Efficacy of three bioinsecticides for control of Colorado potato beetle on potatoes

02033


Introduction
The Colorado potato beetle, Leptinotarsa decemlineata Say (Coleoptera: Chrysomelidae), is native to central Mexico.Larvae and adults mainly feed on the foliage of host plants, with larvae causing large economic losses to farmers.Potatoes are the favorite food source of the Colorado potato beetle, but the beetles can also feed on agriculturally important crops such as cabbage, peppers, tobacco, eggplant, and tomatoes [1].In total, each female Colorado potato beetle can lay up to 600 eggs per season.The main damage to potato leaves is caused by larvae.They can lead to 100 percent defoliation several months before the end of the growing season, reducing the yield of potato tubers by more than 50 percent [2,3].
Plant protection against diseases caused by various pathogens, including Colorado potato beetle, is an economically and socially important problem, as crop production losses in different countries amount to 20 percent of the yield.Therefore, there is high interest worldwide in finding new insecticides for pest control.Chemical control methods (use of chemical pesticides) are widely used to protect plants.Despite high efficiency, chemical preparations have a number of serious disadvantages: insufficient selectivity of action on phytopathogens, environmental pollution, the possibility of resistance development in pathogens and others.The use of chemical preparations in practice has shown that their economic efficiency decreases with time.Pesticide-resistant plant pathogens appear, development of new preparations becomes more expensive, which eventually increases the cost of agricultural products.
In recent decades, interest in the use of biopreparations for plant protection has been increasing.According to expert estimates the market of bioinsecticides has been growing at an average annual rate of 15% since 2010.Currently, bioinsecticides account for about 5% of the global crop protection market.Their current value worldwide is around USD 3 billion.According to some estimates, by 2025 it will grow to 4 billion US dollars.In Russia, only 0.3% of agricultural lands are treated with biopreparations [4,5].
Bioinsecticides are becoming increasingly important as they are an alternative to chemical pesticides in pest control.Bioinsecticides are developed based on naturally occurring living organisms such as animals, plants and microorganisms (e.g., bacteria, fungi and viruses) that can control insect pests that damage plants.Unlike synthetic pesticides, microbial pesticides are specific in action and can be easily obtained without the need for expensive chemicals.Bioinsecticides act through various mechanisms that include inhibition and disruption of plasma membrane and translation of pest proteins.But their pest specificity may be a disadvantage if the aim is to control multiple pests simultaneously [6].
Microbial insecticide based on Bacillus thuringiensis bacteria has been used commercially for more than four decades [7].Bacillus thuringiensis synthesizes crystalline proteins and kills several target species of scale insect pests.Bacillus thuringiensis releases endotoxins during sporulation, which are ingested by the beetle larvae.This leads to the destruction of the larvae's intestinal tissues and their paralysis.These highly effective and specific toxins have great potential in agriculture and for the control of disease vector insects.
Methodology of the study Commercial bioinsecticides "Bitoxibacillin", "Bitoxibacillin + GUMI" and "Bitoxibacillin + Actarofid" based on spores of Bacillus thuringiensis var.thuringiensis were the objects of the study.
Specimens of Colorado potato beetle at the larval stage of 1, 2 and 3 instars were taken as test samples.The larva of the first instar is dark gray, covered with hairs, its length is 1.5-2.4mm; of the second instar -red, with sparsely placed hairs, its length is 2.5-4.5 mm; of the third instar -red-yellow (brick), its length is 4.6-9 mm.Starting from the second instar, larvae are able to eat the entire leaf pulp.Larvae of the third instar cause the greatest damage.The larvae were collected during July-August in sunny weather, at a temperature of 25-30°C, on the leaves of potatoes growing on the territory specially allocated for research.
Collected larvae were kept in a room with necessary conditions for their survival in 3liter containers (3 containers for each drug under study and a separate one for control).Each container contained 50 larvae.Feeding of larvae, cleaning of containers, control of illumination, humidity and temperature were carried out daily for 10 days.
Insects were treated with the studied preparations in a separate room without the possibility of contact of experimental groups with each other and without the probability of overlapping effects.A single treatment of each group of individuals was carried out by irrigating them with the required volume of the drug solution using a sprayer.

Results and discussion
Application of commercial bioinsecticides "Bitoxybacillin", "Bitoxybacillin + GUMI", "Bitoxybacillin + Actarofid" resulted in statistically significant 100% reduction of larval populations during 10-day observation.Moreover, the preparation "Bitoxybacillin" provided 100% mortality of larvae on day 8 at a drug concentration of 10 g/l.During 10-day observation, this preparation at concentrations of 2 g/l and 5 g/l contributed to the death of 62% and 74% of larvae, respectively (Figure 1).Combined preparation "Bitoxybacillin + GUMI", containing spores of Bacillus thuringiensis var.thuringiensis and sodium salts of humic acids, resulted in 100% mortality of larvae on the 7th day of observation.As the drug dosage was reduced to 5 mg/L, its effectiveness decreased to a 92% mortality rate at 9 days (Figure 2).
The complex preparation "Bitoxybacillin + Actarofid" includes a mixture of natural avermectins (0.2% content) produced by the microorganism Streptomyces avermitilis.Synergetic interaction of two components of this bioinsecticide contributed to 100% mortality of Colorado potato beetle larvae of all ages already on the 7th day of observation at a concentration of 10 g/l.In the same time interval, 82% and 94% of larvae were killed at the drug concentration of 2 mg/L and 5 mg/L, respectively (Figure 3).The greatest resistance to the action of bioinsecticides at a concentration of 5 g/l on the eighth day of observation was shown by Colorado potato beetle larvae of 3 instars (Table 1).On the tenth day 100% of larvae of all ages died.

Conclusion
The results of the study show that the investigated biopreparations have great potential for biological control of Colorado potato beetle larvae.Combined biopreparations Bitoxybacillin + GUMI and Bitoxybacillin + Actarofid had the highest efficacy during the ten-day observation.
The third instar Colorado potato beetle larvae showed increased resistance to the action of biopreparations.

Table 1 .
Effectiveness of bioinsecticides on Colorado potato beetle larvae (eighth day of observation).