Abstract: The insecticidal efficacy of Actellic and Sumithion (2%) dusts was assessed against 0-3 day old adult Callosobruchus maculatus (Coleoptera; Bruchidae) on cowpea (Vigna unguiculata) and 0-7 day old adult Sitophilus zeamais (Coleoptera; Curculionidae) on maize (Zea mays) grains in the laboratory at 30±1°C temperature and 70±2% relative humidity. Each insect species was exposed to insecticide/grain admixtures ranging from 0.063 to 0.5 g kg-1 in comparison with untreated controls. Sumithion with LC50 values of 0.073 g kg-1 for S. zeamais and 0.104 g kg-1 against C. maculatus was more effective than Actellic (LC50 = 0.090 and 0.112 g kg-1 S. zeamais and C. maculatus, respectively) in acute toxicity tests. The efficacy of the two compounds declined as storage period increased with Sumithion showing consistent toxicity against S. zeamais. Both test compounds significantly reduced adult emergence in treated grains after infestation was established (p<=0.05). Both Sumithion and Actellic were effective at controlling C. maculatus and S. zeamais, but at admixture concentrations higher than the 0.005 g kg-1 recommended by manufacturers.
INTRODUCTION
Farmers, produce merchants and housewives in Africa store food grains, for which they use various facilities. Storage ensures food security, profit maximization and seed availability for the next planting season. Insects benefit from the presence of large bulks of food and produce successive generations in store. These insect activities are accompanied by varying degrees of damage and consequent losses.
Over 200 insects ravage grains in storage (Haines, 1991; Odeyemi and Daramola, 2000) and among these the cowpea bruchid, Callosobruchus maculatus (F.) and maize weevil, Sitophilus zeamais Motsch, are serious pests because of the importance of the commodity they infest and the severity of the damage they cause, which amounts to several million dollars per year. These insects can cause total loss of cowpea (Vigna unguiculata Walp) and maize (Zea mays) in storage (Osuji, 1985; Jackai and Daoust, 1986; Egwuatu, 1987; Lale, 2002) if the grains are stored without protection for upwards of six months. The protection of grains from insect infestation during storage is therefore very important.
In Nigeria, as in other African countries, grains are protected from insects during storage by chemical (mainly synthetic organic insecticides), non-chemical (including male sterilization, extremes of temperature, admixtures with oils, powders and extracts from plants) and often the integration of these control measures in a compatible manner to reduce their individual negative effects (Delobel and Malonga, 1987; Makanjuola, 1989; Don-Pedro, 1989; Denloye and Makanjuola, 1997; Ogendo et al., 2004). Many of these non-chemical methods are cheap, locally available and easily applicable without need for technical expertise. However they are slow acting and not standardized, their use depending mainly on experience and tradition (Delobel and Malonga, 1987; Belmain and Stevenson, 2001). They are mainly used by subsistence farmers. In addition, the superiority in efficacy of synthetic compounds over non-chemical methods has been shown by recent studies. Okunade et al. (2002) reported that Pyrimiphosmethyl was more potent against Rhizopertha dominica on sorghum in comparison to 12 natural plant products. Similarly, Obeng-Ofori and Amiteye (2005) reported that only pyrimiphosmethyl showed effective control of S. zeamais on stored maize grains when its efficacy was compared with products from three plant species. The foregoing increase support for the use of synthetic chemicals, which are often quick-acting and persistent in the stored grain, thus ensuring long-term protection.
A wide variety of insecticides such as aluminum phosphide, lindane, methyl bromide, ethylene dibromide, Iodofenphos, malathion, actellic, permethrin and sumithion are used against stored product insects either as single applications or as mixtures. These insecticides are marketed for the control of a broad spectrum of stored product insects albeit with blanket dosage prescriptions.
There are problems with the use of insecticides against storage insects. Firstly, Insects react to chemicals differently owing to differences in their physiology, size, life history and behaviour in the stored commodity (Makanjuola, 1989; Bell, 1992). Secondly, the weather and climatic conditions are not uniform throughout the year and even vary from one geographical location to another at any given time, thus their use and consequent efficacy in the tropics in the stable form may not be the same as obtains in temperate countries (Akingbade, 1991; Boroffice and Boroffice, 1993). Futhermore, recent studies have shown the alarming increase in the rate insects become resistant to synthetic insecticidal compounds such that there is no insecticide group that does not have record of resistance to it by insects (Wallbank et al., 2002). Also, the synthetics leave residues as exemplified by the recent studies by Toteja et al. (2006) and Uygun et al. (2007). A continued use of the same insecticide compounds under the same conditions can only result in a continuation or aggravation of the problems associated with their use.
The foregoing justifies the need for a critical assessment and reappraisal of compounds commonly used in tropical countries to protect stored grains from insect attack at the dosage and conditions under which they are utilized. In the present study we assess the activity of two organophosphorus insecticides namely Actellic and Sumithion against C. maculatus (on cowpea) and S. zeamais (on maize) at doses higher than the 5.0 mg kg-1 recommended by the manufacturers under ambient tropical laboratory conditions.
