Population Densities of the Predaceous Mite Amblyseius swirskii on Cotton Plants, and A New Technique to Gather the Mite Individuals

INTRODUCTION

Different insect species can be managed by the predaceous mite A. swirskii Athias-Henriot (Alipour et al., 2016; Riahi et al., 2017). Various pests like thrips, whiteflies, and mites can be controlled by it (Messelink et al., 2005; Arthurs et al., 2009;Stansly and Castillo, 2009; Dogra Maci et al., 2011; Calvo et al., 2015). Since its commercial production, it has been amongst the most effective biological control agents (Buitenhuis et al., 2010).

To carry out various investigations, a great number of A. swirskii mites are required. Accordingly, the primary goal of the current study was to determine the months of the growing period, the plant sections, and the places on the cotton leaf that serve as indicators of mite abundance. Additionally, two insecticides were sprayed, and their impact on the mite numbers was measured. The impact of spraying a defoliant on the mite numbers was also studied.

The methods of discharging predaceous mites across fields have been studied in the past. Different tools, like small packets, can be used to spread mites that are created for commercial purposes. They are tiny units that hang from the plant and unleash predators for a period of time. Predatory mites kept in roughage are usually applied to the foliage. As an alternative, an air blast can disseminate the mites (Opit et al., 2005).

The current study focuses on the contradictory direction, specifically how to collect predatory mites that are present on cotton plants. Accordingly, the second goal of the current study is to discover a way to get the enormous store of A. swirskii that is on the leaves before the end of the growing period in order to use it rather than let it perish.

The restricted dissemination of A. swirskii from the point of discharge was revealed by Buitenhuis et al. (2010). The current study focused on how A. swirskii disperses on piled cotton leaves. This phenomenon might be used to gather predatory mites. The pattern of the mite's dispersal should be the guiding principle to designate the best location of the plant for releasing that mite.

MATERIALS AND METHODS

The current study was carried out in a cotton field in Behera governorate, Egypt over two seasons (2020 and 2021).

Apart from the application of pesticides, standard agricultural procedures were followed. In Egypt, cotton is planted in March, April, and May (El-Debaby et al., 1995; Attia et al., 2021). Consequently, cotton plants remain in some fields till the middle of November. The present study was conducted in a belated cotton field. Individualsof A. swirskii were counted, considering the taxonomic identification.

1. Estimating the population densities of the mites found in the different months from mid-June to mid-November

Replicates were selected at three different field locations. Monthly, ten leaves were picked at random from each location, and the number of A. swirskii individuals present on each leaf was noted. Each location's monthly mite population density was estimated. That was done over the course of two seasons, from mid-June to mid-November (2020 and 2021).

2. Estimating the population densities of the mites found in the different sections of the plant

The plants' average height in September was around 180 cm. Three plants represented the replicates. The bottom, the center, and the top of each designated plant were supposed to be separate sections. The section had a height of roughly 60 cm. From every section, ten leaves were picked at random. Each leaf was picked apart to count the mites found on it. The mite population density was estimated for each of the three sections.

3. Estimating the population densities of the mites found on the two leaf sides and on different leaf parts

In September, ten leaves from the central part of the plants were selected as ten replicates. Each leaf was examined immediately after picking. Certain cotton leaves had five lobes and four pleats. Others had three lobes and two pleats (Figs. 1 and 2). The pleat typically has a convex upper side and a concave underside (Fig. 3).

The underside of the cotton leaf contained either five or three main, noticeable veins. They converge at the base of the leaf. The mites were counted, taking into account the ones that follow:

3. 1. Locations appointed on the upper side  

The number of mites on the upper side of each leaf was recorded, paying particular attention to the base, the pleats, and the remainder of the upperside. The total number of mites on the upper side was then determined.

3. 2. Locations appointed on the underside  

The number of mites on the underside of each leaf was recorded, paying particular attention to the base, the pleats, the edges of all the major veins, and the remainder of the underside. The total number of miteson the underside was then determined.

                  (a) The upper side                                        (b) The underside

Fig. 1 Leaf with five lobes

Fig. 3 Portion of the upper side, comprising a pleat in the center

4. Inspecting the leaves after spraying the pesticides   

In a determined region, Lambda cyhalothrin (5%) E. C. and Chlorpyrifos methyl (48%) E. C. were alternately sprayed. Following the manufacturer's guidelines, each insecticide was applied three times. The interval was 20 days. The initial application (chlorpyrifos methyl) was made on the first of July. The final application (lambda cyhalothrin) took place on August 20th. Accordingly, a total of six sprays were performed. The interval was 10 days. Three sites were chosen to represent three replicates. Furthermore, three pesticide-free sites were chosen to represent the other three replicates. The pesticide application programme ended on August 20^th. Five days later, A. swirskii mites found on each of the ten leaves that were randomly selected from each site were noted. Each site's mite population density was estimated.

5. Inspecting the leaves after spraying a cotton defoliant

Following the manufacturer's guidelines, a cotton defoliant, namely dropp® 50 w. p., was sprayed on the first of October. That was applied on a pesticide-free plot, leaving an area untreated with the defoliant. After a week, there were four-leaf categories:

1. The old leaves underneath the defoliated plants

2. The recently fallen leaves underneath the defoliated plants

3. The leaves remaining on the plants treated with the defoliant

4. The leaves of the plants that haven't been treated with the defoliant

Each category was represented with three plants as replicates. From each replicate, ten leaves were picked and inspected to evaluate the population density of the considered mite.

