The effects of a synthetic insecticide and a mineral oil on alfalfa insect pests

SUMMaRY The effects of the synthetic insecticide Eforia 043 ZK (thiamethoxam+lambda-cyhalothrin), applied alone and at 1/5 and 2/5 reduced doses in a mix with the mineral oil Akarzin, on Tychius flavus Beck. (Coleoptera: Curculionidae), Adelphocoris lineolatus Goeze (Hemiptera: Miridae), Acyrthosiphon pisum Harr. (Hemiptera: Aphididae), harmful thrips (Thysanoptera) and cicadas (Hemiptera: suborder Auchenorrhyncha) were studied. The efficacy of Eforia (applied alone and in reduced doses) was significantly higher against A. pisum , followed by A. lineolatus and harmful thrips, compared to cicadas. The insecticidal action of Akarzin had a better protective effect against A. pisum , followed by A. lineolatus , in comparison with thrips and cicadas. Combined application of the broad-spectrum insecticide Eforia provided good control against insect pests. The mix of Eforia’s reduced 1/5 dose with the mineral oil showed the highest toxicity throughout the reporting period. Eforia applied at the reduced dose of 2/5 with Akarzin followed in efficacy. A synergistic effect was observed in combinations and was defined as subadditive synergism. Reduced dosage of Eforia and decrease in negative environmental impact of the plant protection products is a friendly approach to integrated pest control. It is important to apply innovative formulations of plant protection products that are safer both for plants and the environment.

and on useful organisms in agroecosystems. The adverse effects of these pesticides on human health and the environment have been well documented and proven (Yeo et al., 2003). Also, consumers are exerting increasing pressure to achieve the lowest possible amount of pesticide residues in food (Vu et al., 2007). Using synthetic insecticides can provide excellent control of pests with minimal side effects. However, their sparing use is encouraged within integrated programs to ensure sustainable yields (Dimitrov, 2008) and to reduce the possibility of resistance developing in insect pest populations.
Various studies have aimed not only to look for options and combinations to increase productivity and crop quality but also to improve pest control by combining the use of insecticides with growth regulators, biostimulators and leaf fertilizers in integrated production . Some studies have confirmed a synergistic effect when combining insecticides with mineral oils (Al-Mrabeh et al., 2010) and lower toxicity to beneficial entomofauna compared with other pesticides (McLaren, 2008). Combined treatments not only improve the efficiency of agricultural production but also overcome negative consequences of abiotic and biotic stress (Nikolova & Georgieva, 2015).
Reduced use of plant protection products, and their mixing with various products that stimulate plant growth and development, is another environmentfriendly approach to integrated pest control. It aims to reduce negative pesticide impacts on the environment. It provides not only good plant protection and higher productivity but also an opportunity to increase plant resistance to certain pests (Tsibulko et al., 2000;Demkin, 2007). It is also important to apply selective pesticides which cause minimal side effects on the biocenoses, and to optimize their doses (Nikolova, 2010).
The aim of this study was to investigate the biological activity of the synthetic insecticide Eforia, i.e. the effects of its application alone, and its reduced doses in a mix with the mineral oil Akarzin, as a method of integrated pest control in alfalfa seed production.

MaTeRiaL aND MeTHODS
A field experiment was carried out in an alfalfa crop (cultivar 'Dara') under non-irrigated conditions at the Institute of Forage Crops (Pleven) over the period [2014][2015]. The effects of the synthetic insecticide Eforia 043 ZK, applied alone, and its reduced doses in a mix with the mineral oil Akarzin, on lucerne seed weevil, Tychius flavus Beck. (Coleoptera: Curculionidae), alfalfa plant bug, Adelphocoris lineolatus Goeze (Hemiptera, Miridae), pea aphid, Acyrthosiphon pisum Harr. (Hemiptera, Aphididae), harmful thrips (Thysanoptera) and cicadas (Hemiptera: suborder Auchenorrhyncha) were studied (test variants are listed in Table 1). Treatments were carried out at the beginning of the flowering stage in the second regrowth (ВВСН 59-60) for control of those important insect pests as a method of integrated pest management. A split-plot method was used with three replications and fertilization for soil supply with major nutrients. The efficacy of the insecticides was calculated according to a formula by Henderson & Tilton (1955) by recording insect population: one day before treatment in the control plot, and 1, 3, 7 and 9 days after treatment. The method of sweeping with an entomological net was used.
