Controlling Anomala denuda (Coleoptera: Scarabaeidae), a Maize Pest, Using an Aqueous Extract of Ricinus communis

INTRODUCTION

Cereal is a plant cultivated mainly for its grains used in human and animal food. In 2020-2021, 715 million hectares of cereals were cultivated in the world and 2.714 billion tons of cereals were produced [1]. Among these cereals, maize is one of the 3 most important cereal crops in the world, the other 2 being wheat and rice [2]. It is a strategically important crop for food security in sub-Saharan Africa.

In Côte d’Ivoire, with a national production of more than 840,000 tons, maize is the 2^nd cultivated cereal product after rice [3]. It is a raw material for the agri-food (brewery, oil mill, soap factory) and food manufacturing industries. Such production represents a considerable source of income and employment for rural populations [4]. Although maize is of undeniable economic importance, its cultivation is confronted with multiple biotic and abiotic constraints that greatly reduce yields. Indeed, the major constraints to maize production include low soil fertility, rainfall variability caused by climate change, phytopathogenic diseases, and above all, attacks by insect pests such as termites [5] and maize stem borers [6].

In addition to the pests listed above, a night survey aimed at capturing stem borers in an experimental maize plot in Songon, southern Côte d’Ivoire, Boga et al. [7] unexpectedly observed a very large outbreak of Anomala denuda adult beetles (46,508 individuals/night). Indeed, several studies in the literature have shown that their larvae are known as crop pests. As for the adults, they are little known as important pests.

However, it appeared from Boga et al. [7, 8] that the adults of this beetle attack and feed extensively on the reproductive organs of maize (male and female flowers), causing an unacceptable loss of 32% in maize production.

Given the high incidence of damage by this beetle on maize production, we set ourselves the objective of carrying out experiments to control the A. denuda beetle using an aqueous extract of Ricinus communis (pesticidal plant). Such endeavor aims to reduce the attacks of this beetle and to improve maize production.

MATERIALS AND METHODS

Site of the study

The study was conducted in Songon (5°19'32.3" N 4°15'24.0" W) in southern Côte d’Ivoire.

In 2020, an average temperature of 27.85 °C, an average rainfall of 151.34 mm, and a relative humidity of 82.3% were recorded at the site Tutiempo [9]. The average temperature recorded in 2021 was 26.45 °C with a relative humidity of 79.3% and an average rainfall of 126.61 mm [10, 11].

Material

Biological material

The biological material is composed of A. Denuda adult beetles and of the Zea mays maize species. It is a local variety (PR9131-SR) with an average yield of 2 t/ha and a potential yield of 3 t/ha. It has a short cycle of 90 days (3 months). An aqueous extract of the insecticidal plant R. communis (Euphorbiaceae) was used for field experiments against A. Denuda populations.

Technical equipment

The material used for the extraction of the aqueous extracts of the plant is a mortar, a mixer, a graduated cylinder, a Denver branded electronic precision balance, white poplin fabrics, cotton wool, a funnel, porcelain plates, and an oven set at 50 °C. Two Tropical-branded backpack sprayers with a capacity of 10 liters were used to apply the treatments. Gloves and muffs were used to protect the applicator.

Methods

Experimental set-up (setting up the crops and treatments)

The experimental setup is a plot of 2592 m² (54 m x 48 m). It is subdivided into 3 blocks with 3 m distance between them. The blocks are divided into 3 sub-blocks separated by 2 m. Each sub-block is 48 m long and 16 m wide and contains 9 elementary plots. Each elementary plot has an area of 16 m² and contains 30 maize seedlings. There is a total of 15,147 seedlings on the whole experimental plot. Among the 3 blocks of the experimental setup, one block was reserved for other observations while the 2 others were treated. Each sub-block was randomly assigned 4 treatments (T1, T2, T3, Ti). The control was named To.

Viper 46EC is the reference insecticide used in our experiments. It is a systemic contact and ingestion insecticide. It belongs to the family of neonicotinoids and Oxadiazines.

