Occurrence dynamics and chemical controls of two scale species occurred in orchards in the south of Shanxi province

Abstract:【Objective】Scales are important pests that occurre on forest and fruit trees. Both Drosicha corpulenta (Kuwana) and Didesmococcus koreanus Borchs wre the dominant specie that occurre in orchards in the south of Shanxi province. Their nymphs and female adults feed on the juices of plants and weaken the plant growing and influence the quality of fruit. In recent years, the occurrence areas and population densities of D. corpulenta and D. koreanus increased year by year because of the increase of fruit species, changes of cultivation patterns, transportation of nursery stocks, climate change, etc. Comprehensive control techniques had been vastly investigated for D. corpulenta and D. koreanus, including agricultural control, biological control and chemical control. However, chemical control was still the main technique against these two scale insects. Moreover, the control effects of ordinary pesticides were significantly affected by the biological characteristics of scale insects and the time of control Therefore, exploring the population dynamics, identifying the optimum control time, and finding out the high efficient control insecticides are necessary for the control of D. corpulenta and D. koreanus.【Methods】Field observations and experiments were carried out in the year of 2015 and 2016 to investigate the occurrence period of egg, nymph, maleadult, and female adult and to find out the weak period of growth and development of D. corpulenta and D. koreanusin occurred in walnut orchard and apple orchard, respectively. Furthermore, 4 000 fold diluents of 22% Sulfoxaflor SC, 2 000 fold diluents of 10% Imidacloprid WP, 4 000 fold diluents of 22.4% Spirotetramat SC, and 800 fold diluents of 40% Methidathion EC were sprayed seperately by manual sprayer on walnut trees and apple trees in the field to determine their effects on these two scale insects when they were in stage.【Results】D. corpulenta occurred one generation per year and overwintered in egg stage in the soil at the base of trunk in southern Shanxi. In the following year, the over-wintering eggs hatched from February to April, and the nymphae climbed up the tree to feed on the buds. The peak period of newly moulted nymphae appeared from middle February to early March. The female nymphae emerged as adults molted twice. On the contrary, male nymphae pupated in late March and adults emerged in early May. After mating with the male adults in early and middle May, the female adults started to go down from the walnut trees from the late May to the early and middle June. After entering the ground, female adults laid eggs in white waxiness oocysts. These eggs then over-summered and overwintered till the February or March of the next year. D. koreanus also occurred one generation per year and overwintered as second instar nymphae in the bark cracks and rough skins of the apple tree. In the following year, the over-wintering nymphae crawled out from molting crust to feed on the new branches. Female nymphs emerged as adults molted in early April when apple trees started growing, while the male ones pupated and emerged as adults in the middle April. After mating, male adults later died, while the female ones died after laying eggs under their scales. Eggs of D. koreanus hatched from the mid to the late May. The newly moulted nymphae spread outward from mother shells and then fed on branches, leaves, and fruits.They were gradually covered by the white waxiness. The peak period of newly moulted nymphae was the late May to the early June. And they overwintered after developing as the second instar nymphae in September or October. Chemical experiments showed that the field control efficacy of 800 fold diluents of 40% Methidathion EC was 93.13% 3 days after spraying, which was significantly efficient than other three insecticides. 7days after spraying, the field control efficacies of all these four insecticides increased. And the control effects of 800 fold diluents of 40% Methidathion EC was still the best, which was 94.12%. However, there was no significant difference among the tested insecticides. 14 days after spraying, the field control efficacies of these four insecticides were all above 90%. The control effects of 4 000 fold diluents of 22% Sulfoxaflor SC and 4 000 fold diluents of 22.4% Spirotetramat SC still increased, which were higher than those of 800 fold diluents of 40% Methidathion EC and 2 000 fold diluents of 10% Imidacloprid WP. The control effects of these four insecticides showed the same tendency for D. koreanus.【Conclusion】During the period of the middle February to the early March and the late May to the early June, both 4 000 fold diluents of 22% Sulfoxaflor SC and 4 000 fold diluents of 22.4% Spirotetramat SC could be used to effectively control the newly moulted nymphae of D. corpulenta and D. koreanus, respectively.