Data were subjected to one-way ANOVA, and the means were compared by Tukey's test at 5% probability (p≤0.05). Multiple Regression Analysis of Statgraphics Plus (1995) for Windows Ver. 2.1 was also used.

ReSULT aND DiSCUSSiON
Control of Acyrthosiphon pisum by applying the synthetic insecticide Eforia at the manufacturer's recommended dose, and at 1/5 and 2/5 reduced doses in a mix with Akarzin, was excellent in 2014 (Table 2). Toxic effects ranged from 97.7 to 99.8% with minor differences on the first day after treatment (DAT). Treatment with Akarzin alone also exhibited a high insecticidal efficacy (94.8%), although it was significantly below the effects of the variants with Eforia. Three DAT, the efficacy of the synthetic product in different variants exceeded 90%, and significantly the highest was achieved by the mix of 1/5 reduced dose of Eforia and Akarzin. A similar trend was noted on the seventh day when the toxic effect of Eforia on aphids remained high (83.7 -89.8%). Efficacy was statistically higher after the use of combinations. Nine DAT, a high efficacy of 72.0-75.9% was again exhibited by the combinations and the differences between them were minimal. On the last date, Eforia treatment alone provided a satisfactory protection, considering a reduction in insecticidal activity. A significant reduction in the efficacy of mineral oil was also confirmed.
In 2015, Eforia treatment resulted in weaker protection of plants against pea aphids. Its toxic effect on the first day was high and ranged from 89.0 to 97.0%. The mix with 1/5 reduced dose of Eforia was found to have a significantly higher efficacy than the use of Eforia alone. The efficacy of the insecticide, regardless of dose, remained high, exceeding 80% on the third reporting day (81.4-86.0%), and the differences were insignificant. On the seventh day, the synthetic insecticide manifested a higher toxic effect compared to its use alone and the differences were statistically significant. A satisfactory protective action of Eforia, regardless of the method of use, was observed nine DAT, when the efficacy exceeded 50.0%. Akarzin used alone, showed a low efficacy (28.0%). It was maintained until the ninth day and the differences from the other variants were significant.  The synthetic insecticide used alone and in combinations against Adelphocoris lineolatus exhibited high efficacy from the first (95.3-96.5%) to the seventh DAT, considering also treatments conducted in 2014 (79.6-84.9%) ( Table 3). The products provided good protection against the pest species nine DAT. The combinations showed an extended duration of action and significantly better toxic effect, compared to the other variants, from the third to ninth DAT. There were no significant differences between the combinations. The efficacy of the mineral oil was high one week after treatment, and its insecticidal action decreased significantly nine DAT.
Similar results and high efficacy of Eforia variants were found one and three DAT in 2015. The mix of Akarzin with Eforia at 1/5 reduced dose had the highest toxic effect during the reporting days. The combination had a significantly different effect from the individual use of the insecticide one and three DAT, compared to the other Eforia variants. It provided longer plant protection, including the seventh day in 2015, with significant difference from Eforia applied alone. On the last reporting day, insecticide treatments showed a similar satisfactory effect, exceeding 50%, but the differences were insignificant. The mineral oil, used alone, did not have a satisfactory influence on A. lineolatus density during that year. Eforia at 1/5 reduced dose and Akarzin contributed to significantly greater efficacy than the synthetic insecticide alone from the first to the seventh DAT, including the day of treatment.
