Complex of Aphidophagous Predators of Mealy Plum Aphid Hyalopterus pruni Geoffr. and Their Efficiency in Pest Control ^†

Abstract

Aphids are the dominant pests in the complex of harmful fauna of stone fruits in Ukraine. In plum orchards mealy plum aphid Hyalopterus pruni Geoffr. (Hemiptera: Aphidinae) causes serious economic damage. Insect predators have an important role in the regulation of agricultural pests. Our goal was to determine the role of predation in the dynamics of the mealy-plum-aphid population. Investigations were carried out during the period of mass development of the mealy plum aphid in the experimental plum-orchard garden of the Institute of Horticulture of the National Academy of Agrarian Sciences of Ukraine. For this purpose, model trees were selected and isolators were installed on four branches of each tree with aphid-infested leaves. Different species of entomophagous predators were placed in the isolators and assessments were performed on a daily basis. According to our observations, Adalia bipunctata L., A. decempunctata L., Syrphus ribesii L., S. balteatus Deg, Chrysopa perla L., and Ch. carnea Steph. dominated among aphidophagous predators. Coccinella septempunctata L., Calvia quatuordecimguttata L., and Propylaea quatuordecimpunctata L. were less common. Syrphus and Chrysopa larvae had the highest predation activity. The influence of weather conditions on the efficiency of aphidophagous predators was analyzed. Optimal conditions for A. bipunctata, A. decempunctata, S. ribesii, S. balteatus, Ch. perla, and Ch. carnea during June–July were as follows: mean daily temperature, 17–20 °C, precipitation 0.1–1.5 mm, and relative air humidity 57–67%. The threshold ratios of dominant aphidophagous predators were determined. The high predation activity of Syrphus and Chrysopa larvae and coccinellids indicated that the ratio of entomophages to aphids must be taken into account when making decisions about control measures

1. Introduction

The mealy plum aphid Hyalopterus pruni Geoffr. is an important pest of stone fruits [1]. In Ukraine it is one of the dominated pest species in plum orchards. It reproduces en masse, both in nurseries and in young and fruitful orchards. The aphids concentrate on the leaves, fruits, and shoots, sucking their juice, which causes severe deformation of the fruit and weakening of the fruit trees. The harmful nature of aphids also manifests in reduced yields and reduced frost resistance of the plantations. This pest can also kill the trees [2,3].

An important role in the regulation of aphid numbers is played by aphidophages. Around the world, ladybugs are used against aphids [4,5]. In addition, one of the main regulators of aphid population density is the syrphid larvae [6]. A promising way to develop a biological method of pest control is to study its efficiency.

Understanding the relationships in the plant–aphid–predator (parasite) system is necessary to improve the integrated pest-management program for fruit crops [7].

The goal of this study was to determine the species composition of the natural aphid entomophage population, to determine the daily amount of forage resources eaten, and establish criteria for their efficiency.

2. Materials and Methods

The trophic activity of natural regulators (both in the imaginal and larval stages) of leaf aphids was studied in the experimental garden of the Institute of Horticulture of National Academy of Agrarian Sciences of Ukraine. Determination of the number of aphids eaten by entomophages was carried out in July (the period of mass development of the mealy plum aphid). For this purpose, five model plum trees for each entomophage were selected. Isolators were installed on four branches of each tree, on which aphid-infested leaves were found and different species of entomophages were placed into them: Adalia bipunctata, Adalia decempunctata (both imago and larvae), Syrphus larvae, and Chrysopa larvae. The ratio entomophage:aphid was 1:100. To ensure this ratio, four to five specimens of entomophage was placed in each isolator, depending on aphid density. Isolators were made of film and nylon mesh with small holes. Before placing of entomophages, the aphids were pre-counted. After counting of the aphids and placing of the entomophages, the isolators were tied tightly. Assessments were performed on a daily basis.

The efficiency of entomophages was determined by the formula:

$$E=100·\frac{K_0-K_1}{K_0}$$

where K₀ is the initial number of aphids and K₁ is the number of aphids after entomophage feeding.

Experimental data were analyzed by one-way ANOVA followed by Duncan’s multiple range test (p = 0.05).

3. Results

According to our observations, Adalia bipunctata L., A. decempunctata L., Syrphus ribesii L., S. balteatus Deg, Chrysopa perla L., and Ch. carnea Steph. dominated among the aphidophagous predators of mealy plum aphid. Coccinella septempunctata L., Calvia quatuordecimguttata L., and Propylaea quatuordecimpunctata L. were less common. So, in further experiments A. bipunctata, A. decempunctata, Syrphus larvae, and Chrysopa larvae were used.

