Chromatographic determinations were performed on a HP-1100 Series equipped with UV absorbance detector (Agilent, California, USA). A stainless steel, 250 × 4.6 mm, 5 μm, 120Å C18 column (Merck, Darmstadt, Germany) was used at room temperature. HPLC conditions are as follows. Injection volume 10 μl, flow rate 1.0 ml/min, mobile phase water/acetonitrile (25/75, v/v), wavelength 275 nm.

Water, acetonitrile, and dimethyl phthalate were purchased from Merck (Darmstadt, Germany).

Two sheet type formulations, ISF-I and II, were prepared. 98.6 g of aliphatic polyester resin and 1.4 g of imidacloprid was mixed using tumbler mixer. The mixture was passed through the twin screwed-extruder (Plasti-corder, Brabender, Germany) with cylinder temperatures ranging from 120 to 150^oC. The formulation was passed through the cooling system. These were the form of stripe, 0.3 mm thick and 1.5 cm wide. The formulation recipes were shown in Table 1.

Meanwhile, aliphatic polyester resins used were SG 2109 and SG 1111(SK Chemicals, Korea). The characteristics of SG 2109 and SG 1111 had melting points of 91^oC and 113^oC, intrinsic viscosities of 1.5 and 1.2, and average molecular weights of 31,000 and 25,000, respectively.

About 5 g of sample was crushed using a freeze-crusher. 1.5 g of the crushed sample was placed into a 125 ml cap-stoppered Erlenmeyer flask and 100 ml of acetonitrile: distilled water (1 : 1, v/v) including 1000 ppm dimethyl phthalate as internal standard was added to the flask. The sample was extracted using ultrasonicator (Branson, USA) for one minute. The extract obtained was analyzed by High Performance Liquid Chromatograph (HPLC) and availability of imidacloprid was then determined on the basis of added value of imidacloprid at the stage of formulation.

The formulations were kept in an aluminum package that was sealed using the sealing machine. The package was stored at 50^oC until a given number of days.

Dissipation rate of imidacloprid in the formulations were analyzed following the same procedure used in determining the availability imidacloprid with in a given number of days.

A fixed amount of field soil was placed in a plastic container, 3 cm in diameter. A slice of the sheet formulations are treated in the container. The formulation was covered with small amount of soil

Soil moisture in the plastic container was adjusted to 10 and 60% of maximum water-holding capacity and the plastic container was kept at 50^oC for 25 days. After 25 days, the sample was unpacked and carefully washed. Biodegradation rates were measured by weight loss of the formulations.

The 125 ml cap-stoppered test tube was filled to 100 ml of distilled water. The formulations were then immersed in the water. Released amounts of imidacloprid from the formulation into water were analyzed after selected intervals using HPLC.

The formulations were tested by burying them near the root zones of the rose and chrysanthemum plants. Target insect pests were rose aphids, Machrosiphum ibarae, on rose, Rosa hybrida and cotton aphids, Myzus persicae, on chrysanthemum, Dendranthema grandiflorum, under the greenhouse conditions.

Acephate WP was used as reference formulation and applied ten times during experiment. The number of control plot designates total counts of aphids in ten leaves. After treatment, the control effect and phytotoxicity of the formulations on target insect pests and rose plants were respectively investigated with lapse of time.

As shown in Table 2, the availability of imidacloprid was more than 97%, which suggested thermolysis was negligible. The results suggested that the imidacloprid of the formulations was uniformly dispersed/incorporated on/in the sheet formulations.

Active ingredients of pesticides must be chemically stable during storage although there is no physical transformation of the product. It is annually a basic principle to analyze stability of a.i. in the pesticide formulations by storing the formulations for one year under natural room condition. This is to establish validity of the product (Tanaka, 1981). Generally, the results are analyzed through analogical interpretation using tests on the high temperature condition, because it is difficult to conduct tests over long period. Currently the recommended test durations are two, four, and six weeks in 54 ± 2^oC by Manual on development and use of FAO and WHO specifications for pesticides (2010).

This experiment was conducted to determine the expiry date of the formulations. Samples were kept in a small ampoule with lid immediately sealed. The samples were stored at 50 ± 2^oC until given period of time. The degradation rate of imidacloprid in the formulations recorded less than 1.0% even at 90 days after treatment (Table 3). The formulations were very stable during the experimental periods. Thus, these formulations do not need addition of special stabilizer to stabilize chemically, to extend their expiry dates. The forecasted expiry date of the formulations was more than three years.

This study was conducted to know the pattern of degradation for the formulations applied on soil surface and in soil. The soil moisture was determined during the dry (usually from April to June and October to November) and rainy seasons (from July to September) in Korea.

A slice of the sheet formulations was treated on soil surface and in soil. The degradation was determined after 25 days. The formulation degradation was negligible in 10% water content regardless of site of application.

When the formulations were treated in 60% of water content, the degradation rate was more than 10%. The ISF-II formulation was degraded more 40% when treated in soil. ISF-II was degraded to more about 15% than ISF-I (Table 4). The difference of degradation between the two formulations was possibly caused by the different properties of polyester used. The biodegradable polyester resins are believed to be degraded by hydrolysis and soil-borne microorganisms.

Elution ability of imidacloprid from the formulation has to gush out the pesticide incorporated with the resin by water. Through this, the formulation accomplishes its pesticidal actions. Pesticides that are not gushed out of the product lose their value as pesticides. Therefore, we can say that the elution ability of pesticides is very important.

The amounts of imidacloprid eluted into water were almost 100% at 25 days after treatment for both ISF-I and ISF-II formulations at 20 ± 1^oC (Fig. 1).

Fig. 1. 
Release profile of the insecticide imidacloprid from the sheet formulations into water under static condition.

In case of dispersing controlled release formulations mixed with polymer matrix, release rate of a.i. from formulation is up to concentration of a.i., dispensability, dissolution, and surface area of formulation in polymer matrix of formulation.

It is believed that release rate of a.i. from formulations used in this experiment were not effective due to several factors because the polyester resins used have different properties.

As shown in Table 5 and 6, the control effects of the formulations tested against rose aphid, Machrosiphum ibarae and cotton aphids, Aphis gossypii, were more than 90% even at 173 to 200 days after treatment. Therefore, these results revealed equivalent or superior formulation compared with those applied with Acephate WP 10 times. This is an evidence of a controlled release formulation.

No phytotoxicity to the plants was observed in using the recommended rate or doubled dosage of the formulation during the experimental period.