Seasonal incidence of pests infesting watermelon, Citrullus lanatus (thunb.)

Introduction

Watermelon (C. lanatus) is an economically important cucurbit crop, valued worldwide for its refreshing fruit and rich nutritional profile, including high lycopene content and substantial amounts of vitamin C, vitamin A and B-vitamins (Quek, Chok and Swedlund, 2007) [9] . The fruit is composed of approximately 55.3% juice, 31.5% rind and 10.4% pomace, indicating that a considerable proportion of the fruit constitutes non-edible or processing by-products (Romdhane et al., 2017) [11] . Believed to have originated in Africa, the Middle East and Egypt (Huh, 2008) [5] , it is now cultivated extensively across India, which produced an estimated 8,722.26 thousand tonnes during 2023-24, with Uttar Pradesh, Andhra Pradesh and Tamil Nadu as the leading producing states (NHB, 2023). The Konkan region of Maharashtra, characterized by a warm humid climate, red lateritic soils and high annual rainfall, provides favourable conditions for watermelon cultivation, particularly as a profitable crop grown immediately after paddy harvest under the region's rice-based cropping system (Singh, 2014) [13] . However, watermelon productivity is severely constrained by a complex of insect pests attacking the crop from the seedling stage to fruit maturity, including aphid (A. gossypii), serpentine leaf miner (L. trifolii) and leaf-eating caterpillar (D. indica) Their incidence and severity vary with agro-climatic conditions and unmanaged infestations can cause substantial losses sucking pests alone may reduce yield by 30-70% (Dhillon et al., 2005). L. trifolii, in particular, can damage up to 37% of foliage through mesophyll mining, causing chlorosis and yield decline (Patnaik, 2000) [8] . Since pest build-up is strongly weather-dependent, location-specific studies on seasonal incidence are essential for developing reliable forecasting models and timely management strategies. Such information remains limited for the Konkan agro-climatic zone. The present study was therefore undertaken to investigate the seasonal incidence of major insect pests infesting watermelon (C. lanatus) in relation to prevailing weather parameters in the Konkan region.

Materials and Methods:

The present investigation was conducted at the Vegetable Improvement Scheme (VIS), Central Experiment Station, Wakavali, Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth, Dapoli, during the rabi-summer season of 2024-25 to study the seasonal incidence of pests infesting watermelon. The experiment was conducted under open field conditions using the watermelon variety Augusta. The crop was planted at a spacing of 2.4 m × 0.6 m. The experimental plot measured 0.91 m × 48.46 m. Observations on the incidence of major insect pests were recorded on ten randomly selected tagged vines at regular intervals throughout the crop growth period. The aphid population was recorded weekly intervals on the upper, middle and lower portion of canopy of vines. The mean aphid population per three leaves was calculated for each observation. The leaf miner and cucumber moth infestation was recorded the number of healthy and infested leaves at weekly intervals. The percentage of leaf miner and cucumber moth infestation was calculated using the following formula:

Per cent leaf miner and cucumber moth infestation on leaves = No. of infested leaves Total no. of leaves × 100

The data of pest infestation was correlated with the prevailing meteorological parameters to determine the seasonal occurrence of major pests infesting watermelon.

Results

Seasonal incidence of aphids, (Aphis gossypii Glover)

The aphid populations fluctuated markedly across SMWs, with younger plants showing heavier infestation than mature ones. Aphids were first recorded in the 6 th SMW (5-11 February, 2025) with a mean population of 0.27 aphids per three leaves. Numbers rose consistently from the 7 th SMW onward and reached a peak of 16.16 aphids per three leaves in the 11 th SMW (12-18 March, 2025), coinciding with the crop maturity stage; at this peak the maximum and minimum temperatures were 38.60 °C and 18.40 °C, and morning and evening relative humidity were 87.00 and 42.00 per cent, respectively. Over the season, the population ranged from 0.27 to 16.16 aphids per three leaves, declining gradually thereafter and remaining detectable through the 15 th SMW. These results agree with Sunil et al. (2017) [14] , who reported that A. gossypii incidence on bitter gourd was low during early crop growth, increased progressively from January onward, peaked in March, and declined toward the end of the season a trend closely mirrored in the present study on watermelon. Aphid population showed a significant positive correlation with maximum temperature (r = 0.567*) and a significant negative correlation with evening relative humidity (r = -0.766**); other parameters were non-significant. This indicates that rising temperature favoured aphid build-up, while higher evening humidity suppressed it. Similar results were reported by Sunil et al. (2017) [14] in bitter gourd, confirming that warm, comparatively drier conditions favour aphid multiplication on cucurbits.

