Suitability of cowpea sprouts to optimize rearing of biocontrol agents Phanerotoma syleptae and Liragathis javana

doi:10.21203/rs.3.rs-3338001/v1

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Research Article

Suitability of cowpea sprouts to optimize rearing of biocontrol agents Phanerotoma syleptae and Liragathis javana

https://doi.org/10.21203/rs.3.rs-3338001/v1

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published 06 Aug, 2024

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The legume pod borer Maruca vitrata is a key insect pest of cowpea, Vigna unguiculata Walp. The present study was designed to optimize the efficiency of mass rearing two of its biological control agents, the larval parasitoid Liragathis javana and ovo-larval parasitoid Phanerotoma syleptae, by comparing their performance on M. vitrata hosts reared on sprouting grains of 10 different varieties of cowpea as feeding substrate. Thus, 25 M. vitrata larvae of 24 hours old, 25 M. vitrata eggs of 24 hours parasitized by P. syleptae and 25 larvae of 48 hours parasitized by L. javana were reared on sprouts of 10 cowpea varieties in laboratory at a relative humidity of 70–80% and a temperature ranging from 25 to 26°C till pupae stage. Fecundity of M. vitrata and the two parasitoids were assessed per variety. Experiments were replicated 3 times. Results revealed significant difference between varieties for the development of unparasitzed larvae of M. vitrata and L. javana or P. syleptae-parasitized larvae. The cumulative daily fecundity shows different patterns: higher in IT 84D-449 for M. vitrata and L. javana but higher in Kpodji for P. sylepatae. Mortality of parasitized (by L. javana and P. syleptae) larvae and unparasitized (M. vitrata larvae) was lower in Kpodji variety. No significant differences were observed between male and female proportions (sex-ratio), regardless of insect species and cowpea variety except the varieties Tawa and Vitoco in L. javana. Data were discussed with regard to the optimization of mass rearing of the three insect species for the biological control against M. vitrata using L. javana and P. syleptae.

Phanerotoma syleptae

Maruca vitrata

Liragathis javana

varieties cowpea

larval feeding

Cowpea, Vigna unguiculata (L.) Walp., is a leguminous staple crop widely cultivated in West Africa (Singh and van Emden 1979; Zannou et al. 2004). As a major source of dietary proteins, it is expected to play a key role in human nutrition in this area of the world where few people have access to animal proteins (Phillips et al. 2003). However, cowpea production is limited by various constraints, of which biotic pressure from insect pests remains the major one affecting both yield and grain quality after harvest. The legume pod borer Maruca vitrata Fabricius (Lepidoptera, Crambidae) was reported to cause serious damage to cowpea in tropical and subtropical regions of Asia, Latin America and Africa (Sharma 1998; Ba et al. 2019). Caterpillars induce heavy losses by damaging many plant organs such as flowers, leaves, flower buds, green pods. Maruca vitrata can cause up to 80% yield losses in the absence of control measure (Nampala et al. 2002). Many control methods have been so far developed. But the most used was the application of synthetic chemicals. Chemicals application becomes no more attractive considering the so many side effects including environmental side effects and human hazards. Moreover, alternative control methods such as host plant resistance and cultural control practices were not always effective enough to keep M. vitrata populations below economic thresholds (Tamò et al. 2012). Therefore, the use of living organisms such as natural enemies and effective microorganisms would be an alternative to the use of synthetic pesticides. Many natural enemies of Maruca vitrata have been identified over the world (Arodokoun et al. 2006; Srinivasan et al. 2014)). Of these, Phanerotoma syleptae Zettel and Liragathis javana Bhat and Gupta are two parasitoid species identified in the native area of M. vitrata. These parasitoid species have been imported from Taiwan to Benin for the classical biological control against M. vitrata. One of the key steps when implementing biological control programme, is the mass rearing of the natural enemy. Mass production at lower cost of the biological control agents selected remains a major challenge of any classical biological control programme. The cowpea pod borer M. vitrata has been mass reared using artificial diet that was expensive (Jackai and Raulston 1988). But recently, local rearing techniques have been developed at IITA Benin using sprouts made of pre-germinated cowpea seeds (Wetro et al. 2014). The optimization of such production is found to be affected by cowpea as the pest species showed different susceptibility levels depending on the cowpea variety (Kumar et al. 2021). In order, to optimize the mass production of L. javana and P. syleptae, the suitability of some West African cowpea varieties was assessed in the current study to identify the variety that will improve some of their biological parameters such as development cycle, fecundity and longevity and by lowering larval mortality.