Eforia, applied against another key insect pest in seed production of alfalfa, Tychius flavus, achieved excellent control and the efficacy ranged from 92.9 to 95.0% on the first DAT in 2014 (Table 4). It remained high on the third day, between 83.3-85.4%, and differences were insignificant. On the seventh day, values of less than 71% were noted. A statistically higher efficacy was found after Eforia was used alone and in combination at the dose of 100 ml. Unsatisfactory results were observed in all Eforia variants on the ninth day. The mineral oil had an insecticidal effect against T. flavus, but it occurred only on the first DAT.
In 2015, Eforia was less efficient than in the previous year as its efficacy ranged from 58.0 to 66.0% on the first day. It remained satisfactory on the third day. Differences between Eforia used alone, and 1/5 reduced dose of Eforia with Akarzin were significant on both reporting days. Mineral oil exhibited an unsatisfactory effect on weevils. A decrease in efficacy of less than 50% was observed in Eforia variants on the seventh day and below 40.0% on the ninth day, and the differences were not statistically significant. The number of cicadas and thrips during the study period was sufficiently high to allow evaluation of the impact of products on those two groups of insects. Auchenorhyncha was represented by 7 species -Empoasca pteridis Dhlb.; Erythroneura ribauti Oss.; Peragallia sinuata M.R.; Psammotettix striatus L.; Hyalesthes obsoletus Sign.; Reptalus panzeri P. Lőw; and Philaenus spumarius L. E. pteridis had a relatively high representation among them. A significant reduction in the total number of cicadas was observed in the treated field, compared to the untreated control (Table 5).  The broad-spectrum insecticide Eforia used in combination in 2014 provided good control in the first few days, and product toxicity approached or exceeded 80%. The mix of Eforia reduced 1/5 dose with mineral oil showed the highest toxicity throughout the reporting period. Its efficacy decreased seven and nine DAT, but the effect remained satisfactory. Statistical differences of that variant in relation to the insecticide used alone were detected on the third, seventh and ninth DAT. Eforia alone, and Eforia 2/5 reduced dose + Akarzin exhibited a slightly protective effect on the last reporting day (9 DAT). A satisfactory insecticidal effect of the mineral oil was found only one DAT, and then the cicada counts became comparable to the untreated control.
The impact of products was lower in 2015 than in 2014, but they still provided good protection three DAT. A prolonged insecticidal effect, and a significantly higher efficacy than the other variants were found after treatment with Eforia 1/5 reduced dose + Akarzin (there were no statistical differences, compared to Eforia 2/5 reduced dose + Akarzin on the seventh DAT). Akarzin showed unsatisfactory insecticidal action in that year.
Thrips species found in the second regrowth were: Thrips tabaci Lindeman, T. atratus Haliday, Frankliniella intonsa Trybon, Haplothrips niger Osborn and Odontothrips confusus Priesner. Among those harmful species, T. tabaci had the highest density, followed by F. intonsa. Excellent control and high efficacy, sustaining over nine DAT, were found for Eforia at its 1/5 reduced dose in the mix with Akarzin (within 74.3 -94.1%) in 2014 (Table 6). Differences from the other variants in thrips counts were statistically significant over the reporting period, except for the mix with 2/5 reduced dose on the first and third DAT. Akarzin and Eforia applied at 2/5 reduced dose exhibited a significantly higher efficacy to the insecticide treatment alone in the first three days but on the seventh and ninth DAT the differences were insignificant. Mineral oil had a good toxic effect on the first day after use, satisfactory on the third, but it lost its impact on thrips seven days after that.
Despite the lower density of pest species in 2015, the products exhibited similar effects as in 2014. The efficacy of Eforia variants was high three DAT, followed by a gradual reduction until the ninth day.
