Introduction

The Colorado potato beetle (Leptinotarsa decemlineata Say) has been a very dangerous pest for potato almost 45 years and is an acute problem for the country’s potato industry [1, p. 67]. Insecticides control the number of pest larvae on the crop. However, the parasite quickly adapts to widely used pest control chemicals [2, p. 54]. Without protective measures unstable potato cultivars can lose 25–50 %, in some cases, 80–100 % of tuber resulted from a high number of insects [3, p. 65]. Currently the Colorado beetle has a high level of resistance to most insecticides. Thus, the only economically feasible way to stabilize potato agrocenoses is to develop highly tolerant varieties that can provide a tuber yield of good quality with a minimum level of chemical protection [4, p. 50], [5, p.14].

To successfully create potato cultivars resistant to the Colorado beetle, it is necessary to modify the elements of its interaction with the plant that are critical for the insect's life [6, p. 165]. Breeding potato tolerant to the Colorado beetle is complicated by the fact that this insect is a changeable, genetically and adaptively polymorphic, plastic species. In this regard, when creating resistant varieties, protective genes should act equally on all genotypic forms of the pest in the general population and not cause violations of the population structure [7, p. 85]. The traditional approach to breeding potato cultivars tolerant to the Colorado beetle involves the use of wild potato species S. demissum, S. chacoense, S. commersonii, etc. in crosses. At the same time, the stability of potato hybrids in these combinations is mainly due to the presence of glycoalcoloids in the leaves [8, p. 27]. Zarevo, Peresvet, Bryanskiy nadezhnyy, Nikulinskiy and a number of other cultivars developed on this basis are characterized by the relative resistance against pests at 6–8 points. However, this type of resistance is overcome by the pest, so it is necessary to use insecticides to control the number of insects even on these varieties [9, p. 39].

In this regard, to create potato varieties tolerant to the Colorado beetle, there is a need to find new types of resistance and develop new approaches to creating potato tolerance donors against this type of insects. One of the possible directions of breeding work in this direction is to develop stable genotypes based on the use of a necrotic protective barrier in the form of hypersensitive response of plant leaves to the insect egg clutch. Possible control of the phytophage number based on a necrotic protective barrier has been reported for the first time relatively recently. It was based on the phenomenon observed when individual plants of black mustard (Brassica nigra) in climatic conditions of California valley (significant lack of air humidity) caused the death of eggs laid on the leaves by the cabbage white butterfly (Pieris rapae) and green-veined white butterfly (P. napi). The researchers claimed that the death of the insect clutch occurred as the result of the leaf blade necrosis at the place of the egg attachment followed with leaf drying [10, p. 631]. Insect eggs can’t control their temperature and completely depend on the host plant microenvironment. Thus, necrosis at the clutch place can isolate eggs from the thermoregulation process of the host plant leaf and lead to death [11, p. 3451].

In another example, there was the hypersensitive response of the leaf blade and the necrosis at the place of the Colorado potato beetle egg clutch (Leptinotarsa decemlineata Say) in potato species hybrid Solanum spp. It caused egg detachment, falling on the soil surface and subsequent death [12, p. 655]. Moreover, the selection evaluation of potato hybrids, showed that relatively stable varieties reacted with the necroses on the leaves under the egg clutch, while unstable varieties did not have this reaction [13, p. 10]. However, a detailed study of the effect of this protective barrier on potato tolerance to the Colorado beetle has not been conducted.

It is undoubtedly relevant for further scientific and breeding programs to assess the effectiveness of the necrotic protective barrier on pest resistance and the prospects for its using to develop tolerance of potato varieties against the Colorado beetle [14, p. 69]. Thereby, the authors of the very paper conducted a comparative study of the way the leaf blade hypersensitive response intensity on pest egg clutch decreases the yield of different potato varieties resulted from damage by the Colorado beetle.

