Assessment of the toxic potentials of some plants powders on survival and development of Callosobuchus maculatus

Dried leaves powders of Guirea senegalensis, Piliostigma reticulatum and dried fruit powder of Piper guineense, were tested along side a conventional insecticide, Actellic-2-Dust, to compare their efficacies on survival of cowpea weevil (Callosobuchus maculates) during storage. Both the treated and untreated cowpeas (control) were then infested with newly emerged adult bruchids of both sexes. In all the trials, 83 to 100% mortality rates were observed in the treated samples. The rates in the untreated control ranges from 33.3 to 43.6%. The effects of the powders on fecundity were more pronounced on P. guineense and Actellic-dust treated samples. Similar effects were observed on development of immature stages and emergence of adult weevils. P. guineense had the highest larvicidal effects and lower productivity result especially. P. reticulatum had the lowest mortality rate and higher productivity result. The effects of the different powders were found to be dose related.


INTRODUCTION
The use of naturally occurring plant materials to protect agricultural products against a variety of insect pests is an old-age practice in some parts of the world (Peter, 1985).Extracts from different plants have been shown to possess insecticidal properties against a wide range of insect pests (Golob et al., 1982;Delobel and Malonga, 1987;Srivastava, 1988;Dennis, 1990;Onolemhemhen and Oigiangbe, 1991;Ruskin 1992;Liu, 1994;Hossafay and faisal, 1994;Imam, 1997;Lale, 1992Lale, , 1994;;Yahaya and Magaji, 1997;Yahaya, 2002).Cowpea, a protein rich food, is a staple food in many societies.It is used not only as human food but also as a fodder for livestock feeds.It is subject to attack by a variety of insect pests.However, Callosobuchus maculatus is probably the most common pest of stored cowpea seeds in Nigeria.
Several measures have been adopted to curtail the problems of insect infestation.These include the use of chemical agents/insecticides such as DDT and Lindane (Srivastava 1998).
Despite these efforts, insect infestation during storage still persists.In view of the *Corresponding author.E-mail: sanusi1966@yahoo.com.economic importance of cowpea, the severity of damage caused and the problems associated with the use of synthetic insecticides (Osuji, 1985;Ruskin, 1992) we evaluated natural methods for the control of C. maculatus during storage.

MATERIALS AND METHODS
The materials used are dried leaves of Guirea senegalensis, Piliostigma reticulatum, dried fruits of P. guineense and a synthetic insecticide, Actellic-2-Dust.The dried materials were ground and sieved repeatedly to obtain the finest particles.Local variety of cowpea Vigna unguiculata L. (Walp) was used in this study.The Cowpea seeds with emergence holes or egg debris on the testa were considered infested and removed.The un-infested seeds were sterilized by freezing for three weeks to kill any residual insect.Adult weevils (C.maculatus) used in the experiment were reared in the Physiology Laboratory, Usmanu Danfodio University, Sokoto on previously sterilized V. unguiculata seeds.The second-generation adults that emerged were employed in all the trials.
Four sterilized petridishes (9 cm x 1 cm) containing 50 g of sterilized cowpeas were treated with 1 g of the plant powders separately.The 5th petri dish contained untreated cowpeas, serving as control.The Petri dishes were labelled accordingly and kept in the laboratory.Both dishes were then infested with 10 newly emerged adult bruchids of both sexes.Each treatment and control was replicated three times at 24 h interval.Temperature and relative humidity ranges between 25-28 o C and 50-60%, respectively.
The same procedure was repeated using the same quantity of the grains treated with 2 g and 3 g of the dusts in the second and third trials.Observations were made on adult's mortality, fecundity and emergence of the adult weevils.The weevils were considered dead when there was no response after proving the abdomen with a pin.Eggs laid on 20 seeds randomly selected from each petridishes were counted on the 5 th , 6 th and 8th day post infestation in trials 1, 2 and 3, respectively.These set of seeds were then returned into the Petri dishes containing various dusts and, observations were made on emergence of adult weevils.The data obtained was subjected to analysis of variance.

Adult Mortality
Results obtained have shown clearly that the materials tested affected the longevity or survival of the cowpea weevils.Between 83-100% mortality rates were recorded in trials (Table 1).The mortality rates in the untreated control ranges between 33.3-43.6% in all the trials within the same period.Analysis of variance revealed a significant difference between the treated and untreated samples (P<0.01).

Oviposition
Table 2 shows the mean number of eggs laid in both treated and untreated cowpeas.Oviposition was very Abdullahi and Muhammad 61 much reduced in treated samples as compared with untreated control.Very few eggs were laid in P. guineense treated samples.P. reticulatum and G. senegalensis have the least ovicidal effects with highest mean fecundity of 51-62 eggs in trials 1 and 2. However, at higher concentration (3 g) all the materials used have significant effects on egg-laying capacity of the weevils (Table 2).

Adult Emergence
The effects of the powders on egg development and emergence of adults of C. maculatus is shown in Table 3.
The table revealed that in all the trials between 49.2-59.1% of the total eggs laid in P. guineense treated samples died at various stages of development.Also more than 60% of the total eggs laid, developed and emerged as adults in G.senegalensis and P. reticulatum treated samples.This means that P. guineense powder has the highest larvicidal effects and lowest productivity result (Table 3).G. senegalensis is least efficacious especially at lower concentrations (1 and 2 g).

DISCUSSION
The result clearly revealed that each of the materials tested have distinct effects on survival of C. maculatus.
Treatment of cowpea with various concentrations of the dusts affected the longevity/survival period of the weevils, resulting in higher mortality rates of C. maculatus).Yahaya and Magaji (1997), reported earlier that powders of P. guinnense, significantly affected survival and egglaying capacity of the adult weevils.It was also observed that increased in the concentration reduces the life span of the weevils.The higher ovicidal effects/reduced oviposition rates observed in P. guineense treated cowpeas agrees with the findings of Lale, 1992;Yahaya and Magaji 1997;Yahaya, 2002.The striking effects of P. guineense powder could be attributed to its guineensine 1 component and to its irritating smell which prevents physical contact and caused suffocation among the adult weevils.Though reasons for the higher larvicidal effects are not clear, the significant difference observed in productivity results especially in P. guineense treated samples indicated the possible potentials of the dust as an alternative agent for the control of C. maculatus.The the effect of each powder was dependent upon its concentrations.Thus lower rates were clearly insufficient to have a pronounced effect on oviposition and larval development especially in G.senegalensis and P. reticulatum.When compared with other experimental materials, P. guineense seems to be more efficacious.Conculusively, P. guineense could provide adequate protection to cowpea against C. maculatus during storage.More extensive study might provide useful information on the potentials of these plants.

Table 1 .
Effects of plant materials used on mortality of C. maculates.
Note: Means with the same letter or not significantly different at P>0.05.

Table 2 .
Effects of experimental materials on ovipostion.

Table 3 .
Total number of adult that emerged.