Experimental site

Experiments were carried out in the laboratories of the International Institute of Tropical Agriculture (IITA), Benin Station (6°28N and 2°21E, 15 m altitude), near Cotonou, Benin, at a mean temperature averaging 26 ± 0.50 ºC and a relative humidity of 65.5 ± 5%.

Insect material

Several parasitoid species including Apanteles taragamae, Nemorilla maculosa, Phanerotoma syleptae, Liragathis janava were imported by the International Institute of Tropical Agriculture (IITA) from Taiwan to start the biological control programme against M. vitrata in Benin (Tamò et al., 2013; Srinivasam et al., 2014). Of thiese, L. javana, a larval parasitoid of M. vitrata and P. syleptae, an egg-larval parasitoid of M. vitrata were included in the current study. Maruca vitrata eggs used in all experiments were 24 hours old while larvae were 48 hours old.

Selection of cowpea seed

Differents cowpea varieties from Benin, Togo, Niger, Nigeria and Burkina Faso were included in the current study. Sprouts from these varieties were used to rear M. vitrata larvae, L. javana-parasitized larvae and larvae from eggs parasitized by P. syleptae. In total, ten cowpea vairieties were selected based on their wide cultivation and high susceptibility to M. vitrata as follow: The varieties Kpodji and Tawa from Benin; Vitoco and Vita 5 from Togo; IT84D-449 and IT07K-1-10 from Nigeria; TN 5–78 and TN 27–80 from Niger; Tilligre and Komcallé from Burkina Faso (Table 1). These varieties were obtained with collaboration of national agricultural institutions. Sprouts consisted of germinated seed were used to feed M. vitrata parasitized or non-parasitized larvae.

Table 1

Characteristics of cowpea varieties included in the study
Countries	Varieties	Cycle (Days)	Port of the plant	Seed colour	Yield (Tons)
BENIN	Kpodji guêguê	65	Semi-erected	Greyish	1. 2
BENIN	Tawa	60–65	creeping	white	1
BURKINA FASO	Tilligré	70	Semi-erected	white	0. 850
BURKINA FASO	Komcallé	60	Semi- erected	white	1. 8
NIGER	TN5-78	75	Semi-erected	Brown	1. 5
NIGER	TN27-80	75–80	Semi-erected	white	1. 5
TOGO	VITOCO	90	creeping	white	1. 2
TOGO	VITA-5	90	Semi-erected	white	0. 8
NIGERIA	IT 84D-449	60	Semi-erected	Brown	1. 2
NIGERIA	IT07K-1-10	60	creeping	white	1. 5

Cowpea seed germination process

Cowpea seeds from the different selected varieties were sorted to remove damaged seeds. Seeds were soaked in water for 4 hours after being washed and disinfected with a solution of 5% of bleach. Then, seeds were removed and spread onto towel paper for germination. Germination occurred the following day and sprouts were ready to be used for larval feeding.

Mass rearing of Maruca vitrata

The mass rearing of M. vitrata was established at the insectarium of IITA Benin. Newly emerged adult females and males were placed in a rearing cage for three days to allow mating. They were fed using honey solution at 10%. Only female moths were put in small cylindrical container (3 cm diameter x 3.5 cm height) for oviposition. Eggs were cleaned with 5% bleach solution and containers’ lid, were covered using muslin cloth and kept for hatching. Larvae often hatch three days later. Neonate larvae were fed using cowpea sprouts of the variety Wankoun (from Benin) till pupation. Pupae were collected and put in rearing cages for adult emergence.

Mass rearing of the parasitoids on several varieties of cowpea germinated seed in lab

Phanerotoma syleptae

Eggs laid in small containers (3 cm diameter x 3.5 cm height) were submitted to parasitism for 24 hours by females of P. syleptae. Parasitized eggs were kept till larvae hatching. Hatched larvae were fed using cowpea sprouts till parasitoid pupation. Cocoons were collected and placed in rearing cages for adult emergence. Emerged adults were used to continue the parasitoid mass rearing.

