Residual Efficacy of Certain Insecticides for Protecting Grain Stores from Infestation of Stored Product Insects

INTRODUCTION

Annual post-harvest losses resulting from insect damage, microbial deterioration and other factors such as humidity, temperature, aeration and cleanliness of the bulk storage, are estimated to be 10-25% of production worldwide (Mohan and Fields, 2002). However, insects are the main problem in stored grain because they reduce the quantity and the quality of grains (Madrid et al., 1990).

The red flour beetle Tribolium castaneum (Herbst.) (Coleoptera: Tenebrionidae) is an important worldwide secondary insect-pest of stored products that is observed among several commodities. This pest may cause considerable economical losses if not adequately controlled because it has a very high rate of population increase (Hill, 1990).The red flour beetle is a serious insect-pest species that attacks stored grain products such as flour, cereals, meal, beans and other dried food products; the larvae prefer cereal grain embryos . The female lays tiny white eggs (up to 450/female) that hatch after about 9 days (Sokokoff, 1972).

The rice weevil Sitophilus oryzae (L.) (Coleoptera: Curculionidae) is a major main insect- pest of most stored cereal (rice, wheat, sorghum, barley and maize) worldwide before harvest and in store (Ahmed, 2001; Sabbour, 2012). The adults of the rice weevil are around 2 mm long with a long snout and able to fly. The body color appears to be dark brown, but on close examination, four orange/red spots are arranged in a cross on the wing covers (Halstead, 1964).

          Synthetic insecticides such as lambda-cyhalothrin, primiphos-methyl and chlorpyrifos are currently of the main chemical that can be used to protect stored grains from insects. Spinosad is a new introduced and currently registered compound that can be used in several countries as a grain protectant.

        The spinosyns are a unique family of fermentation-derived insecticides having potent activity and lower environmental effect. Spinosad is a defined combination of the two principal fermentation factors, spinosyns A and D. Structure–activity relationships (SARs) have been extensively studied, leading to development of a semisynthetic second-generation derivative, spinetoram. The spinosyns have a unique mechanism of action involving disruption of nicotinic acetylcholine receptors (Kirst, 2010). Spinosad possesses a unique mode of action in insects and controls insect strains resistant to other grain protectants. When launched globally, spinosad will represent a valuable new addition to the limited arsenal of grain protectants and can positively impact global food security. Its combination of high efficacy, broad insect pest spectrum, low mammalian toxicity, and sound environmental profile is unique among existing products currently used for stored-grain protection (Hertlein et al., 2011). Spinetoram is chemically similar to spinosad, a pesticide approved for use in organic agriculture with an established safety record. Spinetoram is a mixture of chemically modified spinosyns J and L. Formulations are sold under various trade names Delegate^®, Exalt^® and Radiant^®. Spinetoram is a broad-spectrum insecticide used to control crop-damaging insects. It shows high-efficacy against target insects at a very low use rate, with a margin of safety toward beneficial insects. It acts by causing persistent activation of insect nicotinic acetylcholine receptors (Anonymous, 2014). Spinetoram can be an effective alternative to spinosad, and may be used as a grain protectant.

          The objective of this investigation is to evaluate the insecticidal activity of lambda-cyhalothrin, primiphos-methyl, chlorpyrifos and the new insecticide "spinetoram" against both the rust red flour beetle, Tribolium castaneum and the rice weevil Sitophilus oryzae.

MATERIALS AND METHODS

Tested insects

Cultures of S. oryzae and T. castaneum were maintained in the laboratory without exposure to any insecticides on wheat grain and flour wheat, respectively, in glass jars containers kept under the conditions of 25ºC ± 3ºC and 65 ± 5 % R.H., and continuous daily darkness of 24 hrs, except when working inside the rearing cabinet.

 Insecticides

Four formulated insecticides (lambda- cyhalothrin [Lambada- Magic^®5% EC], primiphos- methyl [Actellic^®50% EC obtained from Shora Chemicals, Egypt], chlorpyrifos [Magic-phos^®48% EC] and spinetoram [Radiant^®12 %SC]) were tested. Each insecticide was diluted with water to obtain serial concentrations to be tested against the suggested insects. 

Bioassays

The insecticidal activity of evaluated lambda-cyhalothrin, primiphose methyl, chlorpyrifos and spinetoram was determined by direct contact application. One millimeter of each diluted and prepared concentrations (1 ml) was applied and regularly distributed on filter paper (9cm dia.). Each concentration was replicated 3 times. The filter papers were left over at room temperature to allow the water to evaporate and became dry. Each paper was handled carefully and fixed inside a Petri-dish. Ten adult insects were released into the filter paper and maintained in a Petri-dish that previously treated with the same concentration as that of the filter paper and left at constant room temperature along a period lasted for 72 hours. Mortality determination was done after 24, 48 and 72 hours. The insects were categorized to alive or dead (brittle and showing no movement over a 5 min observation period). This procedure would be easy and rapid method for evaluating the residual activity of a pesticide (Saad et al., 2011).

