Spinosad effectively controls Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae) in organic potato

Abstract

Commercial formulations of spinosad applied at three concentrations of 0.2, 0.1, and 0.05%, and three temperatures of 15, 20, and 25 °C, were tested against Colorado potato beetle, Leptinotarsa decemlineata (Say) under laboratory conditions. Spinosad was toxic by both contact and ingestion. Investigations of interactions between temperatures and doses of spinosad showed that a temperature of 15 °C and a concentration of 0.2% caused a statistically higher mortality of adult insects than did other temperatures and concentrations. This was confirmed in another experiment involving control of the overwintered beetles, in which the mortality of larvae was statistically higher at 0.2% but the temperature had no influence on the results. The Colorado potato beetle was found to be highly susceptible to spinosad and therefore could be effectively controlled in organic potato fields.

Keywords:

• Environment
• mortality
• organic farming
• pest
• Saccharopolyspora spinosa
• susceptibility

Introduction

The Colorado potato beetle, Leptinotarsa decemlineata (Say), CPB, is a very harmful pest of potato in Europe's organic crops. If left unchecked, it can reduce tuber production substantially by defoliating the plants. CPB infestation can be controlled by using crop rotation and synthetic insecticides such as imidacloprid or neonicotinoid compounds, which are acceptable for conventional agriculture. Unfortunately, this pest rapidly develops resistance to insecticides (Stanković et al., 2004; Alyokhin et al., 2007; Cooper et al., 2007). Additionally, use of synthetic insecticides in organic farming systems is forbidden; only environmentally friendly substances are acceptable. They show considerable efficacy in controlling this pest, but their mode of action is slow (Koul et al., 1990; Scott et al., 2003). To control Colorado potato beetle in organic crops many natural substances can be used. One of them is azadirachtin and its efficacy towards CPB has well been established (Kühne et al., 2008). Others such as a pyrethrum, refined rape oil, and slaked lime are under investigation (Trdan et al., 2007; Kühne et al., 2008).

Spinosad is derived from fermentation products of an actinomycete bacterium, Saccharopolyspora spinosa (Sarfraz et al., 2005). It is compatible with the FAO/WHO ‘Codex Alimentarius’ standards, the requirements of the American ‘National Organic Program’, and the Standards of International Federation of Organic Agriculture Movements (IFOAM). In 2008, spinosad was written into Annex IIB of the Regulation No. 2092/91, which includes all substances allowed for protection of organic crops in Europe (Council Regulation, 1991). A considerable advantage of spinosad is that 70–90% of beneficial insects and predatory wasps are unharmed, except for bees (Morandin et al., 2005). It is commonly used to control pests of ornamental plants in greenhouses and orchards. Insects from the Chrysomelidae family present different susceptibilities to spinosad. Worldwide, examples of insect pests controlled by spinosad, including L. decemlineata, are numerous. In Poland, products containing spinosad are registered for use against pests other than CPB. However, it is likely that the recommendations on the label of the spinosad-based product may well be extended by the producer in the near future. Therefore, the results obtained in the experiments presented here can contribute to our better understanding of the susceptibility of CPB to spinosad and may be helpful in improving the strategy of protection of organic potato crops.

The current study was aimed at the evaluation of the toxicity of commercial spinosad formulations to L. decemlineata, since the literature data provided only limited reports on general usage of spinosad for controlling larvae and beetles of CPB (Igrc Barčič et al., 1999; Olson et al., 2000; Mota-Sanchez et al., 2006). As the temperature of the environment may influence both the efficacy of the tested substance and differences in the susceptibility of development stages, the aim of this work was to investigate the impact of post-treatment temperature on mortality of larvae and adults of Colorado potato beetle after application of an aqueous solution of spinosad.

Materials and methods

Laboratory bioassay

In June and July, the individuals for the trial were collected in the Western region of Poland. Larvae of the 3rd and 4th instars and adults (unknown age) were used in the experiments. Bioassays were conducted under laboratory conditions using glass Petri dishes. The bottom of each Petri dish was 7 cm in diameter. The Petri dishes were covered with potato leaves and five individuals of the pest at the same development stage were placed in each dish. Aqueous spinosad was applied to the individuals and leaves using a Kwazar® sprayer (volume 0.5 L) which delivered droplets ranging between 50 and 100 m diameter at a pressure of 0.2 MPa. Experiments were carried out with Biospin SC®, a commercial formulation of spinosad (120 g active ingredient (a.i.)/L product; DowAgroScience). The experiments were arranged with different batches of product. Solutions of product at concentrations of 0.2, 0.1, and 0.05%, containing 0.24, 0.12, and 0.06 g a.i., respectively, were prepared in distilled water. The volume of water (500 L/ha) was equivalent to that applied in the field. The control dishes were treated with neat water. In order to investigate the effect of post-treatment temperature, the glass Petri dishes were incubated at three temperatures (15, 20, and 25 °C) in the dark, at 50% relative humidity in ten repetitions, i.e. in each replicate 50 insects (10 Petri dishes, each one containing 5 insects) were used at each temperature for each developmental stage (L3/L4, and adults). In the years 2005–2008, in the months from April to July (during which time measurements were taken against CPB), temperatures between 9 and 24°C were noted under Polish field conditions. Consequently, the trials were carried out at temperatures from 15 to 25 ^oC. The adequacy of the potato leaves was checked daily and they were replaced if necessary. The mortality was evaluated on the sixth day after application. The criterion for mortality was the inability of the insect to respond to prodding with a fine brush.

