 90 90 80 80 • Percentage mortality of 70 70
60 60 larvae due to intraspecific
50 50
40 40 competition showed a slight
30 30 larvae larvae
competition increase from January to 20 20 10 10 March. 0 0
Jan Feb Mar Apr May June Jul Aug Sep Oct Nov Dec % Mortality dueMortality % intraspecfic to
of containing leaves multipleof
% % Months
Fig. 2: percentage larval mortality due to intraspecific competition in mined leaves
Discussion
The current drought situation, in place since 2015, may be impacting mortality of the moth in the early life
Leaf age is important in the development and survivorship of phytophagous insects, with insect feeding damage concentrated on young foliage
Weather is an important force that limits the length of time that
the food resources are available, affecting insect populations Discussion
Predation is another possible cause for the high rate of mortality of pre- pupae and of the pupal stage. Natural enemies are more likely to limit phytophagous insect populations.
Mortality attributed to parasitism was low, contrary to other studies that parasitism is a major source of mortality in most leaf–mining insects.
Intraspecific competition is another possible cause for high mortality
of leaf-miners. Conclusion
Mortality is high at all stages but the pupal stage is the most prone to mortality
Causes of mortality were not readily determinable but predation, intraspecific competition and desiccation might be contributing factors—especially at the pupal stage Acknowledgements
Agricultural Research Council and University of Cape Town
My supervisors
My colleagues
DEA: NRMP
THANK YOU