The varieties of groundnut developed for their resistance to peanut rosette virus were used for our work (Table 1).

The plant materials used were from several host plants families. All the experiments were carried out with seeds belonging to various leguminous plants. Four of them belong to the family of Ceasalpiniaceae: Tamarindus indica (Linné), Bauhinia rufescens (Lam.), Cassia sieberiana (D.C), Piliostigma thonningü (Schum); five are from Mimosaceae: Acacia senegal (Lin.), Acacia gourmaensis (Lin.), Acacia nilotica var. tomentosa (Lin.), Acacia nilotica var adansonnii; Faidherbia albida (Chev.), and Prosopis africana (Guill.).

The insects used are Caryedon serratus adults reared on groundnut varieties and on wild host plants.

To study C. serratus’ development cycle, the research work was carried out in the laboratory where temperatures ranged about 29˚C ± 3˚C with a relative humidity from 53 to 86 p.c.

C. serratus couples (not older than 48 hours) were maintained on groundnut and placed in three different bottles containing 100 pods of one-seed, two-seeds and three- seeds groundnut, respectively (Figure 1). At the end of three weeks laying, insects were withdrawn from the bottles and eggs were counted for each type of pods. Pods were then observed periodically to notice adults’ emergence. When emerging, insects were measured according to the various types of pods by using an ocular micrometer with a binocular microscope. For the insects’ measurement, their length and width were taken into account. This measurement required the calibration of the stage micrometer placed in the binocular microscope’s objective. After calibration, the stage micrometer is placed in parallel with the reticle’s divisions so that the two scales were superposed. Thereafter, the number of the reticle’s divisions (Y) which are superposed on the intervals of stage micrometer division (X) was determined. Then the micrometric value was determined:

;

I: Value in mm (0.1 mm) of an interval of the stage micrometer.

Length of the insect = A X Interval number of divisions of insect length reticle.

The analyses of variance on the observed parameters were carried out by the Software

XLSTAT version 2007. When the variance analysis revealed significant differences, the Fisher test (LSD) were applied for the comparison of averages at the probability threshold of 5%.

C. serratus larval development study has permitted to show that the larva moves throughout four stages whose durations vary from one stage to another (Table 4).

After laying period, an average time of 4.37 days is observed before the larva passes through the husk to reach the groundnut seed. At stage 1, the larva spends on average

6.20 days to develop. At stage 2, the development duration average was 6.84 days. Stage 3 lasts on average 7.76 days; at this stage, it can be noticed that the larvae are larger. After this stage, some larvae according to feeding support availability may infest new seeds. The fourth stage which precedes the larva entry into nymph stage lasts on average 8.65 days. The larva becomes even larger and become pink; often it gives up its initial pod to enter into nymph stage outside the pod. Nymphal stage duration, in our study, lasted on average 19.30 days.

C. serratus larval development duration from egg to adult stage was observed on whole seeds of the 10 wild host plants which were used in the context of our study. The results are given on Table 5.

In all of the tested host plants, the larval development of the beetle was possible on only five host plants. Results on Table 5 indicate that the time between eggs deposit on various host plants seeds and adults’ emergence varies from 36.40 to 55.33 days according to host plants nature. The longest larval development durations were observed with B. rufescens seeds (55.72 days). It is on F. alblida seeds that the larval development duration was the shortest with only 36.40 days. The number of adult insects emerged varied from 7 to 192 according to host plants. On the other host plants seeds, although eggs were laid, no larval development was observed after more than four months observation. Regardless of host plants, females outweigh males at emergence.

Results on Table 6 showed that eggs deposit was done regardless of pod type. However, the full number of eggs deposited on the pods with three seeds (443 eggs) is significantly higher than that recorded on the other types of pods (P < 0.0001). Similarly, the laying on the pods with two seeds is more important than those obtained on pods with one seed. A total of 443 eggs on pods with three seeds, 310 eggs on pods with two seeds and 245 eggs on pods with one seed are obtained.