MATERIALS AND METHODS
Test Insect and Grains Species
Callosobruchus maculatus on Cowpea (Vigna unguiculata) and
Sitophilus zeamais on maize (Zea mays) were obtained from starter
cultures of the respective insects in the insectary of Nigerian Stored Product
Research Institute (NSPRI) Akoka, Lagos, Nigeria where they had been maintained
over several generations without contact with insecticides. Subsequent cultures
of each test species were established from these cultures. The grains used for
culture maintenance were V. unguiculata c.v Ife Brown (for C. maculatus)
and maize cv flint (for S. zeamais). These grain cultivars were also
used in subsequent experiments. The grains were first disinfested by heating
each grain species in the oven for 4 h at 60°C to destroy any hidden infestation.
The cultures were maintained at 30±1°C temperature and 70±2%
relative humidity in the Toxicology Research Laboratory of the Department of
Zoology, University of Lagos, Lagos, Nigeria (6°28’ N, 3°11’E).
All bioassays were carried out under these laboratory conditions.
Test Compounds
The test compounds were two organophosphate insecticides, Actellic and Sumithion,
both 2% dusts; from Chemical and Allied Products (CAP) Plc., Agrochemicals division,
Ikeja, Lagos, Nigeria.
Bioassays
Acute Toxicity of Test Compounds to C. maculatus and S. zeamais
Lots of 200 g of disinfested cowpea or maize were admixed with Actellic
or Sumithion 2% dust at the rate of 0.5, 0.25, 0.125 or 0.063 g kg-1
in plastic containers that served as exposure chambers. Chambers were shaken
vigorously for 2 min to ensure a good mix. Each admixture lot was inoculated
with 30 0-3 day old adult C. maculatus (mixed sexes). Each lot was covered
with muslin held tightly in place with elastic rubber bands. All treatments
and controls were replicated four times. In the same manner admixtures were
prepared at the same rate using maize grains and each inoculated with 30 0-7
day old adult S. zeamais. Four untreated controls; one for each treatment,
respectively were set up and replicated four times in the same way, but received
no insecticide. The number of dead insect in each treatment and control was
recorded daily for seven days. Insects that failed to move any part of the body
when the exposed part of their abdomen was probed with the sharp end of a tweezer
were counted as dead.
Effect of Storage on the Insecticidal Efficacy of Test Compounds
Lots (1 kg) of cowpea or maize were divided into three batches and admixed
with either Actellic or Sumithion at the rate of 0.5, 0.25, 0.125 and 0.063
g kg-1, respectively and mixed thoroughly by vigorous shaking for
5 min each. Four lots were also prepared with either cowpea or maize grains
but without insecticide. All treatments for cowpea/Actellic, cowpea/Sumithion,
maize/Actellic or maize/Sumithion as well as the untreated controls were replicated
four times and left in a dark corner under the laboratory bench where they were
stored for 4-12 weeks at 30±1°C temperature and 70±2% relative
humidity. At the end of the first four weeks one batch of each treatment was
inoculated with 30 adults of the appropriate insect species (0-3 day old C.
maculatus for Cowpea mixtures; 0-7 day old S. zeamais for maize admixtures)
another batch received insects at the end of 8 weeks while the last batch received
insect species at the end of 12 weeks of storage. The number of dead insects
(determined as described earlier) was recorded daily for seven days. Data obtained
were used to determine LC50 (median lethal concentration) values
of each treatment against each test insect species.
Effects of Test Compounds on Existing Infestation of C. maculatus
and S. zeamais
Twelve 200 g lots of cowpea and maize grains were infested with 20 adult
(10 ♂; 10 ♀) C. maculatus or S. zeamais, respectively.
All adult C. maculatus were removed after 10 days while S. zeamais
adults were removed after 17 days of infestation. Each lot was then admixed
with either test insecticide at the rate of 0.5 or 0.25 g kg-1 (obtained
from results of previous experiments). There were also untreated controls in
each case. Each treatment and control lot was replicated three times. Emerging
adults in each lot was removed and recorded until there was no emergence for
14 days. Data obtained were square root (√x + 0.5) transformed (Sokal
and Rholf, 1995) and means compared to determine differences on adult emergence
at the different treatment levels.
Data Analyses
Data from all dose-response tests were analyzed by probit analysis (Finney,
1971) using a computer soft ware based on maximum likelihood as adopted from
Don-Pedro (1989) after controlling for control mortality following Abbott (1925).
Differences between means were compared by Analysis of Variance (ANOVA) and
ranking was done after LSD test using Statistical Package for Social Scientists
(SPSS,) version 11.0 (SPSS, 2001).
RESULTS
Toxicity of Test Compounds
Both Actellic and Sumithion showed different toxicities to C. maculatus
and S. zeamais as shown in Table 1. The two test compounds
were more toxic to S. zeamais than they were to C. maculatus as
they gave lower LC50 values against S. zeamais. Sumithion
(0.073 g kg-1) was more toxic than Actellic (0.09 g kg-1),
but there was no significant difference between its toxicity and that of Actellic
against S. Zeamais as shown by overlap in 95% fiducial limits (Table
1). The LC50 values for C. maculatus show that Actellic
was similar in potency to Sumithion against C. maculatus. The LC95
values 0.28 g kg-1 (Actellic) and 0.225 g kg-1 (Sumithion)
against S. zeamais were significantly lower than those obtained for C.
maculatus 0.444 g kg-1 (Sumithion) and 0.603 g kg-1
(Actellic).
