The population dynamics of insect pests of little millet, Panicum sumatrense and their natural enemies

Introduction

Little millet (Panicum sumatrense) (miliare) is a key cereal crop belonging to the Poaceae family, especially in the arid and semi-arid regions of Asia and Africa, where it serves as a staple food for many communities. Known for its resilience to drought and poor soil conditions (Sharma et al., 2016) [14] . As global food security becomes increasingly important, enhancing the cultivation of resilient crops like little millet is essential for sustaining rural livelihoods and improving nutritional outcomes. It is known by various local names: Saame or Savi in Kannada, Kutki in Hindi, Samai in Tamil, Samulla in Telugu and Channa in Malayalam (Kundra et al., 2019) [10] . Furthermore, to integrate millets into the mainstream, highlight their nutritional benefits, and encourage their cultivation, the Government of India has designated 2018 as the "Year of Millets." Additionally, the FAO Committee on Agriculture forum has proclaimed 2023 as the "International Year of Millets." Kishore (1996) [8] reported 116 pests that infest pearl millet, in which major pest includes shoot fly (Atherigona approximata), stem borer (Chilo partellus), grey weevil (Myllocerus spp.), leaf roller (Marasmia trapezalis), sugarcane leaf hopper (Pyrilla perpusilla), fulgorid (Peregrinus maidis), hairy caterpillars (Amsacta spp.), ear-head caterpillars (Eublemma silicula and Cryptoblabes gnidiella), white grub (Holotrichia consanguinea) and midge (Geromyia penniseti), pose significant challenges in the field. The shoot fly alone can cause grain losses up to 20-50 per cent. Whereas Ganiger et al. 2024 [5] , worked on taxonomic identification of maggot and adult stages of kodomillet shoot fly, Atherigona simplex.

The highly diverse agro-ecosystem in millet-growing regions directly and indirectly affects grain yield through various biotic and abiotic factors that serve as significant constraints to production. A survey of pest incidence on millets indicated that crops are impacted by 50 to 55 different pest species across various regions and seasons, from sowing to harvesting and storage. Within this complex pest landscape, shoot flies from the genus Atherigona (Muscidae) are particularly harmful, leading to considerable yield losses, especially in little millet during the 4 to 5 leaf stage (Nageshchandra and Ali, 1983) [11] . Fletcher (1914) [3] was the first to report the incidence of shoot flies in South India. Population dynamics refers to the study of how pest populations change over time in response to environmental factors and agricultural practices (Ghosh et al., 2018) [6] . By analyzing these dynamics, patterns can be identified and potential outbreaks can be predicted, allowing farmers to implement timely and effective pest control measures.

Materials and Methods

The research was conducted to collect data on the occurrence of insect pests at various growth stages of little millet and their natural predators during the Kharif season of 2023 and 2024 at ZARS, UAS, GKVK, Bengaluru. The little millet variety TNAU-186 was sown in 3 x 4 m plot with a plant spacing of 30 x 10 cm.

Observations recorded

Observation was recorded every week after 15 days of sowing, where in 10 plants are considered for each observation, such three observations was recorded for deadhearts and natural enemies like predators and parasitoids. The observations were recorded till harvesting stage of the crop. Data obtained on population dynamics of insect pests and their natural enemies in little millet and infestation of major pest was correlated with Standard Meteorological Week (SMW).

Method of observation 1. Shoot fly:

The observations were recorded from 30 plants within the net plot by counting the number of dead hearts to total number of plants selected and the results were expressed as a percentage using the following formula

Grasshoppers:

The population of grasshoppers was observed as the number of grasshoppers per one metre row in a plot.

Weevils:

The weevil population was recorded as number of weevils per plant on 10 randomly selected plants.

Aphids:

The aphid population was recorded as number of aphids per leaf on 10 randomly selected plants.

Natural enemies:

The population of predators i.e., coccinellids and spiders was counted from 10 randomly selected plants at weekly intervals and expressed as number of natural enemies per plant.

Results and Discussion

The population dynamics of insect pests of little millet and their natural enemies during Kharif 2023 and 2024 1. Shoot fly, Atherigona pulla (Wiedemann) During kharif 2023, the shoot fly infestation in little millet ranged from 0.00 to 31.11% dead-hearts, starting in the 36 th SMW with 21.11% incidence. It peaked at 31.11% in the 38 th SMW (third week of September) and declined to 11.11% by the 43 rd SMW (last week of October). The present findings are in corroborates with Saxena et al. (2022) [13] , they revealed that the peak incidence of shoot fly infestation was observed during 29 th and 30 th standard meteorological week (SMW). Whereas, Choudhary et al. (2018) [1] , who revealed that infestations of shoot fly began on the 31 st standard meteorological week (SMW) and lasted throughout the crop season. Later, it gradually increased, reaching a peak in the 33 rd (2014) and 34 th (2015) SMWs with plant infestations of 16.67 and 20.00 per cent, respectively. The variations may be due to change in the time of sowing. During kharif 2024, the shoot fly infestation in little millet ranged from 0.00 to 38.89% dead-hearts, starting in the 28 th SMW (first week of July) with 27.78% incidence. The infestation peaked at 38.89% in the 30 th SMW (third week of July) and declined to 17.78% by the 35 th SMW (last week of August). The present findings were more or less similar to that of Choudhary et al. (2018) [1] , who revealed that infestations of shoot fly began on the 31 st Standard Meteorological Week (SMW) and lasted throughout the crop season. Later, it gradually increased, reaching a peak in the 33 rd (2014) and 34 th (2015) SMWs with plant infestations of 16.67 per cent and 20.00 per cent, respectively. The present findings corroborate with Saxena et al. (2022) [13] , they revealed that the peak incidence was observed during 29 th and 30 th Standard Meteorological Week (SMW).

