Wheat suffers heavy losses during storage due to insect pests. According to the FAO estimate 10 to 25% of the world’s harvested food is destroyed annually by insects and rodent pests [1]. Insect pests cause damage to stored grains and processed products by reducing their dry weight and nutritional value [2]. The confused flour beetle, Tribolium confusum is one of the most serious pests of stored cereals and processed cereal products worldwide [3-5].

Around the world, residual chemical insecticides and fumigation are currently the methods of choice for the control of insects of storedproducts [6]. Their widespread use has led to some serious problems, including the development of insect strains resistant to insecticides [7], toxic residues on stored grains, and health hazards to grain handlers. Increased public concern over the residual toxicity of insecticides applied to stored products, the occurrence of insecticide-resistant insect strains, and the precautions necessary to work with traditional chemical insecticides stress the usage of botanicals to control insects of stored product [8].

Insect pests have mainly been controlled with synthetic insecticides in the last fifty years and the protection of stored grains from insect damage is currently dependent on synthetic pesticides. Botanicals are a promising source of pest control compounds. The plant kingdom can be a rich source of a variety of chemicals with the potential for development as successful pest control agents [9,10]. Secondary compounds from plants including alkaloids, terpenoids, phenolics, and flavonoids can affect insects in several ways. They may disrupt major metabolic pathways and cause rapid death, act as attractants, deterrents, phago-stimulants antifeedants or modify oviposition. They may also slow down or accelerate development [11-13].

There is, therefore, an urgent need to develop environment friendly alternatives with the potentials to replace the highly toxic chemicals. On the basis of experimental studies it was found that the mixture of plant materials with their rapid and slow action, proved to be very effective for the protection of stored grains. It has also been well recognized internationally that some plants derived insecticides can affect a limited range of insect pests, but have no harmful effect on non targeted organisms and the environment. Many of the plant derived materials possess repellant and insecticidal activities against the insects of stored food products, and also confirm their usefulness as potent control agent [14]. The aim of this study is to evaluate the insecticidal activity of ethanolic extracts from Anethum graveolens, Apium graveolens, Eucalyptus glauca, Mentha longifolia, Malva parviflora and Zingiber officinale plants on the last larval stage of Tribolium confusum.

Insect cultures

Mass culture of the confused flour beetle, T. confusum was maintained in the laboratory over 2 years without exposure to insecticides in glass containers (20 cm length×14 cm width×8 cm height) containing wheat flour mixed with yeast (10: 1, w/w) which were covered by a fine mesh cloth for ventilation. The cultures were maintained in an incubator at 30°C and for evaluation of larvicidal activity newly molted last instar larvae of T. confusum insects were utilized for toxicity test.

Plant materials

Fresh leaves of Anethum graveolens, Apium graveolens, Eucalyptus glauca, Mentha longifolia, Malva parviflora and Zingiber officinale were collected from Erbil city. Leaves of these plants were washed with water, shade dried and ground to a fine powder with an electric blender. About 12.5 gm of the powdered material from each plant was soaked separately, in the dark, in a solution of 12.5 ml water and 50 ml ethanol solvent. After one day the solutions were filtered and stored in the refrigerator prior to use [15]. The extracts were tested for their effects on the last larval stage of T. confusum to determine percentage mortality.

Mortality test

To test the mortality rate of T. confusum last instar larvae, the crude plant extracts, (the residue ) was used, in which 1 mL solution of each plant extract was dropped on filter paper (whatman No.1) placed inside a petri dish (6 cm diameter) with the help of a pipette. The filter paper was then air dried for a few minutes. Ten last instar larvae (1-2 days old) were released into each petri dish, and the same number was also confined to filter papers treated with water and ethanol as an untreated check. Three replications were made for each treatment and control [16]. The mortality of beetles was recorded at 0.5, 1.0, 1, 5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, and 5.0 hours after treatment. The mortality was assessed by means of direct observation, and when no leg or antennal movements were observed, the insects were considered dead [17].

Statistical analysis

The mortality (%) was corrected by Abbott’s formula [18] and then subjected to probit analysis from the SPSS software (V. 17.0.0) [19] with time as the explanatory variable to derive the estimated hours for 50% mortality (LT₅₀).

This experiment was conducted in order to determine the insecticidal activity of the plant extracts used on Tribolium confusum. In all cases, considerable differences in insect mortality were shown with different plant extracts.