The main other pests of alfalfa seed crops, beside those listed, were Heliothis maritima Butler (Lepidoptera: Noctuidae), Bruchophagus roddi Guss (Hymenoptera: Chalcididae), Contarinia medicaginis Kieff (Diptera: Itonididae), Gonioctena fornicata Brügg (Coleoptera: Chrysomelidae), and Subcoccinella vigintiquatuorpunctata L (Coleoptera: Coccinellidae). They can cause serious damage and a considerable reduction in yield and/or the quality of alfalfa seed. In the present study, the population density of those pests was low over the period of second regrowth. It made difficult the discovery of significant differences in their numbers after treatment, and resultingly the possibility of establishing the impact of synthetic products against those insect pests.
The products under investigation exhibited the most toxic and long-lasting effects against insect pests in the following descending order: A. pisum, A. lineolatus, phytophagous thrips, T. flavus and cicadas ( Table 7).
The efficacy of Eforia (applied alone and in combination) was statistically higher against A. pisum, A. lineolatus and harmful thrips, than to cicadas, while the insecticidal action of mineral oil had a good protective effect against A. pisum, followed by A. lineolatus, thrips and cicadas.
The pronounced susceptibility of sucking insect pests to Eforia was probably due to their soft body wrap that facilitates insecticide penetration. As the mix of the active substances of contact and systemic insecticides made a strong synergy between them (according to manufacturer's data), Eforia provided a rapid initial effect (knockdown) and high biological efficacy. Using Eforia at the recommended dose provided high or excellent pest control until the seventh DAT, while the product effect on A. pisum and A. lineolatus was more durable. Akarzin exhibited a high insecticidal effect one week after treatment against A. pisum and A. lineolatus, while the control level of harmful thrips was satisfactory over a shorter period. The oil is an emulsive concentrate and its active substance contains 85% mineral oil -paraffin type, which acts asphyxiating on insects.
Mineral oils have been used for a long time for control and management of insect pests, both as adjuvants (allowing an increase in toxicity of pesticides) and as insecticides. They cause insect death by inhibiting their respiratory activity or by insecticidal effects, depending on their physical properties (Gardner-Gee et al., 2013). In Bulgaria, Yankova et al. (2009) revealed а high efficacy of Akarzin in control of green peach aphid, Myzus persicae Sulz., in pepper. Atanasov et al. (2012) reported a combined use of Akarzin with a fungicide for rust control of Tranzschelia pruni-spinosae (Pers) Diet. in a late ripening plum variety. Beside the positive activity of mineral oils, some studies have established a synergistic effect when combining mineral oils with insecticides (Al-Mrabeh et al., 2010), and protection of plants from different viruses by suppression of their transmission (Suranyi, 1999;Margaritopoulos et al., 2009), as well as lower toxicity of the oils to beneficial entomofauna, compared to other pesticides (McLaren, 2008).
Using mineral oils for pest control, according to some authors (Stansly et al., 2002), may cause delays in vegetative plant stages, manifestations of phytotoxicity and yield loss. Various factors determine phytotoxic effects: dose, oil formulation (physical and chemical properties), crop type and environmental conditions, and above all air temperature (Davidson et al., 1991;Аngello, 2002). Most formulations of different oils have been improved over the past decade to achieve more efficient and less harmful products (Al-Mrabeh et al., 2010).
Phytotoxicity was reported in the 1990s when formulations were different. Besides, mineral oils were less harmful to the environment, and caused less adverse effects in pest biological control compared to chemical pesticides (Rae et al., 2006). In this study, there was no delay in phenological stages or any phytotoxic events after Akarzin was used. It complemented the positive insecticide characteristics as a plant protection product and allowed its application in integrated pest control. The high-tech formulation of Eforia, on the one hand, allowed the mixing of the insecticide with various plant protection products, including Akarzin. When combined with insecticides, mineral oils improve the effect of a preparation by reducing surface tension of the working solution, and so creates a uniform film on leaves of treated plants and insect body (according to Bayer Crop Science).
In the present experiment, treatment with 1/5 and 2/5 reduced doses of Eforia and Akarzin increased efficacy and extended the period of plant protection. That was most evident in leaf aphids, as well as plant bugs.