Soil preparation and fertilization

To favor the good growth of maize seedlings, chicken droppings (10 tons/hectare) and urea (150 kg/ha) were used as organic amendments on the experimental plot.

Extraction of the aqueous extract of R. communis

The preparation protocol suggested by Zirihi and Kra [12, 13] was used. Seed capsules of R. communis were collected and dried in the shade for 5 weeks. The organs were ground with a blender. One hundred grams (100 g) of powder from each plant was diluted in 2 liters of distilled water. The resulting mixture was homogenized in a blender for 5 minutes and then filtered through a white poplin fabric. The 1^st filtrate was homogenized in the blender and then filtered for the 2^nd time with Whatman paper. A 3^rd filtration was then carried out using a funnel containing cotton wool. The last filtrate obtained was concentrated by evaporation in an oven set at 50 °C for 48 hours.

Determination of the concentrations of extracts applied in the field

Beforehand, toxicity tests were carried out in the laboratory on 2 plants (R. communis and Azadirachta indica) on the A. denuda beetles. Such tests consisted of evaluating the insecticidal effect of these 2 plants on the mortality of A. denuda to determine the most effective plant and concentration. The results of the laboratory tests showed that the aqueous extract of R. communis was the most effective on the Coleoptera as it induced a mortality rate of 80% on A. denuda at a lethal concentration of 110 g/l [14, 15]. R. communis was, therefore, selected for the field experiments. Based on these laboratory results, 3 concentrations of R. communis extract were determined while taking into account the non-laboratory conditions in the natural environment (field):

110 g/l (80% effective dose in the laboratory): Treatment (T1)

110 g/l + one quarter (1/4) of 110 or 137.5 g/l: Treatment (T2)

110 g/l + half (1/2) of 110 or 165 g/l: Treatment (T3)

VIPER 46 EC (16 Acetamiprid 30 indoxacarb): Treatment) (Ti)

Application of the treatments

Two 10-liter backpack sprayers were used to treat the individual plots. One labeled (Ti) was used to apply the reference chemical insecticide and the other (T) was used to apply the aqueous extract of R. communis seed capsules. The field experiment for the control of A. Denuda beetles were carried out over 2 years (2020 and 2021). For treatments, 4 applications were made:

For the year 2020, a 1^st application was made on the 42^nd day after sowing (DAS) before the arrival of beetles on the plot. The second application was made on the 50^th DAS, i.e. 2 days after the arrival of the beetles. The third application was made on the 56^th DAS and the fourth was made on the 62^nd DAS, which is 6 days between treatment applications.

As for the year 2021, following the preventive treatment on the 42^nd DAS, the other 3 treatments were spaced 3 days apart, namely, 50^th DAS for the second application, 53^rd DAS for the third, and 56^th DAS for the last one.

Efficacy of aqueous extracts of R. communis on the A. denuda beetles

A. denuda beetle is active at night. Its activity is intense between 10:00 p.m. to 12:00 a.m. Individuals gather on the apical flowers and hair of maize cobs on which they feed [7]. This activity only lasts about 2 weeks. From the first application of the treatments, data were collected every night from 10:00 p.m. to 12:00 a.m. on the seedlings until the maize cobs reached maturity. The parameters recorded are 1. The number of seedlings attacked per plot, and 2. The number of beetles per seedling/plot. These data were used to calculate the average number of beetles per seedlings and the various reduction rates in beetle numbers after the application of the treatments:

Txre: Reduction rate in beetle numbers per seedlings

NCT0: Number of beetles visiting the control plot

NCT: Number of beetles visiting the treated plot

Impact of R. communis extracts on yield loss caused by A. denuda

A. denuda is a nocturnal beetle that attacks and devours the male (apical flower cluster) and female (hair) reproductive organs of maize. To assess the impact of this damage on maize production, we counted the maize cobs whose hairs had been completely eaten (wiped out) by the insects. The stems of these cobs were marked with ribbons of different colors according to the time intervals that are: [1 to 3]; [4 to 6]; [7 to 9] [10 to 12] DAS.