The efficiency of the plant extracts on mortality of Tribolium confusum is shown in Table 1 for the different time periods. The value of maximum mortality differs from plant to plant. As observed in this study, the result reached its maximum value in the case of Anethum graveolens, it was 56.67% at 4.5 hrs exposure, Apium graveolens: 93.33% at 5 hrs, Eucalyptus glauca: 90% at 2 hrs, Malva parviflora: 96.67% at 3 hrs, Mentha longifolia: 93.33% at 4 hrs, and Zingiber officinalis 100% at 2 hrs.

Table 1: Cumulative percent mortality of Tribolium confusum with different plant extracts at exposed periods

Mortality was noted with different plants and exposure times. Anethum graveolens was able to induce 50% mortality at 3.5 hrs, whiles Apium graveolens was able to induce more than 50% mortality after 0.5 hrs, and achieved 93.33% at 5 hrs after treatment and Eucalyptus glauca induced more than 50% mortality after 1 hr, and 90% at 2 hrs after treatment. On the other hand, the result of Malva parviflora showed toxicity to Tribolium confusum and induced more than 50% mortality at 0.5 hrs, and reached its maximum value of 96.67% at 3 hrs. Mentha longifolia extract was able to induce more than 50% mortality at 1 hr, and reached its maximum value of 93.33% at 4 hrs. As shown in Table 1 Zingiber officinale was able to induce more than 50% mortality at 0.5 hrs, and reached its maximum value of 100% at 2 hrs. These results agree with those of Chiasson et al. [20] Which showed that the products are generally broad-spectrum, due to the presence of several active ingredients that operate by means of several modes of action? On the other hand, our results didn’t agree with those of Ahmad et al. [21] who studied the insecticidal activity of Zingiber officinale rhizome extracts against Trogoderma granarium (E). At the highest concentration of (6%) Z. officinale was found to be comparatively toxic and killed 9.81% to 22.05% of the larvae after 24 and 96 hrs and they concluded that the extract did not give encouraging results when tested against T. granarium larvae. However, our results indicate that Zingiber officinale was able to induce more than 50% mortality at 0.5 hrs, and reached its maximum value of 100% at 2 hrs. This contradiction and difference could be attributed to the high concentration of the crude plant extract that we used for the Mortality test. Also the results obtained for the other plant extracts against the larvae of T. confusum gave encouraging results.

The toxicity of these crude plant extracts against T. confusum could depend on several factors among which are the chemical composition of the crude plant extract and insect susceptibility. Also Khoshould and Khayamy [22] refer to the insecticidal effects of ethanolic extract from Verbascum cheiranthifolium Boiss against two insect pest species of stored-products and indicated that the mortality of the exposed insects increased with the increase of the exposure interval and dose rate. It was clear from their results that the botanical extract on insects was effective and Iram et al. [23] indicated that the ethanol extract was found to be remarkably more potent than the powder form of same plant. Some other different findings were reported by different scientists. Fang et al. [24] indicated that Tribolium species are classified among the least susceptible insect pests of stored-producst and often more difficult to kill than other stored-product beetles, though the order of toxicity often varies depending on the particular insecticide. The insecticidal activity of monoterpenes, the major component of essential oils, has been reported against insects of stored-products [25,26]. Table 2 shows the LT₅₀ values for the different plants extracts of Tribolium confusum. The LT₅₀ values of T. confusum ranged from 1.111 for Zingiber officinale, and 3.146 for Anethum graveolens but the LT₅₀ values for Apium graveolens, Mentha longifolia, Eucalyptus glauca, and Malva parviflo were 2.451, 1.392, 1.143 and 1.364 respectively .The most toxic plant was Zingiber officinale and the least toxic Anethum graveolens. The results of the present study are in agreement with some earlier studies [16], where it was found that mortality attained was 100% after 12 hrs and LT₅₀ value ranged from 5.83 to 22.14 hrs. From the above results it can be concluded that the insecticidal activity varied with plant species and exposure time.

Table 2: LT₅₀ values for different plants extracts of Tribolium confusum

The findings of the present investigations indicate that botanical derivatives might be useful as store insect control agents for commercial use. All tested plant extracts were effective to some degree in reducing the number of T. confusum. The results of the present study indicated that by considering the mean mortality as a main index, Zingiber officinale proved to be the most effective of the six test plants materials against the larvae followed by Malva parviflora, Apium graveolens, Mentha longifolia, Eucalyptus glauca and Anethum graveolens. A study to improve the effectiveness of botanical derivatives as insecticides will benefit the agricultural sectors of developing countries, as these substance are not only of low cost, but also have less environmental impact in terms of insecticidal hazard.