Seasonal incidence of leaf miner, (Liriomyza trifolii

Burgess)

Leaf miner infestation was first observed in the 6 th SMW with 18.90 per cent infestation and persisted throughout the cropping period, fluctuating slightly in the early weeks before rising steadily from the 11 th SMW onward. Infestation peaked at 24.97 per cent in the 13 th SMW (26 March-1 April, 2025), when temperatures ranged between 36.30 °C and 20.30 °C and morning/evening relative humidity were 91.0 and 54.0 per cent, respectively. Thereafter, infestation declined progressively, falling to 12.64 per cent by the 16 th SMW, a period corresponding to the late fruiting stage. These findings are consistent with Saha et al. (2018) [12] , who reported that leaf miner infestation on cucumber persisted throughout the season and peaked during the later growth stages. Similarly, Tamilnayagan et al. (2017) [15] recorded peak L. trifolii damage (23.7%) in the third week of March, closely matching the present peak period. Archana et al. (2024) [1] further reported that L. trifolii was the first pest to colonize watermelon, appearing within four days of transplanting and peaking during the 11 th SMW broadly corroborating the persistence and seasonal build-up pattern observed here. Leaf miner infestation showed no significant correlation with any weather parameter studied, suggesting its seasonal build-up was governed more by crop phenology than by weather. This agrees with the non-significant trends reported by Archana et [1,4,10] , indicating that L. trifolii incidence is comparatively less weather-dependent than that of aphid or D. indica.

Seasonal incidence of cucumber moth, Diaphania indica (Saunders)

Leaf infestation: The cucumber moth leaf infestation was first observed in the 8 th SMW (19 to 25 February, 2025) and built up gradually, peaking at 15.32 per cent in the 9 th SMW, when average maximum and minimum temperatures were 38.30 °C and 16.07 °C, and morning/evening relative humidity were 84.00 and 42.00 per cent, respectively. Infestation then declined steadily but remained active until the 16 th SMW, with the season-wide range being 3.03 to 15.32 per cent (lowest value, 1.59%, recorded in the 16 th SMW). D. indica infestation showed a significant positive correlation with maximum temperature (r = 0.598*) and a significant negative correlation with evening relative humidity (r = -0.698**), with other factors non-significant. Thus, higher temperature favoured D. indica incidence while high evening humidity curtailed it. These findings are partially supported by Saha et al. (2018) [12] and corroborated by Archana et al. (2024) [1] , who reported significant positive correlations of D. indica with temperature in watermelon.

Conclusion

The seasonal incidence of major pests infesting watermelon was closely associated with weather conditions. Maximum temperature significantly favoured the build-up of aphid and cucumber moth (D. indica) populations, while evening relative humidity significantly suppressed both, indicating that warm, comparatively drier conditions favour these pests, whereas humid evenings act as a natural check. Leaf miner infestation showed no significant correlation with any weather parameter, suggesting its fluctuation is governed more by crop growth stage than by weather. These findings indicate that maximum temperature and evening relative humidity can serve as useful indicators for forecasting aphid and cucumber moth incidence, aiding timely, need-based plant protection in watermelon.

Acknowledgement

AcknowledgementI sincerely express my gratitude to the Department of Agricultural Entomology, Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth, Dapoli, for providing valuable support, guidance during the research work. I also sincerely acknowledge the support provided by the Vegetable Improvement Scheme (VIS), Central Experiment Station, Wakavali, Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth, Dapoli. I am thankful to all the staff members of the Departments of Entomology and Meteorology, Dr. Balasaheb Sawant Konkan Krishi Vidyapeeth, Dapoli-415712, Maharashtra, India, for their valuable assistance and cooperation throughout the research work.

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