By Priyankar Mondal
Our agriculture is being threatened by new pest outbreaks quite frequently in recent times. During July last year, after the very first report of fall armyworm, Spodoptera frugiperda from Karnataka, the destructive pest colonized more than twenty states within a few months munching mainly on corn leaves and stems but occasionally ravaging the sorghum and sugarcane. Till to date, the entomologists are striving hard to tackle this invasive menace and to add to their efforts a massive outbreak of flea beetles was recently reported from New Delhi-National Capital Region and parts of Haryana.
Cruciferous crops constitute a significant portion of the dietary preferences by Indians. It includes the oil-producing Indian mustard and winter vegetables like cabbage, cauliflower, broccoli, knol-khol and radish rich in vitamin A, C, carotenoids, iron, magnesium, protein and dietary fibres. Nevertheless, some of the crops are also exploited for pharmaceutical purposes both in cultural and modern medicines. In India, the yield of these crops is severely limited by a number of insect pests and flea beetle is notable among them. A study published in August, this year in the journal Phytoparsitica addressed a large scale infestation of striped flea beetle Phyllotreta striolata (Fabricius) on radish for the first time from India. It is one among the 12 species in the genus Phyllotreta which was already present in the country but never attained a damaging status in the farmlands.
“The P. striolata is an important pest in the America where most of the studies have been conducted…but in India they were considered as a minor pest of cruciferous crops (until now)”
says Dr. P. R. Shashank, from ICAR-IARI, New Delhi, one of the chief collaborators in the study. On asking what specifically triggered the investigation, Mr. S.S. Anooj from IARI and lead author of the research paper replied,
“We first noticed the damage symptoms of the pest on the infected radish samples (with bore holes on the surface of the tap root) brought to the entomology exhibition stall during the March 2019 Pusa Krishi Vigyan Mela of IARI. As the larvae was not detected initially I thought it could be damage symptoms of some dipteran maggots and decided to rear it for obtaining the adults for identification of the pest and to my surprise after about a weeks’ time lots of striped flea beetles emerged. Thanks also to the then Head of Division of Entomology, Dr. R. K. Sharma who directed a farmer (with similar infested material) to us and then facilitated an initial visit to the farmers field in Palla village of North West Delhi. The visit turned out to be an eye-opener to us, regarding the level of infestation the pest was causing.”
Unlike the uniformly metallic dark blue elytra of Phyllotreta chotanica and P. cruciferae this particular species can easily be identified based on the broad longitudinal yellow stripes conspicuously painting the black elytra along the outer margins with an inward curve at the middle. Male and female of the species are sexually dimorphic with the male possessing antennae with swollen fifth antennomere while the same is slender in the females and the sternite V which is impressed with concave margins in males and with rounded margin in case of females. The team also compared 43 mitochondrial DNA sequences (COI region) of P. striolata around the world including 5 sequences from India to confirm the species identity and found two distinct clusters for the old world and new world populations concluding definitive isolation of the populations (genetic divergence of 4.47%) of the American and Eurasian landmasses. Earlier in 2017, this species has been reported from Kashmir causing moderate loss of turnip but the attack was not severe to turn the heads. According to Dr. Shashank large scale monocropping of radish in the surveyed area during the present study may be the principal cause for the outbreak of this species.
Figure 1. Phyllotreta striolata infestation on radish: A. Adult feeding on leaf and B. Group of adults on a single leaf
"It may be due to both biotic and abiotic factors. Biotic factors may be the availability of ample quantities of hosts during the season, absence of the natural enemies of P. striolata in the study location, varietal preferences and so on. Abiotic factors may be soil type and texture (i.e., sandy loam soil - loose textured soils helps in egg laying, grub growth, development and movement in the soil), climate change, micro and macro environment favourable for the growth and population build-up."- added Dr. KV Raghavendra from NCIPM, New Delhi and co-author of the research paper.
These tiny-shiny beetles feed gregariously on the foliage of radish producing small shot holes leading to huge loss of photosynthetic tissues as the holes coalesce. The whitish grubs feed by scrapping the taproots and drill tunnels into the soft tissue causing discolouration of the marketable produce.
“Northern and Eastern part of India is a major belt of mustard crop and other cruciferous crops (radish, cabbage, cauliflower, turnip, knol-khol, beetroot, etc). The availability of ample amount of host for their survival makes them persist in the localised habitats. This is the reason why we the scientists are very much concerned about since if at all in the near future, if these flea beetles start devastating the mustard crop in a serious manner, we may lose the entire crop and certainly it will become a challenging task for us to protect the mustard crop from this notorious pest since there is no label claim insecticide registered in India till date.” says Dr. Raghavendra.
On asking about the host preference of P. striolata Mr. Anooj, the lead author replied “This itself is a research question. In the Northern Plains, we have observed its affinity towards radish. Anyhow, in absence of radish they prefer turnip and other available brassica crops nearby.”
The weight of experimental evidence from Canada suggests a wide variation in the development and life cycle of this species depending upon prevailing climatic conditions which necessitates further investigation to understand the pest's behaviour in the Northern plains of India. Long term in situ and ex situ research is required to gather substantial data on biology, behaviour and toxicology of the pest. However, the team is determined to restrict the spread of the pest immediately as Dr. Raghavendra confirmed that ICAR -NCIPM and IARI will be taking up IPM studies for management of flea beetles in cruciferous crops and in particular in radish crop in the forthcoming Rabi season.
“The management measures can’t be recommended causally without proper evaluation especially since the produce is consumed raw as salads. Anyhow giving a break to the continuous monoculture of the radish crop is the practical solution as of now.”-says Mr. Anooj.
For further details, please refer to https://link.springer.com/article/10.1007%2Fs12600-020-00825-4
Priyankar Mondal is one of the Student Associate Editor of IE and working as a Researcher Scholar at Bidhan Chandra Krishi Viswavidyalaya, Mohanpur, West Bengal, India. You can contact him @ Email: priyankar.ento@gmail.com