Liragathis javana

Mated females of L. javana were placed in small container (3 cm diameter x 3.5 cm height) containing 48 hours old larvae of M. vitrata to allow oviposition by L. javana. Parasitized larvae were transferred to other small container and fed using cowpea sprouts till pupation. Parasitoid cocoons were collected and put in rearing cages for adult emergence. Emerged adults were used to continue the parasitoid mass rearing.

Experimental set-up

Effects of cowpea varieties on some biological parameters of M. vitrata

Twenty-five (25) one-day old larvae of M. vitrata placed individually in small containers (3 cm diameter x 3.5 cm height) were reared using sprouts of each of the ten (10) cowpea varieties (Table 1). Larvae were reared till pupation. Pupae were kept for adult emergence. The time form egg to larval hatching was noted. Likewise, time from larval stage to pupation was measured and time taken by pupa to emerge as well. Mortality occurring during the whole development time was recorded. Sex-ratio of emerged adults was also determined. Emerged adults were coupled for fecundity study. Longevity of both male and female moths was recorded.

Effects of cowpea varieties on some biological parameters of P. syleptae

Twenty-five 24 hours old eggs of M. vitrata laid on paraffine paper were submitted to mated female of P. syleptae for parasitism. Parasitized eggs were kept for larvae hatching. After hatching, neonate larvae were individually placed in small containers (3 cm diameter x 3.5 height) and reared till cocoon stage using sprouts from each of the ten cowpea varieties (Table 1). Cocoons were put in small containers for adult emergence. Emerged adults were coupled and each couple was offered Twenty-five (25) 24-hours old M. vitrata eggs daily to record fecundity. Development time from parasitized egg to cocoon stages and from cocoon stage to adult was recorded. Mortality was monitored within stages. At emergence, proportion of female was calculated. Experiments were replicated 3 times.

Effects of cowpea varieties on some biological parameters of L. javana

The newly hatching larvae of M. vitrata were reared for two days using sprouts from Wankoun variety. The trial started by offering Twenty-five (25) two-day old larvae of M. vitrata to mated female of L. javana for parasitism. Parasitized larvae were individually placed in small containers (3 cm diameter x 3.5 cm height) and reared using sprouts of each of the 10 cowpea varieties. The development time of parasitized larvae was recorded and time from cocoon to adult stage as well. Fecundity of emerged females was studied by coupling them and by submitting daily to each female 25 two-day old larvae of M. vitrata. Sex-ratio and longevity in both sexes was recorded. Mortality within development stages was recorded for each cowpea variety.

Statistical Analysis

Data were processed using MS Excel. Percentage data were transformed Arcsin \(\surd P\) transformed prior to ANOVA using SAS software version 9.2. Data relative to the parasitism rate, mortality, development time, longevity and fecundity was analyzed by performing ANOVA followed by the Student-Newmans Keuls test (SNK). Chi Square was run to compared cowpea varieties for sex ratio.

Effects of cowpea varieties on some biological parameters of M. vitrata

Mortality

The Fig. 1 shows the percent mortality of M. vitrata larvae when fed using sprouts from different cowpea varieties. The lowest larval mortality was obtained in variety TN5-78. However, no significant differences were observed between this variety and Kpodji and Tawa.

Cumulative daily fecundity

The cumulative daily fecundity of M. vitrata was significantly higher when larvae were reared using sprouts from the variety Vita5 and IT 84D-449 (Fig. 2). These varieties were followed by TN-27, IT 07K-1-10.

Sex-ratio

No significant differences were observed in the percentages of male and female offsprings, regardless of cowpea varieties (Table 2).

Table 2

Sex ratio of *M. vitrata*, when larvae were offered sprouts from different cowpea varieties
Varieties	Male percentage (%)	Female percentage (%)	Pr > ChiSq
Kpodji	44.9	55.1	0.47
Tawa	42.86	57.14	0.31
Vitoco	48.65	51.35	0.82
Vita5	64.29	35.71	0.13
TN5-78	54.72	45.28	0.49
TN27-80	60	40	0.23
IT84D-449	55.88	44.12	0.49
IT07K-1-10	48.15	51.85	0.84
Tilligré	39.29	60.71	0.25
Komcallé	46.43	53.57	0.7

