Natural parasitism of agromyzid leafminer , Chromatomyia horticola ( Goureau ) ( Diptera : Agromyzidae ) on field pea

The leafminer, Chromatomyia horticola (Goureau), (Diptera: Agromyzidae) which infests field pea, is exposed to various parasitoids. Observations were made on the seasonal incidence and parasitization of agromyzid leafminer, C.horticola in Pea crop fields at Pantnagar, Uttarakhand (India) during cropping season of the year 2009 to 2010, 2010 to 2011. Leafminer infested leaves were randomly collected from field and maintained at room temperature (25 to 30°C). Number of emerged adult leafminers and parasitoids were recorded to calculate weekly percent parasitisation. The peak period of parasitization in both years was during 9 th standard week constituted 71.69 and 68.87% of parasitization, respectively. Three hymenopteran parasitoids were recorded viz., two eulophids (Diglyphus horticola Khan, Diglyphus sp.) and one braconid (Opius sp.). Influence of environmental factors viz., temperature, relative humidity, and rainfall on weekly percent parasitization was non-significant.


INTRODUCTION
Leafminers are insects whose larvae live and feed inside the leaves, consuming the mesophyll without damaging the leaf epidermis.Their feeding tracks ("mines") are externally visible in leaves, as whitish or grey areas with variable shapes that range from narrow linear galleries to wide chambers.The leaf mining habit has been developed by a group of over 10,000 species of holometabolous insects, concentrated in four orders: Diptera, Coleoptera, Hymenoptera, and Lepidoptera (Connor and Taverner, 1997).Agromyzid leafminer, Chromatomyia horticola (Goureau) is a highly polyphagous leafminer capable of inflicting severe damage to crops such as field and glasshouse grown vegetables and flowers.It was originated in South America and has spread to other continents where established populations are growing ever more.Due to its hiding nature and resistant development against insecticides makes control measures ineffective.At the same time, natural enemies playing major role in suppressing the leafmining insects in field level.Leafminers are in the phytophagous guild (group of organisms that consume the same resource in the similar manner), which has the greatest number of parasitoid species per host species, and has the highest average rate of parasitoidism (Hawkins, 1994).Over 40 species of parasitoids have been recovered worldwide from the leafminers (Waterhouse and Norris, 1987;Çıkman, 2006;Çıkman and Uygun, 2003;Çıkman et al., 2006;Çıkman and La Salle, 2011;Çıkman, 2012).Thus, the objective of this study was to determine seasonal incidence of agromyzid leafminer, C. horticola and its hymenopteran parasitoids on field pea at Pantnagar, Uttarakhand (India).

MATERIALS AND METHODS
Field surveys of leafminers and their parasitoids on field pea have been made during Rabi season of 2009 to 2010 and 2010 to 2011 at Crop Research Centre, G. B. Pant University of Agriculture and Technology, Pantnagar, Uttarakhand.Weekly, leafminer-infested leaves were randomly collected from the field were brought to the laboratory and placed in plastic bags/rearing jars covered with muslin cloth.Samples were maintained at room temperature (25 to 30°C) and numbers of emerged adult leafminers and parasitoids were recorded daily.All flies and wasps were kept separately in small vials containing 70% ethanol for identification.The weekly percent parasitism was calculated weekly basis according to Mills (1997) and Van Driesche (1983).Weekly percent parasitization correlated with environmental factors viz.temperature ( °C), relative humidity (RH) (%), and rainfall (mm) by using multi-variable regression analysis:

Weekly abundance and parasitization
Weekly visits to pea growing field and rearing leafminer infested leaves over two cropping seasons provided plentiful opportunity to make a general observation and natural parasitization of pea leafminers.
In 2009 to 2010, weekly field surveys of pea crop yielded 2997 adult of agromyzid leafminers and 2463 adult of parasitoids (Table 1) whereas, in 2010 to 2011, 1485 adult of agromyzid leafminers and 1097 parasitoids were recorded (Table 2).In both years, leafminer adults were recorded more than that of parasitoid adults.
During the two years of this investigation, the infestations of agromyzid leafminer were observed more during the 7 th standard week (February, 12 to 18) when limited control was exerted by parasitoids (Figures 1 and  2).The number of leafminer adult emerged during 9 th standard week (February 26 to March 4) was less as compared to the number of parasitoid adult emerged, where parasitoid showed considerable control over Nakakihara (1983) recorded 90% parasitization by two most common parasitoids viz., Diglyphus sp and Halticoptera circulus on Liriomyza species infesting Celery in California.Gratton and Welter (2001)

Statistical analysis
Statistical analysis was made to study the correlation between observed weekly percent parasitization and environmental factors viz., temperature, RH and rainfall.Table 3 shows the weekly percent parasitization and corresponding weekly mean temperature, RH and rainfall for the year 2009 to 2010 and 2010 to 2011.
The data were subjected to multi variable regression analysis by taking weekly percent parasitization as a dependent variable and environmental factors as independent variables: Where, Y 1, Y 2 = weekly percent parasitization, X 1 = mean temperature, X 2 = mean RH, X 3 = rainfall.
Perusal of these equations reveals that during 2009 to 2010 for every increase of one unit of mean temperature and RH, weekly percent parasitization increased by 2.878 and 2.425 units, respectively.Likewise, in 2010 to 2011, every increase of one unit of mean temperature and RH, weekly percent parasitization increased by 3.313 and 3.524 units, respectively.Whereas, in both years (2009 to 2010, 2010 to 2011) every increase of one unit of rainfall, weekly percent parasitization decreased by 1.043 and 2.013 units, respectively.

Figure 1 .Figure 2 .Figure 3 .
Figure 1.Weekly abundance of the leaf miner and its parasitoids recorded on pea crop during 2009 to 2010.

Table 4
shows that environmental factors nonsignificantly influenced the weekly percent parasitization in both the years.However, 40.6% (0.406) and 38.1% (0.381) of variation in the weekly percent parasitization caused by environmental factors during 2009 to 2010 and 2010 to 2011, respectively based on R 2 value.In case of individual factors, variation caused by temperature (5.5%, 12.3%), RH (2.0%, 6.7%) on weekly percent parasitization was less during 2009 to 2010 as compared to 2010 to 2011 whereas variation caused by rainfall (28.3%, 4.7%) was more during 2009 to 2010 than 2010 to 2011.