Competing interest statement
Conflict of interest: the authors declare no potential conflict of interest.
Like other agricultural crops, mulberry is also prone to attack by many insect pests, which cause quantitative and qualitative damage resulting in a decrease in sericultural productivity. According to Narayanaswamy (1996), more than 300 insects and non-insect species of pests are known to attack mulberry. The important pests of mulberry are mealy bugs, leaf webber, thrips, Bihar hairy caterpillar and cut worms (Carsai et al., 2014). Among these, pink mealy bug (Maconellicoccus hirsutus) is a major pest-causing tukra in mulberry. The average incidence and loss in mulberry leaf yield caused by these pests is estimated to be 34.24% and 4500 kg/ha/year (Manjunath et al., 2003). In this study an attempt was made to find the occurrence of five major pests and their natural enemies in mulberry fields at Salem and Namakkal districts of Tamil Nadu.
Materials and Methods
A survey on the incidence of five major mulberry pests and natural enemies was carried outat Salem and Namakkal, the two major sericultural districts in Tamil Nadu (http://tnsericulture.gov.in/ sericultureNov12/BriefNoteOnSericulture.htm). The observations were made at weekly intervals for about three months (December 2012 – February 2013). The study was conducted in Randomized Block Design at Regional Sericultural Research Station (RSRS), Central Silk Board, Vaikkalpattarai in Salem District and Reddipudur village in Namakkal District. In each village, a mulberry gardens was chosen for study. The 100 plants (20each in 4 corners and 20 plants in centre of the plot) were observed for a week in each holding. The natural enemies of mulberry pests were identified with the help of the website http://www.angelfire.com/bug2/j_poorani/. Meteorological data such as temperature, relative humidity and rainfall during the study period were also recorded to assess the influence of these factors on the pest and natural enemies incidence. The correlation coefficients were worked out by using the GNU PSPP statistical software (Version 0.7.9 for Linux Mint 17.1).
Table 1 shows the temperature, relative humidity and rainfall in Vaikkalpattarai and Reddipudur (India) during the study period (December 2012 – February 2013). The lowest (24.77°C) and highest (28.29°C) temperature was recorded in Vaikkalpattarai and Reddipudur during Jan 2013 and Feb 2013 respectively. The relative humidity (RH) was higher in Vaikkalpattarai than Reddipudur (less than 50%). There was light rain (0.09–0.32 mm) in Vaikkalpattarai and no rainfall was registered in Reddipudur during our work (Figure 1).
The incidence of mulberry pests such as Leaf Webber (D. pulverulentalis), Pink mealy bug (M. hirsutus), Papaya mealy bug (P. marginatus), Spiralling white fly (A. dispersus), Thrips (P. mori) was recorded in Table 2. The incidence of pests was higher in Vaikkalpattarai (5 species) than Reddipudur (3 species). Occurrence of P. mori gradually increased from Dec 2012 to Feb 2013 in both stations, similarly the A. dispersus in Vaikkalpattarai, P. marginatus in Reddipudur showed increased trend in the same period (Figure 2).
The occurrence of natural enemies such as Coccinellids and spiders in the mulberry fields of Vaikkalpattarai and Reddipudur was also recorded in Table 2. The occurrence of Coccinellids was highest in first two months of our study period and lowest in the last month. But the spider population was high in Feb 2013 in Vaikkalpattarai and Jan 2013 in Reddipudur (Figure 3).
Table 3 indicated the correlation between weather factors, mulberry pests and their natural enemies at Vaikkalpattarai station. There was no correlation between temperature and relative humidity, but temperature was well correlated with pest A. dispersus (0.70), P. marginatus (0.67) and natural enemy spider (0.63). Relative Humidity showed positive correlation with Rainfall (0.98), Coccinellids (0.96) and negative correlation with all pests and spiders. Rainfall had negative correlation with all pests and spiders and positive correlation with Coccinellids population (0.88). All the mulberry pests showed positive correlation with spiders and negative correlation with Coccinellids in Vaikkalpattarai station.
Temperature showed positive correlation with mulberry pests (D. pulverulentalis 1.00; P. marginatus0.70; P. mori 0.95) and negative correlation with their natural enemies (Coccinellids and spiders, –0.99). In Reddipudur, there was negative correlation between pests and their enemies (Table 4).
A positive correlation between temperature and mulberry pests is in agreement with previous studies (Mahimasanthi et al., 2015). Here it is proved that increase in temperature increases the mealy bug population and infestation, but prolonged high temperature for longer period reduces mealy bug infestation. This is in agreement with well defined temperature limits observable in arthropods, beyond which mortality is observed, as expected in ectoderm animals (Aswathi, 1997; Hemalatha & Shree, 2008). This study demonstrated also that Pink mealy bug may cause damage to the mulberry crop throughout the year, as opposite to other findings (Benchamin et al., 1997) where a maximum damage ranging from 0.79 to 11.69 % of severity was observed from July to August.
In the present study relative Humidity showed negative correlation with all mulberry pests. Similar results were reported by other authors (Mani & Thontadarya, 1987; Narendra Kumar et al., 2006; Rahmathulla et al., 2015) in different areas of Southern India. The infestation of the pest however, was low during rainy and winter season. This could be due to the fact that in cooler climate, females remain inactive in sheltered parts of plants or overwinter in the egg stage.
Rainfall in Vaikkalpattarai station shows negative correlation with all mulberry pests. Venugopalapillai & Krishnaswami (1983) reported that high rainfall and humidity were not favourable for thrips resulting in low peaks of thrips population on mulberry.
D. pulverulentalis infestation in Vaikkalpattarai and Reddipudur was the least serious in the study period. But many workers (Sengupta et al., 1990; Siddegowda et al., 1995; Geethabai et al., 1997; Rajadurai et al., 1999) reported a high percentage of infestation caused by lepidopteran leaf roller D. pulverulentalis in the south of India during rainy and winter months. During the study period the spider population was abundant only in January and February 2013 in both stations. The effect of a spider species on a pest population may be enhanced if the spider population increases rapidly in response to a rich supply of nutritious alternative prey (Jeffries & Lawton 1984; Axelsen et al., 1997). Spiders play a key role, together with the other dominant natural enemies, in suppressing pest populations (Zhang, 1992). Small pests, such as thrips, midges and aphids, may die by being caught in the webs of large spiders, even when they are ignored by the spider (Nentwig, 1987). The spider population might even kill more pests than if the pests were a high-quality preferred prey, because spiders would remain unsatiated (Alderweireldt, 1994). Coccinellids such as Synona rougeti Mulsant, Epilachna vigintioctopunctata Fabricius, Cheilomenes sexmaculata Fabricius, Nephus lentiformis, Illeis bielawskii Ghorpade, Psyllobora bisoctonotata Mulsant) control the pest populations in the mulberry fields of present study. Our results are in agreement with the finding other authors (Cock, 1985; Kumashiro et al., 1983; Ignacimuthu, 2002).