Statistical Analysis of bioassays data

Probit (mortality)/log con. (Dose) regression equations were calculated using the maximum likelihood algorithm described by Finney (1971) adopted as a computer program. Values of LC₅₀ and LC₉₅'s and associated fiducial limits were also calculated by the method described by Finney. The correction of mortality percentages, if there were any control mortality was done using Abbott's formula (1925). Also, the relative efficiency (Toxicity index and Relative potency) of the tested compounds was determined by the formula of Sun (1950) as follows:

                                     LC₅₀ of the most effective compound

Toxicity index (%) =      --------------------------------------------------------      x 100

                                          LC₅₀ of the tested compound

                                 LC₅₀ of the least effective compound

Relative potency =      ---------------------------------------------------------   =   …… Fold

                                         LC₅₀ of the tested compound

RESULTS AND DISSCUSION

1. Efficacy of the tested insecticides against T. castaneum

Data in Table 1 show the toxicity of different tested insecticides (lambda-cyhalothrin [Lambada-Magic^®5% EC], primiphos-methyl [Actellic^®50% EC], chlorpyrifos [Magic-phos^®48% EC] and spinetoram [Radiant^®12 %SC]) against T. castaneum after 24, 48 and 72 hours of exposure. Table 1 show the LC₅₀ (ppm), fiducial limits, slope value and regression equation of each of these tested insecticides against Tribolium castaneum adults. After 24 hour of exposure, spinetoram was proved to be the most toxic insecticide tested against T. castaneum followed by chlorpyrifos, primiphos- methyl and lambda-cyhalothrin. Also, the same trend as that after 24 hrs was achieved after 48 hrs of exposure. Although lambda-cyhalothrin was shown to be the least toxic tested compound within the first 24 hrs (LC₅₀= 86.29 ppm₎, it was shown that after 72 hrs of exposure that lambda-cyhalothrin was the most toxic compounds as compared with the other tested insecticides.   The calculated LC₅₀ of spinetoram was found to be 4.12 ppm (Table 1).

 From the previous results, it could be also seen that spinetoram was highly toxic against T. castaneum after 24 and 48 hrs of exposure. Vice –versa lambda-cyhalothrin was highly toxic against T. castaneum followed by spinetoram; chlorpyrifos and primiphos-methyl after the exposure period of 72 hrs. Nevertheless, it could be noticed herein that the toxic effect of spinetoram  (LC₅₀=4.12ppm) was merely equal to that of lambda-cyhalothrin (LC₅₀ =3.28ppm) after that same period of 72 hrs exposure.

Table (1): Response of T. castaneum to primiphos- methyl, lambda- cyhalothrin, chlorpyrifos and spinetoram

* x=log concentration

 These results supported the obtained results by Huang and Subramanyam (2003) who reported that spinosad at 0.5 or 1 mg/kg on white wheat was very effective against all the tested species except the red and confused flour beetle T. confusum). Also, Arthur (1992) mentioned that Sitophilus zeamais or Tribolium castaneum did not survive in the case of the application of deltamethrin + chlorpyrifos to corn. Meanwile, results of Khashaveh et al. (2008) revealed that the application of spinosad dust formulation at higher rates and for longer exposure intervals could control T. castaneum in different oilseed types.

Denloye et al. (2008) reported that both Sumithion^® (fenitrothion) and Actellic^® (primiphos-methyl)  were effective for controlling C. maculatus and S. zeamais at concentrations higher than that of 5 mg/kg which have been recommended by manufacturers.

Table 2 shows the LC₅₀ (ppm), Toxicity index (%) and Relative potency (fold) of the four tested insecticides against the rust red flour beetle adults (T. castaneum) after 24, 48 and 72 hours.  

Table (2): LC₅₀ values, Toxicity index and Relative potency of the tested insecticides against T. castaneum adults (after 24, 48 and 72 hrs bioassay)

After 24 hrs bioassay, it was confirmed that lambde-cyhalothrin was the least efficient toxicant (LC₅₀ = 86.29 with toxicity index equal to 21.6% and relative potency of 1.00 1fold, respectively). After 48 hrs, spinetoram still had a strong action on T. castaneum (LC_{50 =} 9.49 ppm, toxicity index 100% and relative potency 2.42 fold), followed by chlorpyrifos (LC₅₀ = 14.63 ppm, toxicity index 64.87 % and relative potency 1.57 fold). Primiphos-methyl was the lowest efficient toxicant (LC₅₀ = 22.93 ppm with a toxicity index of 41.39% (of spinetoram) and relative potency of 1.00 fold). After 72 hrs, each of spinetoram and lambde-cyhalothrin gave strong action on T. castaneum represented by their high toxicity and reduced LC₅₀ values(Toxicity index of 79.61 & 100% and relative potency of 3.58 & 4.49 folds, respectively.)