Development time

The development time of M. vitrata larvae was significantly shorter when larvae were fed using sprouts from the variety IT 84D-449 compared to the other varieties, exception for Kpodji, Tawa and TN5-78 (Table 3). The time taken by adults to emerge from pupae was significantly shorter on the variety Kpodji compared to the other varieties. Likewise, the development cycle was significantly shorter when M. vitrata larvae were reared using sprouts from Kpodji compared to the other varieties. The emergence rate was significantly higher in TN5-78 and Kpodji compared to Vita5. However, no significant differences were obtained between these two varieties compared to the other ones. Adults lived longer when larvae were reared using sprouts from Kpodji and Tawa compared Vita 5, TN5-78, IT 84D-449 and IT 07K-1-10. However, no significant differences were observed between the two varieties (Kpodji and Tawa) and the remaining varieties.

Table 3

Effet of sprouts from different cowpea varieties on key biological paramaters of M. vitrata
Varieties	Larvae development length (Day)	Development time from pupae to adults (Day)		Percent emergence (%)	Longevity (j)
Kpodji	11.26 ± 0.19bc	5.8 ± 0.10a	19.82 ± 0.21a	99.33 ± 0.60b	10.46 ± 0.33d
Tawa	11.5 ± 0.17c	6.42 ± 0.16b	20.66 ± 0.27bc	97.33 ± 0.88ab	10.36 ± 0.34d
Vitoco	10.56 ± 0.09ab	7.37 ± 0.14c	20.97 ± 0.17c	95.66 ± 0.66ab	10.27 ± 0.27cd
Vita5	10.6 ± 0.09ab	6.67 ± 0.10b	20.42 ± 0.18bc	90.33 ± 3.17a	8.25 ± 0.51ab
TN5-78	11.5 ± 0.09b	6.43 ± 0.12b	20.94 ± 0.18c	99.33 ± 0.60b	7.26 ± 0.31a
TN27-80	10.45 ± 0.10ab	6.57 ± 0.11b	20.25 ± 0.16bc	98 ± 1.00ab	9.28 ± 0.36bcd
IT84D-449	10.32 ± 0.17a	6.76 ± 0.11b	20.02 ± 0.16bc	93 ± 5.36ab	8.64 ± 0.60abc
IT07K-1-10	10.56 ± 0.16ab	6.55 ± 0.11b	20.33 ± 0.22bc	96 ± 0.88ab	8.25 ± 0.58ab
Tilligré	10.5 ± 0.27ab	6.7 ± 0.14b	20.23 ± 0.26bc	99 ± 0.33ab	9.76 ± 0.41bcd
Komcallé	10.66 ± 0.09ab	7 ± 0.10bc	20.63 ± 0.13bc	97.66 ± 0.88ab	9.15 ± 0.44bcd

Means affected by the same letter in each column are not significantly different with ANOVA followed by SNK at 5%

Effect of cowpea varieties on some biological parameters of Liragathis javana

Mortality

The lowest percent mortality was obtained when L. javana-parasitized larvae were reared with sprouts from the variety Kpodji compared to the other varieties (Fig. 3). Kpodji was followed by Tawa and Vitoco. However, no significant differences were observed between these varieties and the remaining varieties, except Vita5.

Cumulative daily fecundity

The cumulative daily fecundity of L. javana varied when larvae were fed using sprouts from the different cowpea varieties (Fig. 4). The curves illustrating the cumulative daily fecundity were substantially higher for the varieties IT84D-449, Komcallé, Vitoco, Vita5, Kpodji and TN27-80 than the remaining varieties.

Sex-ratio

The sex-ratio was male-biased when parasitized larvae were reared using sprouts from the varieties Tawa and Vitoco (Table 4). However, no significant differences were observed between the percentage of male and female offsprings for the remaining varieties.

Table 4

Sex ratio of *L. javana*, when parasitized larvae were fed using sprouts from different cowpea varieties
Varieties	Percentage of male (%)	Percentage female (%)	Pr > ChiSq
Kpodji	46.51	53.49	0.64
Tawa	84.62	15.38	0.01
Vitoco	81.25	18.75	0.01
Vita5	70	30	0.2
TN5-78	66.67	33.33	0.12
TN27-80	52.94	47.06	0.8
IT84D-449	64.71	35.29	0.22
IT07K-1-10	60	40	0.37
Tilligré	42.11	57.89	0.49
Komcallé	53.33	46.67	0.79