Statistical analysis

The numerical data were subjected to analysis of variance (ANOVA). The ANOVA test was performed for two treatment variables. Therefore, the tests were limited to the analysis of interactions between temperature and spinosad application rate. The life stages of insects used in the tests were analysed separately, as they were characterized by differing susceptibility to the insecticide. The significance of differences from the main trend was examined by using Tukey's multiple range test at P<0.001. The data are presented in the form of untransformed means±standard error (SE).

Results and discussion

The ANOVA showed a significant interaction between temperatures and concentrations of spinosad for adult insects (F =13.87, df ₁=6, df ₂=227, P<0.001) as well as for larvae (F=8.06, df ₁=6, df ₂=226, P<0.001).

After 6 days, at concentrations of 0.05, 0.1, and 0.2% and at a temperature of 15 °C, mean larval mortality was 9, 23, and 49, respectively, whereas adult mortality was 9, 26, and 49, respectively. At concentrations of 0.05, 0.1, and 0.2% and temperature of 20 °C, mean larval mortality was 18, 24, and 46, respectively, whereas adult mortality was 16, 36, and 39, respectively. And finally, at concentrations of 0.05, 0.1, and 0.2% and a temperature of 25 °C, mean larval mortality was 17, 37, and 47, respectively, whereas adult mortality was 21, 31, and 30, respectively. The results are presented in Table I.

Table I. Mortality of different development stages of Colorado potato beetle to spinosad, six days after treatment.

Spinosad concentration (%)	Mean combined mortality of adults±SE/replicate	Mean combined mortality of larvae±SE/replicate
Temperature 15 °C
0.2	49±1.9a	49±1.8a
0.1	26±1.9bcd	23±1.8c
0.05	9±1.9e	9±1.8d
Temperature 20 °C
0.2	39±1.9b	46±1.8ab
0.1	36±1.9b	24±1.8c
0.05	16±1.9ed	18±1.8cd
Temperature 25 °C
0.2	30±1.9bc	47±1.8a
0.1	31±1.9bc	37±1.8b
0.05	21±1.9cd	17±1.8cd
Untreated-mean mortality of adults (A) = 0.0f, mean mortality of larvae L3/L4 (L) = 2.0d. Within the column for adults, values carrying the same letter do not differ significantly (P<0.001, Tukey's test). Within the column for larvae, values carrying the same letter do not differ significantly (P<0.001, Tukey's test), n=50.

Spinosad may be variously toxic depending on the insect species, stage of development, and mode of application. This insecticide is effective also against insect species that are resistant to some synthetic insecticides (Mota-Sanchez et al., 2006) and has a limited impact on nontargeted organisms (Mertz & Yao, 1990). Among the beetle species, toxicity varies; for example, spinosad was over 400-times more toxic to adult Rhyzopertha dominica [Fabricius] than to adult Tribolium castaneum [Herbs] (Fang & Subramanyam, 2003).

Investigations of temperature dependence on insecticide efficacy show negative correlation between temperature and mortality of pests or prove the opposite (Shipp & Zhang, 1999; Arthur & Dowdy, 2003). This factor is the most important in the case of using biological agents in a strategy of protection (Vassilakos et al., 2006; Kim & Alston, 2008). The effect of spinosad depends on the mode of application, temperature, and exposure time (Amarasekare & Edelson, 2004). Treatment with spinosad has resulted in a similar mortality of grasshoppers, Melanoplus differentialis [Thomas] at all temperatures (ranging from 10 to 35 °C), except for 10 °C. Spinosad dust can be used as an alternative to traditional grain protectants, but its effectiveness is highly determined by the target species, commodity, dose, and temperature (Athanassiou et al., 2008). The results of the present investigation indicate that temperature has an influence on efficacy of spinosad tested against different stages of development of CPB. This fact is the most important in field application compared with laboratory conditions. Therefore, special attention should be paid to this phenomenon. The interaction between temperature and dose of spinosad showed that a temperature of 15 °C and a concentration of 0.2% caused statistically higher mortality of the tested pest adults compared with other temperatures and concentrations tested. This fact could be confirmed in controlling the overwintered beetles. Mortality of larvae was statistically higher at a concentration of 0.2% compared with other concentrations, and temperature had no influence on the results. Later stages of CPB, which are more difficult to control, are highly susceptible to spinosad and can be effectively controlled in organic potato fields.

Study of interactions between temperature and spinosad doses has shown that a temperature of 15 °C and a concentration of 0.2% of formulation of spinosad causes a statistically higher mortality of L. decemlineata adults compared with temperatures of 20 and 25 °C. The obtained results clearly indicate that under field conditions the first beetles on the plants should be controlled using a concentration 0.2% of Biospin SC® when the temperature of air is below 20 °C. No differences at 20 and 25 °C between mortality of adults at 0.2 and 0.1% concentration of formulation were found. In the field, at the range of temperatures between 20 and 25 °C, a lower concentration (0.1%) of the product could be used. It was observed that a 0.2% concentration of Biospin® caused a statistically higher mortality of larvae than do other concentrations, whereas temperature had no influence. Also, temperature had no influence on mortality of CPB adults and larvae at 0.05% concentration of Biospin®.

Acknowledgements

This work was financially supported by the Polish Ministry of Science and Higher Education, Grant No. NN 310 4358 33 for the years 2007–2010.