Cochineal Scale Dactylopius opuntiae controls Opuntia dillenii in Bundala National Park, Sri Lanka

: Opuntia dillenii (Ker Gawler) is an invasive plant species in Bundala National Park. The outstanding success of the cochineal insect, Dactylopius spp. in bio-control of the prickly pear, Opuntia spp. has been investigated worldwide. The present study reports the emergence of D. opuntiae as a pest of O. dillenii after several decades of its introduction to Sri Lanka. The intensity of infestation was 100% along three of the five transects surveyed and the overall damage was more than 50%. This recent outbreak of D. opuntiae may have resulted from an unnoticed existing population of D. opuntiae in the vicinity.


INTRODUCTION
Opuntia dillenii (Ker-Gawler) Haworth (prickly pear) belongs to family Cactaceae and is native to the Caribbean Islands, North and South America (USDA, 2020; Anderson, 2001). Opuntia dillenii is a succulent perennial shrub, forming low clumps or tall branched bushes and sometimes with distinct trunks. Opuntia dillenii was introduced to Sri Lanka as an ornamental plant in the 19 th century and subsequently became an invasive species along the coastal area between Hambantota and Yala National Park, and especially in Bundala National Park (Dilshan et al., 2017). It has invaded several hectares of sand dune areas, adjacent scrub forests and pasture lands in Bundala. Certain areas are densely covered by O. dillenii that have developed into large prickly patches completely inaccessible to humans and animals. The seeds are spread by macaque monkeys and some birds (Gunasekara, 2009). Bambaradeniya et al. (2006) Moran and Zimmermann (1984) report five species of cochineal insects used successfully for biological control of several cactus weeds around the world without any adverse consequences on environment. The first record of a herbivorous insect for biological control of a weed, anywhere, involved the release of Dactylopius ceylonicus (Green) onto Opuntia vulgaris Miller in Sri Lanka during 1863 (Tyron, 1910). Later, there had been several incidents in which Dactylopius spp. were introduced as a bio control agent to limit the spread of cactus species in Sri Lanka. According to Goeden (1988) D. ceylonicus has been introduced again in 1865 for the successful control of Opuntia monacantha and it is known as the first international transfer of a natural pest for biological control of weeds. Jepson (1930) recorded the position of O. dillenii (Cockerell) in Ceylon after introduction of D. opuntiae and also the study revealed that the introduction to certain areas has failed to control O. monocantha in these regions. Further, it was also revealed that the introduction of D. opuntiae to control O. dillenii populations in India has also been unsuccessful (Muniappan et al., 2009).
In 2017, Opuntia dillenii was the most highly invaded species and covered approximately 9% (567 ha) of the total area of the Bundala National Park (Suraweera and Dahanayaka, 2017). According to Marambe et al. (2010) several programs have been implemented by the Department of Wildlife Conservation (DWC) to manage the spread of O. dillenii in Bundala National Park. However, there has been no progression in these programs to control the spread of O. dillenii up to now. The present study reports a recent event of natural bio control of O. dillenii by D. opuntiae in Bundala National Park, Sri Lanka.

Study area
Bundala National Park is a RAMSAR wetland site that lies on 6° 12ˈ 50" N and 81° 13ˈ 30" E, along the southern coastal region of Hambanthota district, Southern province    of Sri Lanka. The wetland consists of 6,216 ha of low land area, and five lagoons with a total surface area of 2,250 ha that are located at 10 m above mean sea level (Green et al., 2008).

Study of the intensity of infestation
Dactylopius opuntiae infesting O. dillenii was observed in the Bundala National Park and adjacent area while in Bundala National Park the heavy infestation led to the death of cactus plants. The initial inspections were conducted to identify the infected areas and to categorize the intensity of infestation on 28 th and 29 th of August, 2020. The intensity of infestation was categorized by visual examination according to the scale given bellow, 0% -no infestation.
Two field trips were conducted on the 30 th of August and the 1 st of September in 2020 to investigate the intensity of D. opuntiae infestation in cactus in several localities in and around Bundala National Park (Figure 1). Five line transects (100 m) were randomly selected along the boundary of National Park where O. dillenii were abundant. GPS locations of each site were recorded. The percentage intensity of infestation was measured using the above four infestation scale along each line transects at each 10 m interval using one square meter quadrat at each point (Moussa et al., 2017). Data gathered was analyzed by IBM SPSS Statistics 22 software and Microsoft Excel 2010 software. Mean percentage values of ten quadrates in each transect were used to plot bar charts to show intensity of infestation.

Collection and identification of the insect
The heavily infested cladodes of cactus were hand collected from adjacent areas near to Bundala National Park and the insects were removed from the cladodes with a fine brush and were preserved in 70% alcohol. Few representative specimens of adult females were slide mounted using standard entomological techniques (Nye, 1947).
Photographs of the damage and insects were initially sent to Prof. J.P. Edirisinghe, Emeritus Professor, Department of Zoology, University of Peradeniya. Slide mounted specimens were further identified using the key by Lotto (1974) to confirm the identity.

Dactylopius opuntiae, the pest of Opuntia dillenii
The collected specimen belonged to D. opuntiae. Live specimens of D. opuntiae were oval in shape and of dark maroon colour [ Figure 2 (a)]. Several days after specimens were preserved in 70% alcohol, the alcohol turned into a beautiful light maroon-red colour, the colour of the cochineal dye. Mounted adult female had an elongated and oval shaped body with short and stout legs. Dorsal, ventral and lateral modified body setae are short, cylindrical and moderately to strongly stout; rather numerous [ Figure  2 Figure 4 (e)] while it was little more than 50% along the Transect 2 [ Figure 4 (b)]. However, after several weeks of moderate infestation, the intensity of damage became high. According to Figure 4 (f), the infestation in the whole study area was severe (> 50%) and was observed in nearly 60% of the total of 50 one square meter quadrates observed in the study area. Furthermore, the intensity of infestation was very high where O. dillenii grows as an understory in Prosopis juliflora shades than in open areas. There were plants such as Salvadora persica, P. juliflora, Lantana camera and Ziziphus mauritiana around the areas having infected Dactylopius opuntiae. However, these plants were found to be not infested by O. dillenii. Though there were several previous records of anthropogenic introduction of Dactylopius spp. to control cactus in Sri Lanka (Tyron, 1910;Goeden, 1988;Muniappan et al., 2009), the present study revealed the emergence of D. opuntiae as a pest of O. dillenii after several decades of its introduction to Sri Lanka. However, there are no published records of recent introduction of D. opuntiae to control cactus in Sri Lanka. Therefore, this recent outbreak may have resulted from an unnoticed existing population of D. opuntiae in the vicinity.