In each year, Syrphus and Chrysopa larvae had the highest predation activity—an average of 73.7% and 71.2%, respectively. A. bipunctata and A. decempunctata were less effective (

Table 1. Efficiency of entomophages against mealy plum aphid, %.

Entomophages	2017	2018	2019	2020	Mean
Adalia bipunctata	75.5b 1	65.1b	54.5ab	74.6b	67.4
Adalia decempunctata	63.8a	47.4a	65.2b	52.0a	57.1
Syrphus larvae	87.2c	70.8b	49.1a	87.8c	73.7
Chrysopa larvae	88.4c	81.2c	59.6b	55.5a	71.2

¹ Within each column, means followed by the same letter do not differ at p = 0.05.

).

It is necessary to say more about the dependence of the efficiency of useful fauna on weather conditions. Optimal for the activity of Syrphus larvae (efficiency of 87.2–87.8%), were average daily temperatures of 17.6–19.9 °C, with a maximum temperature of 22.6–25.4 °C; and minimum humidity of 42.6–48% with an average of 57.9–67.8%. With decreasing temperature and increasing humidity, compared with the above values, the efficiency of the larvae decreased to 49.1%.

The efficiency of Chrysopa larvae was influenced by a minimum temperature of 11.2–12.9 °C and an average daily temperature of 17.6–19.0 °C, precipitation of 0.1–1.0 mm, and average humidity of 57.9–74.9%. Under such conditions, their efficiency was 81.2–88.4%. At elevated temperature and humidity and increased precipitation, the efficiency of Chrysopa larvae decreased to 55.5–59.6%.

An average daily air temperature of 17.6–19.9 °C, with a maximum of 22.6–25.4 °C and a minimum of 11.2–14.1 °C, and a minimum humidity 42.6–48% favored feeding of A. bipunctata. Under such conditions, their efficiency was 74.6–75.5%. Cool weather reduced the efficiency of beetles to 54.5–65.1%.

Moderate temperatures—an average daily temperature of up to 17.6 °C, maximum up to 22.6 °C, and minimum up to 11.8 °C, precipitation of 0.1–1.5 mm, and average humidity of up to 69% stimulated the activity of A. decempunctata. Its efficiency was 63.8–65.2%. High humidity and air temperature had a negative effect on the life of the beetles, reducing efficiency to 47.4–52.0%.

The final stage of the study was to determine the threshold ratio of entomophagous predators to mealy plum aphid. Since different insects (imago or their larvae) kill different numbers of aphids, the thresholds was determined for each of them. The threshold ratio of plum aphids in the presence of A. bipunctata was 25–40, for A.decempunctata, 20–30, for Syrphus larvae, 30–50, and for Chrysopa larvae, 10–30.

4. Discussion

Many species of aphidophagous ladybirds are important predators in biological pest control. A. bipunctata has a high predation rate. In greenhouse experiments it completely eradicated the green-peach-aphid population in sweet peppers within two days of release. The predation per adult female was 122.7 aphids per day [8]. For mealy plum aphid we found the threshold ratio was 1:25–40.

Our investigations showed that under favorable weather conditions aphidophages can reduce the population of mealy plum aphids by 74–88%. The best results were obtained for Syrphus and Chrysopa larvae. Average daily air temperature of 17.6–19.9 °C, with a maximum of 22.6–25.4 °C and a minimum of 11.2–14.1 °C, precipitation of 0.1–1.5 mm, and average air humidity 57.9–67.8% were optimal for the feeding of A. bipunctata, A. decempunctata, Syrphus larvae, and Chrysopa larvae.

Cool weather reduced the efficiency of A. bipunctata. Jalali et al. [9] showed that the lower developmental threshold for this insect was 9–11 °C and the best temperature for its rearing was around 23 °C [9,10].

The feeding potential of S. balteatus varied from 202 to 986 depending on the aphid species [11]. S. balteatus can control development of an Aphis pomi population when maintained at a ratio of 1:50 for the initial infestation and at 1:200 at a later period of infestation [12].

In many crops, Chrysopa carnea plays an important role as an aphidophage. Releases of its early-second-stage larvae at the predator/prey ratios of 1:5, 1:10, 1:20, and 1:40 successfully controlled Myzus persicae on sugar beet [13]. A ratio of one adult C. carnea per 5–30 aphids is recommended in greenhouses [14]. Our investigation showed that at a ratio of 1:10–30, Chrysopa larvae can efficiently control the mealy plum aphid. This ratio was the lowest among the four investigated entomophages.

5. Conclusions

In plum orchards A. bipunctata, A. decempunctata, S.ribesii, S. balteatus, Ch. perla, and Ch. carnea dominate among entomophages of mealy plum aphid. High predation activity of Syrphus and Chrysopa larvae and coccinellides indicates that the ratio of entomophagous to aphids must be taken into account when making decision about control measures.