Development time

Larval development was significantly shorter when L. javana-parasitized larvae were fed using sprouts from the varieties Kpodji, Tawa, Vitoco, TN-78 and Tilligré compared to the other varieties exception for Vita5, TN27-80, IT84D-449 and Komcallé (Table 5). Time taken by adult parasitoids to emerge from cocoons was significantly shorter in Vitoco, but was not significantly different from the other varieties, except IT 07K-1-10. The whole development cycle of L. javana was significantly shorter in Vitoco, but not statistically different from the other varieties, except for IT 07K-1-10. The parasitism rate of M. vitrata larvae by L. javana was significantly higher in Kpodji compared to Tawa but not statistically different from the other varieties.

Table 5

Effet of sprouts from different cowpea varieties on key biological paramaters of *Liragathis javana*
Varieties	Larvae stage duration (day)	Cocoon stage duration (day)	Developpment cycle (day)	Parasitism rate (%)
Kpodji	10.48 ± 0.07a	6.53 ± 0.13a	17 ± 0.18ab	77.33 ± 10.66b
Tawa	10.46 ± 0.14a	6.23 ± 0.28a	16.69 ± 0.39ab	33 ± 9.33a
Vitoco	10.38 ± 0.11a	6.33 ± 0.22a	16.22 ± 0.47a	40 ± 8.32ab
Vita5	10.7 ± 0.19ab	6.9 ± 0.28a	17.63 ± 0.45ab	46.66 ± 5.81ab
TN5-78	10.52 ± 0.11a	6.53 ± 0.17a	17.09 ± 0.25ab	49.33 ± 5.33ab
TN27-80	10.76 ± 0.10ab	6.41 ± 0.22a	17.17 ± 0.28ab	46.66 ± 12.71ab
IT84D-449	10.76 ± 0.10ab	6.7 ± 0.18a	17.47 ± 0.27ab	40 ± 6.11ab
IT07K-1-10	11.00 ± 0.00b	6.8 ± 0.21a	17.8 ± 0.21b	40 ± 6.11ab
Tilligré	10.47 ± 0.14a	6.42 ± 0.23a	16.89 ± 0.34ab	45.33 ± 9.33ab
Komcallé	10.86 ± 0.09ab	6.33 ± 0.23a	17.2 ± 0.27ab	48 ± 2.30ab

Means affected by the same letter in each column are not significantly different with ANOVA followed by SNK at 5%

Effect of cowpea varieties on some biological parameters of Phanerotoma syleptae

Mortality

Figure 5 gives the mortality rate of larvae hatching from eggs parasitized when they were fed using sprouts from different cowpea varieties.

Cumulative daily fecundity

Figure 6 shows the curves that describe the cumulative daily fecundity of P. syleptae females when larvae hatching from parasitized eggs were reared using sprouts from different cowpea varieties. The curves describing the cumulative daily fecundity for Kpodji and Tilligré were above those describing the cumulative daily fecundity for the other varieties, suggesting higher fecundity on these two varieties. Moreover, these two varieties induced a longer oviposition period compared to the others.

Sex ratio

There was no significant difference between proportions of male and female wasps, regardless of cowpea varieties (Table 6).

Table 6

Sex ratio of *P. syleptae*, when larvae hatching from parasitized eggs were fed using sprouts from different cowpea varieties
Varieties	Percentage of male	Percentage of female	Pr > ChiSq
Kpodji	56.25	43.75	0.22
Tawa	59.09	40.91	0.39
Vitoco	65.63	34.38	0.07
Vita5	56.67	43.33	0.46
TN5-78	55.71	44.29	0.33
TN27-80	61.54	38.46	0.23
IT84D-449	61.11	38.89	0.18
IT07K-1-10	63.64	36.36	0.36
Tilligré	44.44	55.56	0.37
Komcallé	64.71	35.29	0.22

Development time

The development time of larvae that hatched from P. syleptae-parasitized eggs was significantly lower when fed using sprouts from the varieties Kpodji Tawa and Tilligré (Table 7). No significant differences occurred between varieties for the time taken by adults to emerge from cocoons. However, the total development cycle was significantly shorter on Tilligré, Kpodji, Tawa and TN27-80. The highest parasitism rate was observed in TN27-80 when parasitized larvae were fed using sprouts from this variety.