The adult emergence rate per type of pods was more important on pods with three

seeds compared to the other types of pods. On pods with three seeds, the rate of emergence was 73.36% compared to 68.36% on pods with two seeds and 56% on pods with one seed. For adults emerged weight from the three types of pods, no significant difference was observed among them.

The results on C. serratus biology under laboratory conditions are similar to those

Averages (± standard deviation) with the same letters in the same column do not differ significantly at the threshold of 5%.

The averages (± standard deviation) with the same letters in the same column do not differ significantly at the threshold of 5%.

reported by many authors [13] [14] . The incubation period of eggs is similar to that reported by Robert [7] but it is the half of what was reported by Rama and Venugopal [13] who found an incubation time of 8 days. This difference could be related, not only to groundnut variety used, but also to their study’s conditions. As far as larval stages are concerned, four distinct larval stages were given as reported [12] . Our results indicate that larval stage duration varies from one stage to another; which was not the case with the results reported by some authors who found a 4 days average duration for the first three larval stages [13] . This constant duration has something to do with the climatic conditions, mainly with temperature [15] ; they consider the latter as the main factor which determines insects’ larval development. In addition to this factor, insect used variety and biotype can also influence this development period. Variations in larval stages duration or their number may suggest the presence of a biotype [16] . The sex-ratio in favor of females can be explained by a sufficient food supply in quantity and quality for insects during their development [17] . This tendency of C. serratus to produce more females explains the scope of infestations observed in groundnut stocks. A 78.9 eggs average laying at 30˚C suggesting thus that temperature plays a crucial role in groundnut beetle’s laying [18] .

On seeds exposed to C. serratus females, the beetle larval development was possible only on 6 (six) plants species which are T. indica, P. thonningii, B. rufescens, A. hypogea (seeds and pods) C. sieberiana and F. albida. Similar results were reported [19] [20] [21] ; however, these authors do not report a C. serratus larval development on F. albida’s seeds. For these authors, F. albida permits only the development of the Caryedon species called Caryedon excavatus which in fact exploits the latter under natural conditions. Lale and Igwebuike [22] explain that F. albida’s pods in Nigeria Savannas are infested by the C. serratus species. This situation indicates that this C. serratus species can lay and develop on F. albida’s seeds. C. serratus larval development failure on other plants’ seeds can be justified by the structure of these plants’ teguments. In fact, certain plants seminal tegument rich in tannins and lignin can constitute a chemical barrier against beetle larvae penetration [23] [24] [25] . The results on the development time show the importance of the laying substrate on larvae development. The development duration is faster on F. albida’s seeds compared to other seeds, suggesting that this plant can be a natural host plant for C. serratus. C. serratus laying behavior reveals that whatever the conditions, C. serratus females lay on more than half of the species to which they are exposed. These results suggest that C. serratus females seem to have little requirement as far as the laying substrate is concerned [7] [10] . However, 90% of the insects from Bruchidae family species to which the C. serratus belongs lay and develop at the expense of leguminous plants [2] . In our results, the majority of laying is observed on leguminous plants. Laying comparison according to the botanical family indicates that C. serratus females laid more eggs on Ceasalpiniaceae than on other host plants families. This female’s attraction by Ceasalpiniaceae is due to the weakness of the chemical defenses met in this family [26] but also to the chemical composition is the major index which intervenes in the host selection process by insects [27] .

Considering the aspect of the groundnut pods, the laying was possible preferentially on three-seeds pods. These observations are similar to those by Mitchell who observed the same laying behavior with the cowpea beetle Callosobruchus maculatus [28] . The choice of groundnut three-seed pods by females can be explained by a more important availability of feeding support for larvae [9] . With the cowpea beetle, female’s fertility is related to seeds number offered to them [29] . In the case of the groundnut beetle, it can be supposed that females laying respects conditions observed in cowpea beetle case. As far as the survival rate of larvae is concerned, more than half of laid eggs evolves to give adults, whatever the pod size; suggesting that this parameter does not have an influence on emergence rate.