Seasonal occurrence of the corn leaf aphid, Rhopalosiphum maidis (Fitch) (Hemiptera: Aphididae) and coccinellids on maize crop

Introduction

Maize (Zea mays L.), which originated in Mexico, is a member of the Poaceae family. It is a crop with a wide range of adaptability and a high potential output (Tali et al., 2018a) [10] . It is the most significant cereal crop, grown all over the world and has the biggest production of any grain. Due to its numerous uses, maize is frequently referred to as the "Queen of Cereals" (Tali et al., 2018b) [11] . Maize grain contains protein (10%), oil (4%), carbohydrates (70%), fat (5 to 7%), fibers (5 to 7%) and minerals (2%) (Tali et al., 2018a) [10] . Because the maize crop has lush growth, soft, succulent foliage, an endless supply of food, space, and shelters, it attracts a large number of insect pests. Among them, R. maidis is a significant agricultural polyphagous, multivoltine pest, also one of the most severe pests of maize.

Although it has Asian origins, it may found in all tropics and temperate regions of the planet (Hill, 1987; Blackman and Eastop, 2000; Kuo et al., 2006) [6,2,7] . It is found in barley, oats, maize and occasionally in wheat. It's most common wild host is Jhonson grass. Both are connected to orchards and fields of cereal. Two weeds, Chromolaena odorata and Cynodon dactylon, served as alternate food sources and maintained the population alive from October to March. The main food plants are wheat, barley and sorghum (Ganguli and Raychaudhary, 1985) [4] . It produces a large amount of honeydew, which causes inflorescences to be sterilized and leaves to become malformed (Hill, 1987; Bing et al., 1991; Blackman and Eastop, 2000) [6,1,2] . The sucking cell sap by sharp needle like stylet into cell that result into stunted growth of plant and secreting honeydew like sweet sugary substance by nymphs and adults gives the plant its sticky appearance, while also luring black ants. It is well known that the abundance of insect pests and natural enemies depends upon abiotic and biotic factors at a particular space and time.

Materials and Methods

The present evaluation of the Seasonal occurrence of aphid, Rhopalosiphum maidis (Fitch) and coccinellids on maize, were carried out on the Entomology farm, BACA, AAU, Anand during rabi 2021-22. To record the seasonal occurrence of the aphid, R. maidis and its natural enemies like coccinellids, the maize crop was sown in area of 120 sq.m. It was grown using all conventional agronomic practices, with the exception of using pesticides. The plot was divided into six equal quadrates. Observations of each quadrate (4×5m) were recorded at weekly interval starting from the commencement of the pest incidence till the harvest of the crop. For this 30 randomly selected plants were tagged and observations on aphid were recorded from the plant by visual count during morning hours. The observations on population of aphid were recorded on randomly selected shoot, tassel and cob from each tagged plant and mean population of aphid per shoot, tassel and cob was worked out. Weekly meteorological data recorded at the meteorological observatory, AAU, Anand were collected and correlation coefficient values were calculated in order to determine the impact of weather parameters.

Results and Discussion

The aphid, R. maidis was found to be the major insect pest causing damage to the maize crop started in the fourth week of December, 2021 and continued until the crop was harvested. The data on the R. maidis population observed on maize plants are provided in Table 1 and Fig. 1. Aphid activity started in December and continued upto March. The aphid infestation on the tassel began in the fourth week of December, 2021 (52 nd SMW). Aphid population increased gradually in small counts (54.7 aphids/tassel), progressively expanded and peaked (561. 8 aphids/tassel) in the fifth week of January (5 th SMW). Once the crop reached maturity, the population gradually reduced until the last week of March (1 st SMW). Between the second to fifth weeks of January, there was higher infestation of aphid (2 nd SMW to 5 th SMW). The maximum aphid population on maize tassel reached during January, 2022. Similar to above, an aphid infestation on cob began in the first week of January, 2022 (1 st SMW). Its population was initially low (47.3 aphids/cob), but it gradually increased in numbers and peaked in the fifth week of January with 472.2 aphids/cob (5 th SMW). Once the crop reached maturity, the population gradually decreased until the first week of March (1 st SMW). Between the second to fifth weeks of January, there was maximum infestation of aphid (2 nd SMW to 5 th SMW). Ghortale (2015) [5] from Rahuri also observed aphid incidence in December, 2014 to February, 2015. According to the report of Varadharasu et al. (2019) [12] , the incidence of R. maidis began to develop in December (0.57/leaf) and persisted through March. However, Aphid population increased in the second and third weeks of February after increasing in the second and third weeks of January as reported by Singh (2011) [9] . According to Choudhary et al. (2017) [3] , the aphid, R. maidis population peaked in the first week of February after first emerging in the first week of January. As per the report of Patel (2021) [8] came to the conclusion that the aphid first occurred in the second week of January and gradually increased and goes down up to March, 2021. The aforementioned reports thus slightly differ from the current findings. The variance could result from many environmental factors at various sites. The correlation coefficient value indicated that bright sunshine hour exhibited a non-significant positive correlation with aphid population on tassel. The maximum temperature was determined to be non-significantly negative correlation. The minimum temperature had a very strong negative correlation (r=-0.83**). Aphids and relative humidity correlated positively in the morning and negatively in the evening. Significant negative association was established between the morning vapour pressure (r=-0.58*) and evening vapour pressure (r=0.55*). Aphid population and wind speed showed a weakly positive correlation. Bright sunshine hour had a non-significant positive correlation in the aphid on cob study. A non-significant negative correlation was noticed at the maximum temperature. The lowest temperature had a very strong negative correlation (r=-0.83**). Aphids and relative humidity correlated positively in the morning and negatively in the evening. Significantly negative correlations established between the vapour pressure in the morning (r=-0.61*) and evening (r=-0.60*). Aphid population and wind speed showed a weakly positive correlation. Similar to the current findings, Ghortale (2015) [5] also noted a highly significant negative correlation between aphid infestation and the minimum temperature and non-significantly positive correlation with humidity. [4] likewise came to the conclusion that the maximum temperature showed a significant negative association (r=-0.76) and relative humidity showed a significant positive correlation (r=0.80).

However, Varadharasu et al. (2019) [12] found a positive correlation between the occurrence of aphids and the minimum and maximum relative humidity. (1995) where in R. maidis population was negatively correlated with temperature and it was positively correlated with relative humidity. Aphid infestation had a nonsignificantly positive correlation with humidity and a highly significant negative correlation with lowest temperature (Ghortale, 2015) [5] . According to Choudhary et al. (2017b) [4] the highest temperature showed a significant negative association (r=-0.76) and the relative humidity showed a considerable positive correlation (r=0.80) with the aphid population.

Conclusion

The present study revealed that the first incidence of aphid on maize tassel was observed during 2 nd week of December and on cob during 4 th week of December. Aphid population were dynamically fluctuated on maize during the crop period. Apparently, prevailing weather parameters were found to be exclusively responsible factors to oscillate the pest population. The maximum and minimum temperature were found negative correlation with aphid population while, aphids and relative humidity correlated positively in the morning and negatively in the evening.

References

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