The observed synergistic effects of the combinations were defined as subadditive synergism as it did not exceed the cumulative toxic effects of Akarzin and Eforia individually. Adding Akarzin probably enhanced the penetration of Eforia into leaf tissue, and enabled a more rapid translaminar activity. It resulted in an increase in insecticide efficacy when the active substance was reduced to 1/5 and 2/5 of the recommended dose. The results are consistent with data from other studies (Carvalho Guedes et al., 1995;Pluschkell et al., 1999), according to which the use of mineral oils with insecticides improved their penetration into plant tissue and contributed to better insect pest control. Mujica et al. (2000) found that addition of plant oils to the active substance abamectin increased its efficacy to the extent that the active ingredient may be reduced between a half and three-quarters of its normal dose against the leafminer fly, Liriomyza huidobrensis Blanchard. Similar findings have been reported for other pests, such as Tetranychus urticae (Schuster & Everett, 1983) and Plutella xylostella (Abro et al., 1988). In our country, Yankova et al. (2009) reported an increased efficacy of insecticides at reduced doses in combination with Akarzin. Another factor affecting the insecticidal activity of test products, besides application type (either alone or in combination), was the weather in experimental seasons ( Table 8). The tested plant protection products had a weaker protective effect in 2015 because of a lower average daily air temperatures in combination with a considerable amount of rainfall. Especially indicative were the periods 11-20 June 2014 and 21-31 June 2015, when treatments were performed, and the amount of rainfall in 2015 was 44.0 mm more than in 2014 over the period. Also, despite an oily film that formed on leaves of the treated plants, the retention period was different in the two seasons. In 2015, the oily film on leaves after treatment with Akarzin (alone and in combination) lasted for 15 days. Compared with the previous year of 2014 (18 days), the retention period was shorter due to more rainfall, which had a negative impact on the efficacy of products and their after-effects. Supporting the results were trends revealed by some authors which showed that, under field conditions, oils remained on leaf surface for 10-14 days after application (Simons et al., 1977). Also, small amounts of rainfall (<25 mm per day) did not remove the oily film from treated leaves (Simons & Zitter, 1980).
The results of the analysis of variance (Table 9) showed that all four factors: year, insect species, products and their effects (DAT) had different degrees of impact. Factor C (product type) had a dominant influence and a significant impact on efficacy -40.7%. The effects of factor D and factor B were weaker (22.0 and 13.7%, respectively) but statistically significant. Meteorological conditions over the years had also lower values but the effect on efficacy variation was significant -8.4%. Regarding the possible 11 interactions among factors A, B, C and D, only the combination of year, insect species and product (A x B x C) had a statistically significant impact of 4.2%.
From the aspect of alleviation of negative environmental impacts of plant protection products, the use of the synthetic insecticide Eforia at reduced optimum doses in combination with the mineral oil Akarzin is an environment friendly approach to integrated pest control. It is also important to apply innovative formulations of plant protection products that are safer both for plants and for the environment.

CONCLUSiONS
The efficacy of Eforia (applied alone and at 1/5 and 2/5 reduced doses in a mix with Akarzin) was statistically higher against A. pisum, as well as A. lineolatus and harmful thrips than to cicadas.
The insecticidal action of the mineral oil Akarzin had a better protective effect against A. pisum, followed by A. lineolatus, in comparison with thrips and cicadas. Combined use of the broad-spectrum insecticide Eforia provided good control of the test insect pests. The mix of 1/5 reduced dose of Eforia with mineral oil achieved the highest toxicity throughout the reporting period. Eforia used at the reduced dose of 2/5 with Akarzin followed it. A synergistic effect was observed in combinations and was defined as sub-additive synergism.
To reduce the negative environmental impact of plant protection products, the synthetic insecticide Eforia may be used at doses below label rate and in combinations with the mineral oil Akarzin as an ecological approach to integrated pest control.