Weevil, Myllocerus discolor (Schoenherr) and

Myllocerus viridanus (Fabricius) During kharif 2023, the weevil populations started at 0.33 per plant in the 37 th SMW (second week of September). The peak population of 0.89 per plant was observed during the 39 th SMW (fourth week of September). The population gradually declined to 0.11 per plant by the 42 nd SMW (third week of October). Whereas in 2024, Weevil populations began at 0.11 per plant in the 28 th SMW (first week of July), peaking at 0.56 per plant during the 30 th SMW (third week of July). The population gradually declined and reached a minimum of 0.11 per plant by the 33rd SMW (second week of August).

Grasshopper, Oxya hyla (Serville) and Chrotogonus oxypterus (Blanchard)

In Kharif 2023, Grasshopper populations began in the 36 th SMW (first week of September) at 0.33 per metre row. The population peaked at 0.67 per metre row in the 37 th SMW (second week of September) and declined to 0.11 in the 43 rd SMW (fourth week of October). At harvest, the population stabilized at 0.22 grasshoppers per metre row. Whereas in 2024, Grasshopper populations started at 0.44 per metre row in the 28 th SMW (first week of July). The population peaked in the 31 st SMW (last week of July) and declined to 0.22 per metre row by the 36 th SMW (first week of September). At harvest, the population stabilized at 0.22 per metre row.

Aphid, Hysteroneura setariae (Thomas)

During kharif 2023, aphid populations were began with 0.22 per leaf in the 36 th SMW (first week of September). The highest population of 0.56 aphids per leaf was recorded during the 37 th SMW (second week of September). The population declined to 0.11 aphids per leaf by the 41 st SMW (second week of October), with natural enemies such as coccinellids and spiders observed feeding on aphids. Whereas in kharif 2024, the aphid populations commenced at 0.22 per leaf in the 28 th SMW (first week of July). The population peaked at 0.56 aphids per leaf during the 30 th SMW (third week of July) and declined to 0.11 per leaf by the 33 rd SMW (second week of August), with coccinellids and spiders feeding on them.

Natural enemies Coccinellids, Coccinella transversalis (Fabricius) and

Coccinella septempunctata (Linnaeus) Spiders, (Thomisus sp.) In Kharif 2023, Coccinellids were first observed in the 36 th SMW (first week of September) at 0.22 per plant, peaking at 0.56 per plant during the 38 th SMW (third week of September). Their activity persisted until the 43 rd SMW (last week of October) at 0.11 per plant. Spiders first appeared at 0.11 per plant in the 36 th SMW, reaching a peak of 0.89 per plant in the 39 th SMW (fourth week of September). Whereas in Kharif 2024, Coccinellids were first observed in the 28 th SMW (first week of July) at 0.11 per plant, peaking at 0.44 per plant in the 30 th SMW (third week of July). Their activity persisted until the 35 th SMW (last week of August) at 0.11 per plant. Spiders began at 0.11 per plant in the 28 th SMW and peaked at 0.56 per plant in the 30 th SMW (third week of July).

Correlation matrix between dead-heart and meteorological variables during Kharif 2023 and 2024

During kharif 2023, the per cent infestation due to shoot fly revealed a significant positive correlation with minimum temperature (r = 0.778), a non-significant positive correlation with afternoon relative humidity (r = 0.574) and total rainfall (r = 0.509) and a non-significant negative correlation with maximum temperature (r = -0.491), morning relative humidity (r = -0.617) and sunshine hours (r = -0.269). The present findings more or less showed similar to that Saxena et al. (2022) [13] revealed that dead-heart (r = 0.68) and adult flies (r = 0.79) showed positive significant correlation with minimum temperature. Karibasavaraja and Balikai (2006) [7] , who reported positive association between egg load and dead-hearts with rainfall. Delobel and Lubega (1984) [2] , Gahukar (1987) [4] and Kumar et al. (2015) [9] , also reported similar results. However, Rai and Wakgari (2012) [12] revealed that rainfall had very poor nonsignificant positive correlation with infestation (r = 0.15). Whereas during kharif 2024, The per cent infestation due to shoot fly revealed a significant negative correlation with minimum temperature (r = -0.690*), a non-significant positive correlation with morning relative humidity (r = 0.537) and total rainfall (r = 0.203) and a non-significant negative correlation with maximum temperature (r = -0.396), afternoon relative humidity (r = -0.185) and sunshine hours (r = -0.255). The present findings are similar with reports of Kumar et al. (2015) [9] which revealed that the minimum temperature during 14 days after emergence of the crop showed negatively significant. Whereas, Swathi et al. (2022) [15] showed significant negative correlation with minimum temperature, evening RH (r = -0.513, r = -0.476) and non-significant negative correlation with maximum temperature (r = -0.307) and non-significant positive influence with morning RH (r = 0.298) in little millet.

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