6. Gathering the mites on some leaves 

The mites were gathered from the pesticide-treated region by following three successive steps:

1. The defoliant, namely, Dropp®, was applied at the on the first of October.

2. Some of the recently fallen leaves were gathered on October 8^th and transferred to the laboratory right away.

3.The mites were accumulated on some leaves in the laboratory as follows:

The petioles of the leaves as well as the tips of the broad leaves were removed. In a glass jar, fifteen leaves were stacked on top of one another, as seen in Fig. 4. Nine replicates of the leaf-filled jar were made. The margins of the leaves made no direct contact with the inside surface of the jar wall. The jar rim had been previously treated with vaseline mixed with camphor oil. The exterior surfaces of the jar sides were covered with an opaque fabric. The jars were left open in a continuously lit laboratory.

Three replicates were considered on the next day to count the mites that were present on each of the leaves. The same inspection was performed on three replicates every day. Consequently, the nine jars were considered within three successive days. Five groups, each of which comprises three leaves, were thought to make up the 15 leaves of the jar. Following that, the numbers from one, which stands for the highest group, to five, which stands for the lowest group, were used to signify the five groups. Each group's mite population density was estimated. The laboratory experiment to assemble the mites was done at a temperature of 30 ± 5° C and a relative humidity of 70 ± 5%.

Fig. 4 A jar's top view before the sides are covered

7. Statistical analysis

An ANOVA was used for examining the data. The computer application COSTAT software was used to determine the least significant differences (L.S.D.) at the 0.05 level.

RESULTS

1. Population densities of the mites in the different months of the season

According to Table 1 and Fig. 5, the highest population density occurred in September, while the lowest occurred in June.

Table 1. The mites' average numbers, per leaf, in the differentmonths

Three replicates were used for the samples.

The order of the ranks is downward.

Averages in the same column that are followed by the same letter do not differ significantly at P < 0.05 level.

Fig. 5. Average numbers of the mites, per leaf, during the months of the season

2. Population densities of the mites in the different sections of the plant

According to Table 2, the leaves located in the central section of the plants had the highest mite population density (an average of 9.967 mites per leaf), while the leaves located in the top section of the plants involved the lowest mite population density (an average of 1.633 mites per leaf).  

Table 2. The mites' average numbers, per leaf, in the different sections

Three replicates were used for the samples.

The order of the ranks is downward.

Averages in the same column that are followed by the same letter do not differ significantly at P < 0.05level.

3. Population densities of the mites on the two leaf sides and on different locations of each side

Considering the undersides of ten leaves, the average number of mites detected in the pleats amounted to five, as indicated in Table 3. Simply said, there were 50 mites on the undersides of all the pleats of ten leaves. Hence, most mites were observed in that location.

The average number of mites detected on the undersides of the ten leaves amounted to 9.2. Simply said, there were 92 mites on all the locations of the undersides of ten leaves. Comparatively, the average number of mites detected on the upper sides of the ten leaves amounted to just two. Thus, the mite population was substantially greater on the undersides of the leaves than it was on the upper sides.

Table 3. The mites' average numbers, per leaf, in different locations

Ten replicates were used for the samples.

The order of the ranks is downward.

Averages in the same column that are followed by the same letter do not differ significantly at P < 0.05 level.

4. Population densities of the mites after spraying the pesticides

Leaves that had been exposed to pesticides had an average of 4.567 mites per leaf, while leaves that had not been exposed to pesticides had an average of 5.60 mites per leaf. Accordingly, the two pesticides had no significant impact on the mite population density (Table 4).

Table 4. The mites' average numbers, per leaf, on the pesticide-untreated leaves and the pesticide-treated leaves

Three replicates were used for the samples.

Averages in the same column that are followed by the same letter do not differ significantly at P < 0.05 level.

5. Population densities of the mites after spraying a cotton defoliant

In spite of applying the defoliant, a few leaves lingered on the stems' lower portions for a few days.

According to Table 5, The average numbers of mites per leaf, considering the four-leaf categories, i.e., the old leaves underneath the defoliated plants, the recently fallen leaves underneath the defoliated plants, the leaves remaining on the plants treated with the defoliant, and the leaves of the plants that haven't been treated with the defoliant, were 1.433, 13.500, 7.367, and 5.433 mites per leaf, respectively, with a ratio of 1: 9.42: 5.14: 3.79. Hence, the recently fallen leaves underneath the defoliated plants had the highest population density of mites. Simply said, those greenish and freshly fallen leaves were home to the greatest number of mites.

Table 5. The mites' average numbers, per leaf, on the four-leaf categories

Three replicates were used for the samples.

The order of the ranks is downward.

Averages in the same column that are followed by the same letter do not differ significantly at the P < 0.05 level.

6. Gathering the mites on some leaves

The uppermost leaves in the jars acquired the mite most densely.

Table 6 demonstrates the mite population densities after three days of stacking the leaves in the jars. An average of 39.89 mites were detected on the uppermost three leaves of group 1. In contrast, the mite population density on the defoliant-untreated plants in October 2021 was just 4.867 mites per leaf.

As regards the elapsed days, the total mite numbers in the jars have declined day after day.

As far as the prey is concerned, no living prey was observed on the jar leaves at the end of the experiment.  

Table 6. The mite's average numbers, per leaf, in each group and on each day

Three replicates were used for the samples.

The order of the ranks is downward.

Averages in the same column that are followed by the same letter do not differ significantly at P < 0.05 level.