Thus, all the attacked cobs were marked with ribbons and the others that were not attacked were left untouched. At maturation (91 days), the cobs were harvested and classified into two batches labeled by plot and by treatment. For each plot, the cobs were divided into 2 labeled bags: One bag containing the cobs whose flowers had been wiped out and the other containing the ears that were not attacked. In the batch containing the wiped cobs, the cobs were divided into 4 bags according to the time intervals ([1 to 3]; [4 to 6]; [7 to 9] [10 to 12] DAS in which the cobs were wiped out by the insects. The cobs were transported to the laboratory, stripped, and dried at room temperature for 21 days before they were dehulled. The different batches of maize grains were weighed according to the treatments using a Denver electronic precision balance.

The yield loss caused by insect attacks as well as the rate of loss reduction after treatments were calculated according to the following formulas [16].

Txre: Reduction rate

Statistical analysis

Data processing was done using 2 software programs. Microsoft Office 2010’s Excel software for data entry and graphical representations, on the one hand, and Windows SPSS 21.0 for statistical data analysis, on the other hand. Data were subjected to analysis of variance (ANOVA) and means categorized by the Student-Newman-Keuls (S-N-K) test at a 0.05 significance level. Pearson’s correlation test was used to study the relationships between attack rates caused by the A. denuda beetle and the average number of individuals that visited the plot.

RESULTS AND DISCUSSION

Results

Effect of R. communis aqueous extracts on the reduction in numbers of A. denuda beetles

Crop year 2020

The analysis of the control curve shows 2 phases. A phase in which the number of A. denuda visiting the plot increases sharply (from 48^th to 58^th DAS). Another phase (from 58^th DAS) where the number of insects per seedling progressively decreases until the number of insects per seedling falls below 5 (64^th DAS). Then, the flow of insects visiting the maize seedlings is annihilated from 76^th DAS (Figure 2). After the 2^nd application at 50^th DAS, analysis of the nightly post-application surveys revealed that no insects visited the plots treated with the reference insecticide. However, an average of 0.59 ± 0.003 insects were recorded on plots treated with R. communis aqueous extracts compared to 6.28 ± 1.08 insects on control plots (Figure 2). Four days following the 2^nd application of the treatments (at 50^th DAS), the number of beetles progressively increased, reaching 54^th DAS, the day before the 3^rd application, with average numbers of 19.25 ± 5.12; 14.25 ± 2.09; and 14.08 ± 0.99 insects/seedling, respectively on treatments T1, T2, and T3. The control recorded a peak of 36.64 ± 4.6 insects/seedling on average. Compared to the control, these average numbers correspond to average reduction rates of 47.44 ± 7.23% (T1), 61.57 ± 5.11% (T2), 61.11 ± 11.5% (T3), and 96.66 ± 2.57% insects/seedling for the treatment (Ti). Statistical comparison (Anova test followed by the Newman-Keuls test at the 5% threshold) revealed that there was a significant difference (p < 0.001) between the insect population reduction rates for treatments T1, T2, T3, and the control (T0). This statistically significant difference (p < 0.001) is also expressed when the rates of insect population reduction recorded on treatments T1, T2, and T3 are compared to those induced by the treatment (Ti).

The 2^nd application of the treatments did not annihilate the flow of insects onto the maize seedlings. Nevertheless, it kept the numbers low compared to the control (Figure 2).

