Abstract

Red Palm Mite (RPM), Raoiella indica Hirst (Arachnida: Acari: Tenuipalpidae), is a pest of coconut, areca, date palm and many other ornamental as well as commercial palm species. The mite feeds on the underside of palm fronds. This pest, first reported in India from Coimbatore, Tamil Nadu during 1924, has gained considerable economic significance since it has been reported as an invasive species in many new-world countries. As the name implies, the mite is bright red in colour. The body is flat with long spatulate setae, and droplets on the dorsal body setae. Females of R. indica are on an average 245 microns (0.01 inches) long and 182 microns (0.007 inches) wide and are larger than males and less active. The life cycle from egg to adult typically requires 23 to 28 days for females and 20 to 22 days for males. Though this species is indigenous to India, it is an invasive species to Dominican Republic, Guadeloupe, Puerto Rico, Saint Martin, Trinidad and Tobago, the US Virgin Islands, Granada, Haiti and Jamaica causing severe damage to coconut, banana and many other crops. The aim of the project was to study the population dynamics of the mite and to survey the natural enemies associated with it in its native range to develop a biocontrol strategy to tackle the problem.

The host list of RPM is extensive; however, reports indicate that Areca catechu and Cocos nucifera are the major host plants in India. They affect seedlings of both areca and coconut, especially during the summer season. Trees grown in conditions of poor drainage, irrigation and low mineral and organic matter are particularly affected compared to plants grown in well irrigated shaded nurseries. Life cycle is closely linked with monsoon and increase in population occurs after the end of rainy season.

Field and laboratory studies were carried out in order to assess the relationships between RPM, its natural enemies and other factors such as climate and host. Spatial and temporal surveys were carried out in coconut and arecanut during 2008 and 2009. Two sites were chosen for each species (Palakkad and Peechi for coconut and Kunnamkulam and Nilambur for arecanut), that are historically drier and wetter, climatically. Surveys for each treatment were conducted once in a month in order to obtain temporal data.

In each survey area, 20 trees were randomly selected within a 10km strip ( only ten sites in Kunamkulam). A start point for each area was chosen and trees were selected randomly by driving for 30 seconds, stopping and selecting the nearest palm. Young palms not taller than 4 metres were selected and one lower frond was sampled. Each frond sampled was divided into 3 sections, upper (tip), middle and lower (base) and a leaflet was removed from close to the rachis giving 3 leaflets in total for each frond. When the leaflet was removed, a visual inspection for RPM and natural enemies was carried out. Natural enemies were removed using a paintbrush and placed into 70% alcohol (1st sample) then the remaining members of same species were collected alive for testing in the laboratory. The cut leaflets were labelled using the letters ‘L’ for the lower section, ‘M’ for the middle section and ‘U’ for the upper section. Leaflets were stored separately in cotton bags. The total number of leaflets on the frond was then estimated in order to gain an estimate of leaf area sampled. Upon reaching the laboratory, the leaflets were closely inspected under a stereomicroscope for the presence of RPM and its natural enemies.

In general, RPM populations were initially very low in November and December and high in February and March. All sites showed a significantly higher RPM population in March than in December. In most cases, the number of sites infested with RPM rose every month, with Palakkad having a larger proportion of RPM infested sites compared to the other sites. This showed that the RPM are either dispersing within the survey areas or breeding to population levels which are more detectable.

Surveys were also carried out on alternative host plants like banana, pandanus, and on many other palm varieties. Though solitary individuals were located on banana from Palakkad area, no direct evidences of colonisation were observed, indicating a lack of suitability of the host, or other potential factors limiting the RPM establishment.

Along with RPM, various predators were also recorded. The majority of predators collected during the surveys were phytoseiid mites belonging to the genus Amblyseius Apart from this coccinellids were also recovered which were relatively low in number. Two different genera of coccinellids including Stethorus keralicus were observed. Other predators recorded belonged to the orders, Thysanoptera, Neuroptera, Hemiptera and Diptera.

The influence of local weather parameters and predators on RPM population were investigated. It was found that there was a significant positive effect of site temperature on RPM population. It was also found that there was a significant effect of host plant species on population levels of RPM. Even though there was no significant effect of site humidity on RPM number (F=0.37, p=0.56) there was a trend whereby higher humidity levels were related with lower RPM numbers.

Predator (phytoseiid) number was not related to site temperature, but it was slightly related with site humidity. There was, however, a very significant correlation between average phytoseiid number and rainfall of the previous month (F=23.49, p<0.01), although no correlation was seen between phytoseiid number and rainfall in the current month (F=0.37, p=0.55). These results indicates that the increase in populations of RPM is not only linked to temperature, but also to the host plant, number of predatory mites present, humidity and rainfall. Laboratory studies proved that phytoseiid feeds red palm mite, but rearing and bioassay was difficult because phytoseiids always showed escaping behaviour from the arena.

The rapid spread of RPM throughout the other parts of the world (New World) demonstrates the ability of the mite to disperse effectively between plants. However, the method of transfer between plants is unknown, along with the factors that trigger the dispersal of the mite between plants. To address this problem, wind-dispersal traps were installed in the field to study the mechanism of dispersal of RPM. Tailor made traps were set up in the field and periodic observations were made. Besides, to assess the mite density on each trap site, four coconut palms were randomly sampled each month (different trees were selected each month), from which three leaflets were collected from a lower frond and mite density was estimated. RPM was caught in April and May. RPM density was also high during that time. The study indicated that RPM dispersed through the wind current. Aerial dispersal occured when the populations were dense on the tree canopy. Results also showed that the number of solitary females found increased throughout the season. Leaf nutrient analysis revealed that Phosphorus content of the leaves and RPM numbers were related.

To summarise, the results showed that the most abundant predator associated with RPM is the phytoseiid mite. There were high numbers of phytoseiid mites during the months of December and January but there was a significant drop in numbers in the later period. Phytoseiid mites were highly correlated to rainfall of the previous month, and negatively correlated to RPM populations, even though laboratory data has shown that these mites do feed on RPM. From this information it could be postulated that the predator is indeed adapted to feeding on RPM but it is poorly synchronised. RPM on the other hand, has an abundance of suitable host plants and ideal weather conditions for population expansion