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

: Aphids are the dominant pests in the complex of harmful fauna of stone fruits in Ukraine; in addition to fruit-damaging species. In plum orchards mealy plum aphid Hyalopterus pruni Geoffr. (Hemiptera: Aphidinae) cause 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 dy-namics of mealy plum aphid population. Investigations were carried out in period of mass development of mealy plum aphid in the experimental plum orchard garden of the Institute of Horticulture of Ukrainian Academy of Agrarian Sciences of Ukraine. For this purpose, model trees were selected and isolators were installed on 4 branches of each tree with aphid-infested leaves. Different species of entomophagous were placed in isolators and assessments were performed on daily basis. According to our observations, Adalia bipunctata L., A. L., dominate among aphidophagous predators. Coccinella septempunctata L., Calvia quatuordecimguttata L., Propylaea quatuordecimpunctata L. were less common. Syrphus and Chrysopa larvae have 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, 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 to aphids were determined. High predation activity of Syrphus and Chrysopa larvae and coccinellids indicates that the ratio of entomophages to aphids must be taken into account when making decision about control measures


Introduction
Mealy plum aphid Hyalopterus pruni Geoffr. is an important pest of stone fruits [1]. In Ukraine it is one of the dominated species in plum orchards. It reproduces en masse, both in nurseries and in young and fruitful orchards. They concentrate on the leaves, fruits and shoots, sucking their juice, which causes severe deformation, oppression, weakening of fruit trees. Harmfulness of aphids is also manifested in reduced yields and frost resistance of plantations. This pest can cause death of trees [2,3].
An important role in the regulation of aphid numbers is played by aphidophages. Ladybugs against aphids are used around the world [4,5]. Also, one of the main regulators of aphid population density is considered to be the syrphid larvae [6]. A promising way to develop a biological method is to study their efficiency. Understanding the relationship 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, daily amount of forage resources eaten by natural aphid entomophages and establish criteria for their efficiency.

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 amount of aphids eaten by entomophages was carried out in July (the period of mass development of mealy plum aphid). For this purpose, 5 model plum trees for each entomophage were selected. Isolators were installed on 4 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, Chrysopa larvae. The ratio entomophage:aphid was 1:100. To ensure such ratio 4-5 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, aphids were precounted. The ratio entomophage:aphid was 1:100. To ensure such ratio 4-5 specimens of entomophage was placed in each isolator depending on aphid density. After counting the aphids and placing the entomophages, the isolators were tied tightly. Assesments were performed on daily basis.
The efficiency of entomophages was determined by the formula: K0-the initial number of aphids; K1-the number of aphids after entomophages feeding. Experimental data were analyzed by one-way ANOVA followed by Duncan's multiple range test (p = 0.05).
In all years Syrphus and Chrysopa larvae have the highest predation activity-in average 73.7% and 71.2% respectively. A. bipunctata and A. decempunctata were less effective (Table 1). The final stage was to determine the threshold ratio of entomophagous to mealy plum aphid. Since different insects (imago or their larvae) kill different numbers of aphids, the thresholds has been determined for each of them. The thresholds ratio of plum aphids is the presence of A. bipunctata was 25-40, A.decempunctata-20-30, Syrphus larvae-30-50 and Chrysopa larvae-10-30.

Discussion
Many species of aphidophagous ladybirds are important predators in biological pest control. A. bipunctata have high predation rate. In greenhouse experiments it completely eradicated the green peach aphid population in sweet pepper within 2 days following release and predation per adult female was 122.7 aphids per 24 h [8]. For mealy plum aphid we found the rate 1:25-40 as threshold.
Our investigations showed that under favorable weather conditions aphidophages can reduce the population of mealy plum aphid by 74-88%. The best results were obtained for Syrphus and Chrysopa larvae. Average daily air temperature 17.6-19.9 °C, with maximum of 22.6-25.4 °C and minimum 11.2-14.1 °C, precipitation 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 is 9-11 °C. And the best temperature for its rearing is around 23 °C [9,10].
Feeding potential of S. balteatus varies from 202 to 986 in dependence of aphid species [11]. It can control development of Aphis pomi population when maintained at ratio from 1:50 for the initial infestation level to 1:200 at later period of infestation [12].
Chrysopa carnea plays the important role as an aphidophage on many crops. 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 1 adult C. carnea per 5-30 aphids is recommended in geenhouses [14]. Our investigation showed that at ratio 1:10-30 Chrysopa larvae can efficiently control mealy plum aphid. This ratio was the lowest among four investigated entomophages.

Conclusions
In plum orchards A. bipunctata, A. decempunctata, S.ribesii, S. balteatus, Ch. perla, 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.