After the 3^rd application of treatments on 56^th DAS, the upward flow of beetle numbers on treated plots was interrupted and forced downward, while it remained high on the control (30.78 ± 3.65 insects/seedling). On the 58^th DAS, T1, T2, T3, and Ti treatments reduced insect numbers by 93.88 ± 20.29; 96.51 ± 09.65; 97.15 ± 9.24%, and 100% per maize seedling, respectively. The ANOVA test followed by the Newman-Keuls test at the 5% threshold revealed a significant difference at the 5% threshold (p < 0.001) between the rates of reduction of insect numbers/seedlings of treatments (T1, T2, and T3); T0, and Ti. Treatments T2 and T3 induced statistically similar reduction rates to the synthetic chemical insecticide Viper on 58^ème DAS (Figure 2).

During the critical period of attack in the 2020 crop cycle, the treatments kept the amplitude of insects/seedlings low (5.17 vs. 22.48 insects/seedlings) and reduced the duration of their presence on the plot by 6 days.

Crop year 2021

The analysis of the control curve shows three (3) peaks in the average number of insects per seedling. The first peak is recorded on 51^st DAS (21.47 ± 1.16 insects/seedling), i.e., one day after the first treatment. The second peak was observed on the 56^th DAS (30.78 ± 2.99 insects per seedlings) and the third on the 60^th DAS (24.78 ± 1.04 insects/seedling). This curve shows a phase of a strong increase in the insect population (from 48^th to 56^th DAS). The 2^nd phase is observed from the 56^th DAS where the number of beetles progressively decreases until it falls below 5 insects/seedling on 64^th DAS. These numbers are annihilated by the 72^nd DAS (Figure 3).

After the 2^nd application of R. communis extract on 50^th DAS, treatments T1, T2, and T3 recorded average numbers of 7.57 ± 0.63; 4.24 ± 0.9; and 0.98 ± 0.07 insects/seedling on 51^st DAS, respectively. These average numbers correspond to average reduction rates of 64.74 ± 13.87%; 80.25 ± 18.5%; and 95.44 ± 13.6% for insects/seedlings, respectively. The effect of the T3 treatment at the concentration of 165g/l is almost similar to that of the reference insecticide (Ti) which reduced the insect population by 97.90 ± 11.6% (Figure 3).

Statistical comparison of insect/seedling population reduction rates reveals a significant difference (p = 0.0001) between treatments T1, T2, T3, and T0; but also, between T1, T2, and Ti.

After the 3^rd application of the treatments on the 53^rd DAS, beetle numbers continued to increase in the control plots until a peak of 30.78 ± 2.99 insects/seedlings was reached on the 56^th DAS. At this peak, treatments T1, T2, T3, and Ti recorded 13.47 ± 0.9; 6.78 ± 0.06; 0 and 0 insects/seedling, respectively. At this date, T1, T2, T3, and Ti treatments reduced to 56.23 ± 6.25; 77.97 ± 11.79; 100 and 100% insects/seedlings, respectively. Statistical analysis to date shows that the mean numbers of insects recorded on the treated plots are significantly lower (p = 0.0001) compared to the control. The average number of insects recorded at T3 is almost similar to that of the synthetic chemical insecticide with p > 0.05 (Figure 3).

The number of insects that visited the plot was maintained at a low level of 05.14 insects/seedling on average before canceling from 64^th DAS compared to 19.72 insects/seedling in the control, a reduction rate of 73.93%. Although at low numbers, the insects remained on the maize plants longer compared to the year 2020 (Figure 3). The combination of treatments reduced A. denuda presence time on the plot by 04 days.

Correlations between numbers of A. denuda visiting the plot and attack rates

This study showed that there is a very strong positive correlation between average A. denuda numbers and attack rates. The different correlation coefficients are 0.88 for the year 2020 (Figure 4a) and 0.92 for the year 2021 (Figure 4b). This means that damage increases with insect numbers. Therefore, reducing insect numbers on the plot would reduce attacks and in turn, probably improve maize yield.