Methods

The research was conducted in the Cis-Ural climate zone (northern forest-steppe) on the lands of the Birsk scientific division of the Bashkir Research Institute of Agriculture. The experiment was performed in 2016, 2017 and 2019. Meteorological conditions in the research area for all years of studies were within the climate norm. Annually the period from the moment the Colorado beetle females lay eggs on potato leaves to the development of larvae of the IV age was from the beginning of the 2^nd decade of June to the end of the 3^rd decade of July. This period in 2016, the average daily temperatures were 19.6–23.9 °C and the amount of precipitation was 60 mm. At the same time, there was insufficient water due to a lack of rainfall in the first three decades of this period. In 2017, during the embryonic development of phytophage larvae, the average daily temperatures were within 16.8–21.5 °C, the amount of precipitation was 166 mm, which fell evenly. In 2019, the mean daily temperatures in the studied period were 16.1–20.6 °C; there were 95 mm of precipitation that fell relatively evenly.

The experiments were based on 30 different potato cultivars. Samples were planted on two row plots of 10 plants in three-fold repetition. The planting scheme was 70 × 30 cm. Pest dispersal and egg laying by female Colorado beetles on experimental potato plants took place naturally. At the time of mass larvae hatching from eggs, one row of each plot was fenced off with a protective screen and treated with 0.005 % solution of Regent insecticide at the rate of 200 ml, which is a hectare rate of 10 g/ha. The other row of this plot was not treated with insecticide. The biological effectiveness of the insecticide against larvae of age 1–2 was at the level of 95–98 %. The degree of damage to the potato tops on the unprotected half of the plot was assessed visually 15 days after hatching, when the pest larvae reached age IV and fed intensively before going into the soil for pupation. The damage degree on each Bush plot was evaluated visually on a 9-point scale:

9 points – high stability, weak damage up to 10 %;

7 points – relatively high stability, damage from 10 to 24 %;

5 points – average stability, damage from 25 to 49 %;

3 points – weak stability, damage from 50 to 79 %;

1 point – lack of stability, severe damage from 80 to 100 %.

Each row of the plot was harvested separately. The yield was recorded in the 1st decade of September. The extent of losses from pest damage was estimated by comparing the yield of potato tubers from the protected row and the yield from the row without the use of insecticide.

The development of a leaf blade hypersensitive response to a Colorado potato beetle clutch was evaluated on 3–5 day after the clutch placement on the following scale:

0 points – no hypersensitive response;

1 point – protrusion of the leaf blade at the place of the clutch attachment;

2 points – necrosis on 25 % of the area occupied by the clutch;

3 points – necrosis on 50 % of the area under the clutch;

4 points – necrosis on 100 % of the area under the clutch (fig. 1);

5 points – perforation of the leaf blade at the place of the clutch attachment.

Fig. 1. Development of leaf blade necrosis on 100 % of the area occupied by the Colorado beetle egg clutch (view from the upper (a) and lower (b) sides of the leaf blade), Bashkirskiy variety

The development of the leaf blade hypersensitive response was analyzed under ten clutches found in a row on the plot. Then the average value of the necrosis development extent was deduced in points.

Mathematical processing of the obtained data was performed by calculating the arithmetic mean of 10 observations and the standard error of the average. Correlation analysis of the obtained data was done according to the accepted methodology [15. p. 269].

Results

The results of three years of field experiments indicate that the Colorado beetle is a particular danger as a pest of potato culture. Annual losses from the insect, depending on the variety, reached up to 93 % of the yield (tables 1, 2, 3, 4). At the same time, varietal characteristics were a decisive factor in the level of yield reduction when the crop was damaged by the Colorado beetle. Thus, the data on the reduced yield of potato tubers without pest control by insecticide, in comparison with the conditions of using protection means, prove that the most resistant cultivars had 8–18 times lower yield reduction than the least resistant varieties.

Table 1

The effect of the hypersensitive response of potato leaves to the Colorado beetle egg clutch on potato tolerance to the pest damage, 2016

Table 2

The effect of the hypersensitive response of potato leaves to the Colorado beetle egg clutch on potato tolerance to the pest damage, 2017

Table 3

The effect of the hypersensitive response of potato leaves to the Colorado beetle egg clutch on potato tolerance to the pest damage, 2019