Ecological factors affecting the establishment of the biological control agent Gargaphia decoris Drake (Hemiptera: Tingidae) : a thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Science at Massey University, Manawatu, New Zealand
The Brazilian lace bug (Gargaphia decoris Drake (Hemiptera:Tingidae)) was released in
New Zealand in 2010 for the biological control of the invasive weed woolly nightshade
(Solanum mauritianum Scopoli (Solanaceae)). Currently there is scarce information about
the potential effect of ecological factors on the establishment of this biological control
agent. This study investigated: 1) the effect of maternal care and aggregation on nymphal
survival and development; 2) the effect of temperature, photoperiod and humidity on G.
decoris performance; and 3) the effect of light intensity on S. mauritianum and G. decoris
Maternal care and aggregation are characteristic behaviours of G. decoris. These
behaviours have an adaptive significance for the offspring and are key determinants for the
survival of the species under natural conditions. Maternal care is reported to increase the
survival and development of offspring under field conditions, and higher aggregations to
increase the survival of the offspring. However, in this study, maternal care negatively
affected the survival and development of the offspring, and higher aggregations had no
significant impact on offspring survival. The availability of host plants under laboratory
conditions may have influenced the expression of these behaviours.
Climate is a factor that constrains insect development and therefore establishment.
In this study, temperature affected the survival, nymphal development, life cycle, adult
longevity, female reproductive success (i.e. total number of eggs, number of eggs laid per
female, number of egg batches, number of eggs per batch, pre-oviposition period, percent
females that oviposited successfully, number of eggs in the first batch and percentage of
eggs that hatched from the first batch) and population growth parameters (i.e. life table).
Temperatures between 20 – 25 °C were the optimal temperatures for G. decoris
establishment. Photoperiod affected the mean percentage of egg hatch (i.e. emergence of
nymphs in egg batch collected from colony) and total nymphal survival (i.e. egg to adult
emergence), adult longevity and population growth parameters. The photoperiod 16L:8D
was the optimal photoperiod for insect establishment. Humidity affected the mean
percentage of egg hatch, adult longevity and population growth parameters. G. decoris
population growth was highest at 70 ± 10% RH but the population growth was faster at 50
The CLIMEX model predicted that G. decoris could occupy broader regions not
only on its native range (i.e. Brazil and Argentina) but also other regions where S.
mauritianum is considered invasive (i.e. New Zealand and South Africa). G. decoris is
predicted to be able to establish optimally in most of New Zealand North Island, except in
regions with altitudes higher than 1300 meters above sea level. Most of the South Island is
considered unsuitable for G. decoris establishment, except parts of the West Coast, Nelson
and the Tasman region, which are predicted to be moderately to marginally suitable.
Light intensity and plant age (i.e. day of harvest) affected host plant quality and had
an indirect impact on insect establishment. Light intensity and plant age affected key
physiological, morphological and defensive traits of S. mauritianum. Three compounds
appeared to be involved, and were positively identified as glycoalkaloids: α-solamargine/β-
solamarine, solauricine/solasonine, and unknown-954. The reproductive performance of G.
decoris was affected because females avoided ovipositing on unshaded plants. The
presence of trichomes and an increase in concentration of glycoalkaloids in the second
harvest affected the nymphal performance and was reflected in adults, which had smaller
bodies and wings.
The results of my study have implications for using the Brazilian lace bug G.
decoris in biological control programmes. The ecological factors included in this study
work synergistically rather than independently and are important to consider when deciding
the best locations in which the insect could be liberated.