Effect of Storage
The efficacy of Actellic and Sumithion, respectively decreased with increasing
period of storage as shown in Fig. 1. The figure shows that
both compounds were most toxic at 4 weeks storage and least toxic at 12 weeks
for both C. maculatus and S. zeamais, respectively. Actellic was
similarly toxic against both C. maculatus and S. zeamais at 4
and 8 weeks storage prior to insect introduction but more toxic to C. maculatus
than S. zeamais at 12 weeks storage. Sumithion on the other hand gave
similar toxicity against the two test insect species at the various storage
periods tested (Fig. 1).
| Table 1: | Toxicity of Actellic and Sumithion to C. maculatus and S. zeamais |
| Table 2: | Adult insect emergence from grains treated with Actellic or Sumithion after established infestation |
Each datum is a mean of three replicates, Data were square root (/x + 0.5) transformed before analysis of variance, Differences between column means bearing different superscripts are significant following Least Significant Difference (LSD) test p≤0.05 |
|
| Fig. 1: | Toxicity of Actellic and Sumithion 2% dusts to (A) Callosobruchus maculatus and (B) Sitophilus zeamais in treated grains stored for various periods before insect introduction |
Adult Emergence
The emergence of adults of each test insect species increased as the treatment
concentration reduced in all cases tested (Table 2). ANOVA
(p≤0.05) show that more adults emerged at successive treatment concentrations.
More adult C. maculatus emerged than did S. zeamais (Table
2).
DISCUSSION
The study show that both Sumithion and Actellic were toxic to S. zeamais and C. maculatus, respectively, but at concentrations higher than the 5.0 ppm (0.005 g kg-1) dosage recommended by the manufacturers. The results demonstrating the efficacy of Actellic has further confirmed the findings of earlier workers who have shown that Actellic is an effective insecticide for the control of S. zeamais and C. maculatus (Caswell and Akibu, 1980; Obeng-Ofori and Amiteye, 2005; Asawalam and Emosairue, 2006). Earlier studies carried out by Obeng-Ofori and Amiteye (2005) shows that Pyrimiphosmethyl, the active compound in Actellic was effective for the control of S. zeamais on maize grains at one-quarter of the recommended dose, a contradiction of findings in the present study where Actellic demonstrated effectiveness with LC50 value of 0.09 g kg-1 against S. zeamais.
The LC50 values for the test compounds increased as the post treatment period preceding introduction of each of the two test insect species increased (Fig. 1). This suggests that both Actellic and Sumithion may have broken down and having their efficacy reduced as storage time increased. Mensah et al. (1979) reported that the organophosphate insecticides degrade under most storage conditions. If the organophosphates had degraded in the present study, they may have resulted in compounds that are less toxic to the test insects than either Sumithion or Actellic. Furthermore, it has been noted that the efficacy of organophosphate insecticides decrease with increasing temperature and humidity (Matthews and Maliphant, 1993). The laboratory temperature (30±1°C) and relative humidity (70±2%) under which the experiments were carried out may therefore have contributed to the degradation of the suspected in the test compounds. There is need to further investigate the influence of these laboratory conditions and the breakdown products of organophosphates on the survival of C. maculatus and S. zeamais, respectively.
The progressive increase in LC50 values is notable, but more profound is the fact that the LC50 values obtained after 8 weeks are above 1.0 g kg-1, which is X20 of the recommended dosage for the two test compounds. This indicates that high amount of the insecticidal dusts would be applied in typical storage systems, heightening the possibility of residues on the protected grains in agreement with the reports of Hilton and Banks (1997), Lee et al. (2003), Toteja et al. (2006) and Uygun et al. (2007). In Nigeria several reports including those of Adeola and Fafunsho (1998) have shown insecticide residues in food stuff from different parts of the country.
Futhermore, that the LC50 values (Fig. 1) given by both Actellic and Sumithion are more than X4 of the recommended dosage is a pointer to the fact that treatment should be repeated more frequently. The results shown in Fig. 1 is suggests the need for repeat treatment every 8 weeks in order to adequately protect grains from these insects for the 3-6 months period that they (food grains) are normally stored in Nigeria (Egwuatu, 1987; Lale, 2002; Iken and Amusa, 2004).
Table 2 shows that the two compounds caused a significant reduction of adult emergence of the two test insect species when treatment was carried out after infestation. The two compounds can therefore be conveniently recommended for use in typical storage systems, insect infestations in stored grains are usually hidden until damage becomes noticeable.
ACKNOWLEDGMENTS
We gratefully acknowledge the supply of samples of test insects as starter cultures from the insectary of the Nigerian Stored Product Research Institute (NSPRI), Lagos. We thank Mr. Ajibade, M.A. of the Zoology Laboratory, Department of Zoology, Lagos State University (LASU) for technical assistance.