The climate of the study locations is undergoing changes especially temperature is increasing significantly. The increase in maximum temperature and its extremes have already been reported from the northeastern region of India (Jhajharia and Singh 2011; Chakraborty et al. 2014; Saha et al. 2022). Though there were changes in the minimum temperature, the pattern varied among the locations. This difference in minimum temperature trends has also been reported by Deka et al. (2009) and Jhajharia and Singh (2011). The changes in monthly and seasonal rainfall also had differences among the locations but it was mostly on the negative side during the tomato growing season. Statistically significant decreasing trends during winter and pre-monsoon months were also observed in rainfall over different parts of the northeastern region (Saha et al. 2015; Chakraborty et al. 2017). All these changes are taking place simultaneously and making the bio-climate of the region change which in turn has a more cumulative impact on the crops and its pests/diseases. The bioclimate of the region has also undergone changes in the recent past which have been reported by the trends in the heat index (HI), Temperature-Humidity Index (THI), Compensated Summer Ombrothermic Index, etc. (Chakraborty et al. 2014; Saha et al. 2016). Such variations are definitely having impacts on the occurrences of pests/diseases in the region.
The incidence of fruit borer among the selected locations had two broad patterns. At Location I & II it commenced on the 10th SMW (first week of March) while at Location-III it appeared on the 13th SMW (fourth week of March). Consequently, the peak incidence was noticed between the 13th to 14th SMW (the last week of March to the first week of April) in Location-I and II, while for location-III it was on the 15th SMW (second week of April). As location-I and II are quite warmer (mean monthly Tmax of March 26.4°C and 25.5°C, respectively) compared to location-III (mean monthly Tmax of March 21.3°C), the incidence of the pest was early. Hence, the incidence of this pest is highly dependent on the thermal regime of the location and also the coincidence of the suitable crop growth stage. Similar findings indicating the highest infestation of H. armigera on tomato in March were reported by Singh and Gupta (2017). Kumar et al. (2017) observed the presence of H. armigera in tomato crops as early as the first week of February (Tmax of 27.7°C), with peak infestation during the third week of March (Tmax of 34.1°C) in an experiment conducted in Udaipur, Rajasthan, India. Therefore, it is very clear that the thermal regime played a crucial role in the incidence as well as the severity of the pest attack. With respect to the crop growth stage, it is seen that the initial population of fruit borer gradually expanded, primarily staying within the vegetative growth stage. However, a rapid increase occurred during the fruiting stage of tomatoes (Singh 2013). The fruit damage pattern followed the trend in the pest population i.e. larvae/plant. The fruit damage started during the 11th SMW (2nd week of March) at Location-I and II while it was at the 13th SMW (4th week of March) at location-III. In all locations, the peak fruit infestation was recorded 2–3 weeks after the initiation. This coincided with the maximum fruiting stage and temperature (Tmax) rise of 1.5°C to 2°C compared to initiation of pest occurrence. These two factors play a crucial role in the build-up of the pest population and its associated damage intensity. The increase in temperature shortens the life cycle of the pest and the availability of food further hastens it. These combinations increase the fruit damage intensity. The peak population of pests was comparable for Location-I and II while it was much lower for Location-III, which contributed to the higher extent of fruit damage in Location-I and II (26–29%) compared to location-III (9%). This could be due to the marked difference in the temperature between the locations. The Tmax of Location-III is almost 4.5°C to 5°C lower as compared to Location-I and II. Therefore, it is quite clear that with the increase in temperature, the pest population as well as damages increase.
The correlation analysis revealed that Tmax and Tmin appear to be the most influential weather parameters in pest incidence and fruit damage across the studied locations, while relative humidity and rainfall showed weaker or non-significant correlations. The result clearly suggests that higher Tmax is associated with higher larval populations across all the locations. However, higher Tmin is associated with increased pest incidence and fruit damage at Location-III. This may be due to the fact that the minimum temperature of Location-III is a bit lower as compared to the other locations (by almost 2°C to 3°C ranging from 11.1°C to 13.9°C during March and April) and as it increases in the crop growing season, the pest infestation also increases. The findings of the present study are in line with the study of Jat et al. (2017) who noted a significant positive correlation of H. armigera larval population with Tmax followed by Tmin. The results are further supported by Keval et al. (2017), who found that relative humidity had a substantial negative correlation with the population of H. armigera in pigeon pea but a significant positive correlation with Tmax. The findings are in partial agreement with the results of Saini et al. (2017) who reported that the fruit borer exhibited a negative and significant correlation with relative humidity and total rainfall while temperature showed a positive but non-significant correlation. As it is seen, due to rainfall, the temperature decreases, and the pest takes more time to complete the life cycle thereby reducing the generations and population build-up which in turn affects the fruit damage. Further, the climatic trends of temperature at all the studied locations are on the increasing side during the tomato growing season, which indicates that in the future the pest population and its associated damages will increase in the region making it a matter of concern.