2. Efficacy of the tested insecticides against S. oryzae

Table 3 shows the extracted parameters of the toxicity of different tested concentrations of evaluated insecticides expressed as the LC₅₀ value (ppm), fiducial limits, slope value and regression equation of each of  these tested insecticides against Sitophilus oryzae adults after 24, 48 and 72 hours of exposure. After 24 hrs of exposure, chlorpyrifos and spinetoram were equally high toxic against S. oryzae adults showing merely the same LC₅₀ values of 14.56 and 13.38ppm, respectively.

Again, the further exposure of the adults of therice weevil S. oryzae to the tested insecticides up to 48 and 72 hrs revealed that chlorpyrifos was as toxic as spinetoram and comparatively were more toxic and superior to the other tested compounds (LC₅₀=5.73 and 5.29 ppm, respectively) after a 72 hrs bioassay versus lambda-cyhalothrin which was the least toxic compound (40.62ppm). Moreover, chlorpyrifos was as toxic as spinetoram.

   From the previous results, it could be concluded that spinetoram was the utmost highly toxic insecticide against S. oryzae followed by chlorpyrifos, and primiphos-methyl after 24 and 48hrs. Also spinetoram was the superior and highly toxic one against S. oryzae, followed by chlorpyrifos, lambda-cyhalothrin and primiphos-methyl after 72hrs.

Samson and Parker (1988) found that deltamethrin was not effective against Sitophilus spp. Our results agree with those results reported by  Kljajic et al. (2007) who found that the most toxic insecticides to S. oryzae adults were bifenthrin and dichlorvos, and the least toxic was pirimiphos-methyl. Also, Getchell and Subramanyam (2008) reported  on the comparison of the time required for killing 50% (LT₅₀) and 95% (LT₉₅)  and showed  that R. dominica adults were consistently and significantly more susceptible and died quickly than S. oryzae adults when exposed to spinosad treated commodities.

Kavallieratos et al. (2010) stated that the lowest dose of spinosad was highly effective (>90%) against R. dominica and S. oryzae. In the case of T. confusum combination of longer exposures with higher doses was required for each formulation to be effective. Our results disagree with those arrived at by Rumbos et al. (2013) who found that Sitophilus species were highly susceptible to two pirimiphos-methyl formulations, since complete mortality (100%) was achieved while the present investigation showed that S. oryzae was more susceptible to spinetoram and chlorpyrifos.

Table (3): Response of Sitophilus oryzae to primiphos-methyl, lambde cyhalothrin, chlorpyrifos and spinetoram

*x=log concentration

The exhibited results in Table 4 show the calculated values of LC₅₀ (ppm), Toxicity index (%) and Relative potency (fold) of the four tested insecticides against the rice weevil adults S. oryzae after 24, 48 and 72 hours. After 24 hrs, chlorpyrifos had the strongest action against S. oryzae (LC_{50 =} 17.09 ppm with a toxicity index of 100% and relative potency of 4.80 fold). Spinetoram was as effective as chlorpyrifos LC_{50 =} 17.22 ppm with a toxicity index of 99.24% and relative potency of 4.76 fold) followed by primiphos-methyl (LC₅₀ = 35.05 ppm with toxicity index equal to 48.76% and relative potency of 4.27 fold). Lambde-cyhalothrin was the lowest efficient toxicant (LC₅₀ = 82.04 with toxicity index equal to 20.83 and relative potency of 1.00 fold). After 48 hrs of exposure, spinetoram had a strong action on S. oryzae (LC_{50 =} 9.33 ppm with toxicity index of 100% and relative potency of 4.35 fold), while lambde-cyhalothrin was still the lowest efficient toxicant (LC₅₀ = 40.62 ppm, with toxicity index of 22.97% and relative potency of 1.00 fold).  Furthermore, after 72 hrs, spinetoram had a strong action against S. oryzae showing its high toxicity (LC_{50 =} 5.29 ppm with a toxicity index of 100% and relative potency of 2.67 fold). On the other hand, primiphos-methyl was the lowest efficient toxicant (LC₅₀ = 14.13 ppm with toxicity index 37.44%  and  a standard relative potency considered for the least efficient compound that equal 1.00).

Table (4): LC₅₀ values, Toxicity index and Relative potency of the tested insecticides against S. oryzae adults (after 24, 48 and 72 hrs bioassay).

          From the afore-mentioned results, it could be revealed that the spinosyns having potent activity, lower environmental effect and unique mode of action toward insects and can control insect strains resistant to other grain protectants (malathion, chlorpyrifos …etc).Therefore, the application of spinetoram would be realty valuable as a good protectant against stored-grain insect-pests. Though, its application could be recommended and involved within integrated stored- product pest management programs for protecting grain stores from the insect infestation.