Impact of R. communis extracts on yield loss caused by Anomala denuda

Attacks that occur in the [7^th to 12^th DAS] period have no impact on yield [7, 17]. Therefore, in this part of our study, the impact of attacks on yields will only be assessed in the [1-3]; and [4-6 DAS] periods. The cob attacks that occurred (mostly) between the 1^st to 6^th DAS, caused abortion of the unfertilized grains, followed by a partial filling of the cobs, unlike the cobs whose development was not disturbed by the insect attacks. The estimation of the yield loss was, therefore, based on the difference in weight between the grains from the wiped cobs and those from the healthy cobs (in the period from 1^st to 6^th DAS). The more effective the treatment applied, the lower the yield loss and vice versa.

Crop year 2020

In the control plot, the production of unattacked cobs was estimated at 2.1561 t/ha and that of grains from attacked cobs was 1.4793 t/ha. The difference between the 2 expresses the yield loss due to the attacks of the A. denuda beetles. Thus, on the control plot, a yield loss of 0.6768 ± 0.056 t/ha was recorded, corresponding to a loss rate of 31.39 ± 2.58%. The yield loss rates obtained on treatments T1, T2, T3 are respectively 13.58 ± 2.47; 9.54 ± 1.79; 5.14 ± 0.89% (Figure 5a). These yield loss rates are low and significantly lower than the control (p < 0.001). The most effective treatment was T3, which had the lowest yield loss rate (5.14 ± 0.89%). This loss was statistically identical to that of the reference chemical insecticide (3.48 ± 0.28%) with (p > 0.05). The yield loss in the control was 0.6768 ± 0.056 t/ha and that in the T3 treatment was 0.1108 t/ha. Treatment T3 resulted in a production gain of 0.5659 t/kg, an improvement in maize yield of 26.25%. This production would have been lost if the treatment was not applied. The treatment significantly reduced insect damage to maize and consequently yield losses, resulting in higher maize production.

Crop year 2021

In the control plot, the weight of grain from unattacked cobs was 2.1183 t/ha and that of grain from attacked cobs was 1.6743 t/ha. The yield loss calculated from these values was 0.444 ± 0.0637 t/ha, a loss rate of 20.96 ± 3% recorded on the control plots. The lower the yield loss (or yield loss rate) induced by insect damage, the more effective the treatment applied. The data analysis showed that treatments T1, T2, T3, and Ti had low yield losses compared to the control. Such treatments T1, T2, T3, and Ti resulted in yield loss rates of 6.41 ± 0.46; 5.7 ± 0.88; 3.85 ± 1.4%, and 2.1 ± 1.04%, respectively (Figure 5b). These yield losses were significantly different from the control (p < 0.001). The lowest rate of yield loss was noted on treatment T3 (3.85 ± 1.4%), almost similar to that induced by the reference chemical insecticide Ti (2.1 ± 1.04%) according to the analysis of variance (p > 0.05).

The yield loss of the control was 0.444 t/ha (loss rate 20.96 ± 3%) and that of treatment T3 was 0.0815 t/ha. Treatment T3 resulted in a production gain of 0.3624 t/kg or an improvement in maize yield of 17.80% in 2021. This production would have been lost if the treatment was not applied.

Analysis of the impact of treatments on the 2 crop cycles (2020 and 2021)

All treatments applied were relatively effective compared to the control (p < 0.05). They reduced the number of insects visiting the treated plots to an average of 5.16 insects/seedling compared to 21.10 insects/seedling noted on the control, i.e., an average reduction rate of 75.54%. Among these 3 treatments, the most effective was treatment T3 (165 g/l). It recorded the lowest number of insects per seedling 3.30 in 2020 and 2.85 insects per seedling in 2021, for an average of 3.03 insects/seedling. Treatment T3’s action with R. communis aqueous extract at 165 g/l induced the lowest production loss rates in 2020 (5.14%) and 2021 (3.85%). Over the 2 years of the experiment, an average loss of 4.49% of production was recorded on T3 compared to a loss of 26.17% on the control. Thus, the yield was significantly increased by 26.25% in 2020 and 17.80% in 2021 on the plots treated with T3, i.e., an average of 22.03%.