Browsing by Author "Hemachandra, K. S."

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Developing a scale to assess the Spiralling Whitefly, Aleurodicus disperses Russel (Hemiptera: Aleyrodidae), infestation on cassava (Manihot esculenta)
(4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Gunawardana, D. U. M.; Warnasooriya, P. G. A. S.; Hemachandra, K. S.
Spiralling whitefly, Aleurodicus dispersus Russel (Aleyrodidae) is a highly polyphagous quarantine pest, spread in tropical and subtropical region of the world. As direct feeders of cell sap whiteflies affect adversely on growth and act as vectors of viruses and spread plant diseases, of many agricultural and non-agricultural plants. This pest can be managed by the means of biological control with the use of coccinellid predators. In this process, it is necessary to make samples and assess the whitefly infestations in relation to many laboratory and field studies. In order to facilitate the assessment of whitefly populations, a study was conducted to develop a scale to measure the whitefly infestation in cassava. Whitefly infected cassava leaves were collected from cassava fields in Dholuwa-Kandy for one cropping season. Leaflets of collected leaves were separated (leaflet length 14.29 ± 0.43 cm, width 3.53 ± 0.21 cm and leaf area 57.86 ± 6.34 cm2 ) (n= 80). Each leaflet was examined separately under the binocular light microscope (10x4) and each growth stages (eggs, instar 1, instar 2, instar 3 and instar 4) of the whitefly were counted based on the four whitefly infestation levels (25%, 50%, 75% and 100%). The variation of eggs appears to have a more or less linear relationship with the area of infestation, but not the other growth stages. When total number of eggs is considered, a significant linear relationship (r2 = 87.1%, F (1, 78) = 524 p < 0.001) with infestation level was found which can be expressed as y =2.58 x +8.33. When total number of instars is considered, a significant linear relationship (r2 = 95.1%, F (1, 78) = 1521 p < 0.001) with infestation level was found which can be expressed as y =3.332 x +2.275. Eggs represented 80%, out of the total number of counted individuals. Significant relationships were observed between the area of infestation and instar 1, instar 2 and instar 3 but not with instar 4. Having considered these relationships, number of whitefly stages or the egg numbers can be predicted by examining the area of the infestation of the leaf, in this case cassava. There is a possibility, this pattern may be true for other crops but exact relationship is needed to be established, with a separate study
Evaluation of leaf quality on food search behavior of Cryptolaemus montrouzieri larvae (Coleoptera: Coccinellidae), a predatory beetle of mealybugs
(4th International Research Symposium on Pure and Applied Sciences, Faculty of Science, University of Kelaniya, Sri Lanka, 2019) Wijekoon, W. M. S.; Warnasooriya, P. G. A. S.; Hemachandra, K. S.
Several groups of insect pests such as mealybugs, whiteflies and scale insects are successfully controlled by the predatory beetle mealybug destroyer, Cryptolaemus montrouzieri (Coleoptera: Coccinellidae). Both adult and larvae of C. montrouzieri feed on all the stages of mealybugs. Food searching behaviour is a key aspect for a predator which is influenced by the morphological features of the leaves such as trichome density, waxy nature of leaf lamina. Hence, the objective of the study was to identify the variations in food searching behaviour of larval instars of C. montrouzieri with references to different leaf morphology characters of five different plant species; cassava (Manihot esculenta), guava (Psidium guajava), brinjal (Solanum melongina), crotan (Codiaeum svariegatum) and hibiscus (Hibiscus rosa-sinensis). Mealybug egg mass was attached to the underside of each detached leaf near to the apex and enclosed within a Petri dish. Each starved larval stages of C. montrouzieri was introduced to the prepared setup, 4 cm away from the egg mass and observed the behaviour. The time taken to detect the egg mass, feeding, walking and staying were recorded in every three minutes over a period of two hours under the laboratory conditions of 28 ± 2 °C and 55-85 % relative humidity. Each treatment was replicated 20 times. The initial larval instars shown variations among the observed time durations while the search behaviour of the L4 instar was not significantly affected by leaf types (P>0.05). The time spent on walking was not significantly affected by the leaf type (F (4, 95) =1.167 P=0.33). However, feeding time was significantly different among leaves (F (4, 95) =2.945 P=0.024) and the resting time of L4 larvae was significantly different among the leaf types (F (4, 95) =2.86 P=0.027). It was found that L4 larvae having longer feeding time duration on cassava leaves (75.5 ± 8.6 minutes) whereas feeding time on other leaves ranged between 37-55 minutes. However, the lowest resting time period was observed on cassava (26.1 ± 8 minutes) while other treatments ranged between 52–60 minutes. This study confirms the impact of leaf morphological features towards the behavioural variations of C. montrouzieri larvae.