Author : Kyaw Min Htut

Summary : The suitability of different plants, namely banana, mangosteen and taro as host of
Aspidiotus rigidus (Reyne) was quantified. This was done by comparing demographic
parameters such as development rate, survivorship and reproductive potential of A.
rigidus on coconut to those observed on test plants. The study insect was collected from
an outbreak area in Basilan, Mindanao and the experiment was carried out in U.P. Los
Baños under laboratory condition with the temperature ranging from 25°C to 34°C.

This study showed that the development time of immature stages, adult longevity,
survival, growth and fecundity of the scale insect were significantly different among the
host plants tested. Aspidiotus rigidus completed its life cycle on the leaves of all test
plants except on taro. Total immature development time of both male and female scale
insects on banana and mangosteen were significantly longer as compared on coconut;
34.77 ± 4.32 and 32.18 ± 4.10 days on banana, 39.35 ± 6.30 and 39.20 ± 7.82 days on
mangosteen while only 29.03 ± 2.65 and 28.46 ± 3.69 days long on coconut for male and
female, respectively.

Settling rate of the crawlers was also considerably lower on banana and
mangosteen, i.e., only 57.46% and 56.62% on banana and mangosteen while 87.18% of
settling was observed on coconut. Regardless of the host plant, the lowest survival rate
was observed in the first instar nymph. The highest survivorship was always observed on
coconut, followed by banana and mangosteen and the lowest was on taro. Summing up,
56% of original cohort survived on coconut, 33% on banana and only 10% on
mangosteen.

The data showed that A. rigidus female on coconut had the earliest age of
reproductive activity (16.23 ± 1.63 days), followed by banana (17.34 ± 2.28 days) and the
latest activity was observed on mangosteen (20.56 ± 5.00 days). Significant variability
was also found on total fecundity among the hosts where 27.86 ± 6.39 eggs laid on
banana, 17.11 ± 9.12 eggs on mangosteen while 45.83 ± 9.01 eggs were laid on coconut.

Life table estimates of the population parameters of A. rigid us based on the
laboratory data ranked the various host plants as follow: coconut> banana> mangosteen. The highest rm (0.07) was observed on coconut, which is almost 1.7 and 7 times larger
than on banana (0.04) and mangosteen (0.01), respectively. Even though all the
population parameters collectively indicated that A. rigidus on banana would also lead to
a potential to increase in its population, the nature of the leaf morphology as well as the behaviour of crawler show that the outbreak of this insect is not guaranteed in banana.

Leaf morphological characteristics were significantly different among the host
plants tested; stomata density and vascular bundle distribution of the leaf were directly
correlated with the population growth of A. rigid us on its respective host plant. However,
the variation in the nutrient contents of each host plant did not directly support the
population growth of A. rigid us on its respective host plant.

Shorter pre-reproductive period and increased reproductive capacity of an insect
on a host reflect the suitability of the plant which could bring A. rigidus into outbreak
situation. This is confirmed by the current study although secondary metabolites of the
test plant species were not measured or analyzed in the laboratory, they may also be in
inhibiting the reproductive success of the scale insect hence limiting the performance of
this insect as well.