Climatic conditions play a significant role in shaping the observed plant characteristics. Higher plant heights in some varieties may indicate favourable conditions for growth, including optimum thermal regime as well as sunlight. Due to these factors, the plant height was considerably higher at Location-I and II as compared to Location-III where the average temperature remains almost 4.5°C to 5°C lower during the crop growing season. The plant height, number of branches per tomato plant, leaf area expansion rate, and leaf area index were positively influenced by the warmer environment (Pandey et al. 2004). The optimum daily temperature ranges from 25°C − 30°C during the day at different growth stages of tomato; and this range of temperature is significant for maintaining a normal net assimilation rate (Camejo et al. 2005; Laxman et al. 2013). As the temperature at Location-I and II are within the optimum range, the growth of the plants was higher. Varieties/genotypes that exhibited the earliest flowering across all locations suggest that these varieties are adapted to quicker flowering, possibly as a response to specific climatic conditions. These varieties/genotypes may be better adapted to locations with shorter growing seasons or cooler temperatures, as they can initiate fruit production sooner. However, it is clearly seen that the days to flowering were quite higher at Locaion-III as compared to the others due to the lower temperature prevailing in the area taking more time to fulfill the cumulative growing degree days (GDD) requirement for the particular variety/genotype (Laxman et al. 2013). Cherry tomato and MT-2 have relatively lower fruit damage percentages across locations, suggesting they may possess natural resistance or adaptability to local pest challenges. Varieties with higher fruit damage percentages are more susceptible to the fruit borer pest. MT-3 and MT-2 consistently yielded the highest across all locations. These varieties may have traits that make them well-suited to the local climate and soil conditions, resulting in higher yields. Cherry tomatoes and LE-626 consistently yielded lower, which may be due to factors like plant characteristics (smaller fruit sizes), higher fruit damage, and sensitivity to the local environment. Yield variations reflect the ability of certain varieties/genotypes to thrive in the prevailing climate, including resistance to local environmental stressors. Overall, the differences in plant characteristics among tomato varieties in different locations can be attributed to a combination of genetic traits and the influence of local climatic conditions.
The relationship between biochemical parameters and H. armigera incidence is influenced by the specific climate conditions in each location and the variety/genotypic characteristics. The study clearly showed that TSS was positively correlated to the tomato fruit borer incidence across all the locations indicating that higher TSS would make the tomatoes more attractive for the pest. Mallikarjun et al. (2011) reported that the total soluble protein and total soluble sugar content exhibited a positive correlation with the infestation of the bruchid (Callosobruchus theobromae L.) population on fieldbean. Haralu et al. (2018) reported a significant positive correlation between total sugar levels and pod borer infestation during both the vegetative (r = 0.824) and reproductive (r = 0.919) stages of chickpea. Sharma et al. (2009) observed that resistance to H. armigera was associated with lower sugar content and higher concentrations of tannins and polyphenols. Jat et al. (2021) found that high levels of total soluble sugars and crude protein were linked with increased pest infestation. The current findings are supported by Sharma et al. (2009) that resistance to H. armigera in wild pigeon pea relatives is associated with reduced sugar content. Similarly, the results were also consistent with the studies of Jagtap et al. (2014) and Blaney and Simmonds (1990) who demonstrated that higher total sugar content promoted H. armigera incidence and significantly affected the feeding behavior of H. armigera larvae in pigeon pea. Dodia (1992) also documented an increase in feeding activity of H. armigera larvae in pigeon pea as the quantities of total soluble sugars in leaves, pods, green seeds, and dry seeds increased. Varieties/genotypes with low TSS like MT-2, MT-3, and VL Tomato 4 (TSS ranging from 4.52°B to 4.7°B) would be more tolerant to the fruit borer pest.
Acidity consistently showed a strong negative correlation with pest incidence in all studied locations, indicating its potential role in tolerance to fruit borer. A negative correlation between fruit infestation and the levels of ascorbic acid, phenols, and acidity was reported by Usman et al. (2012). Similar findings were also reported by Kashyap and Verma (1987). Therefore, tomato genotypes characterized by high levels of ascorbic acid, phenols, and acidity may serve as marker traits for the development of resistance against H. armigera. A similar observation was made by Srivastava and Srivastava (1990) who found that susceptibility to H. armigera is associated with lower acidity levels in leaves. Additionally, Selvanarayanan and Narayanasamy (2006) reported that resistant tomato genotypes exhibit higher acidity content compared to susceptible varieties. Vitamin C showed a positive correlation though it was not statistically significant with pest incidence in the present study. Goggin et al. (2010) studied a wide range of plant-insect interactions and found that the presence of vitamin C (L-ascorbic acid, ascorbate, or AsA) in plants played a pivotal role in determining their vulnerability to insect feeding. These effects are influenced by the multiple roles of AsA, i.e. its importance as a vital dietary nutrient, its function as an antioxidant within the insect midgut, and its role as a substrate for plant-derived ascorbate oxidase which can lead to the creation of potentially harmful reactive oxygen species. Ascorbic acid, commonly referred to as vitamin C, is a water-soluble antioxidant and an enzyme co-factor crucial for the metabolic and developmental processes of both animals and plants (Carr and Frei 1999; Institute of Medicine (US) Panel on Dietary Antioxidants and Related Compounds 2000). There is a statistically significant positive correlation between the lycopene content and the incidence of pests as reported by several researchers (Rasheed et al. 2018, El-Sitiny et al. 2022) while studying fruit damage caused by pests including Tuta absoluta in various tomato genotypes. These findings emphasize the importance of considering both biochemical and climatic factors for identifying suitable varieties/genotypes to develop eco-friendly pest management strategies for tomato crops in different regions.