Table 7

Effet of sprouts from different cowpea varieties on key biological paramaters of *Phanerotoma syleptae*
Varieties	Larvae stage duration (Day)	Cocon stage duration (Day)	Cycle Development (Day)	Parasitism rate (%)
Kpodji	9.66 ± 0.19a	5.54 ± 0.12a	18.18 ± 0.23a	44 ± 6.11ab
Tawa	9.79 ± 0.18a	5.41 ± 0.12a	18.2 ± 0.25a	38 ± 1.33ab
Vitoco	11.81 ± 0.08d	5.45 ± 0.12a	20.27 ± 0.14de	29 ± 2.66ab
Vita5	10.91 ± 0.37bc	5.25 ± 0.17a	19.16 ± 0.51bc	16 ± 2.30a
TN5-78	11.3 ± 0.08cd	5.22 ± 0.08a	19.5 ± 0.14de	66.66 ± 15.37b
TN27-80	10.55 ± 0.17b	5.13 ± 0.09a	18.68 ± 0.22ab	38.66 ± 1.33ab
IT84D-449	11.88 ± 0.08d	5.23 ± 0.10a	20.11 ± 0.14de	22.66 ± 1.33a
IT07K-1-10	11.96 ± 0.17d	5.58 ± 0.13a	20.54 ± 0.24e	36.66 ± 6.66ab
Tilligré	9.72 ± 0.24a	5.18 ± 0.06a	17.87 ± 0.26a	44 ± 9.23ab
Komcallé	11.96 ± 0.07a	5.12 ± 0.13a	20.12 ± 0.15ab	33.33 ± 11.85ab

In the same column, the averages followed by the same alphabetical letters are not significantly different.

The results of this study allowed assessing the effect of ten cowpea varieties on the development of two biocontrol agents L. javana and P. syleptaeand their obligate host, the pod borer M. vitrata. Our study shows that the larvae of M. vitrata or parasitized larvae were able to develop by feeding on sprouts of all tested cowpea varieties. However, the development cycle of M. vitrata ranged between 16 and 20 days, slightly shorter compared to that on artificial diet (21 days), adapted from Jackai and Raulston (1988). Considering the development cycle, adult emergence rate, longevity and larval mortality, the cowpea varieties Kpodji and Tawa were identified as optimizing rearing of M. vitrata larvae compared to the other varieties. Such differences may be explained by differences between these cowpea varieties in chemical content/antinutritional factors ((Ileke 2014; Omenna et al. 2016) as hypothesized by Dannon et al. (2012). Thus, cowpea seeds were reported to contain high amounts of antinutritional factors including enzymes inhibitors that may act against insect pests feeding on cowpea seeds or sprouts (Kumar et al. 2021;). However, during germination, seeds antinutritional compounds content was reduced in the different cowpea varieties tested (Kumar et al. 2021), enabling then the successful development of M. vitrata larvae when fed using sprouts. Beside secondary metabolites, larval mortality rates may also be influenced by environmental factors such as temperature and relative humidity (IITA 1991; Rurema 2007, Adati et al. 2004, Brown and al. 2004; van Lenteren and al. 2006; Bale and al. 2008).

Likewise, the larval parasitoid L. javana performed better when parasitized M. vitrata larvae were reared using sprouts from Kpodji and Tawa when with regard to development time, parasitism rate, cumulative daily fecundity, and mortality rate. On the other hand, L. javana sex-ratio was male-biased when parasitized larvae were reared using sprouts from the variety Tawa. Also, a lower cumulative daily fecundity was observed when L. javana-parasitized larvae were reared with sprouts from Kpodji. A similar trend was observed for the egg-larval parasitoid P. syleptae when M. vitrata larvae were fed using Kpodji and Tawa sprouts with regard to development time, parasitism rate, mortality rate and cumulative daily fecundity. However, lower cumulative daily fecundity and higher mortality rates were obtained in Tawa as compared to Kpodji. Our observations on the two parasitoid species support the previous findings that the nutritional quality of the substrate offered to the host insect affected the biology of koinobiont parasitoids (Eben and al. 2000; Lill and al. 2002 ; Uçkan and Ergin 2002; Hunter 2003; Harvey 2005; Parmesam 2006; Gols et al. 2008; Gols and Harvey 2009; Dannon et al. 2012). In fact, koinobiont parasitoids developing on hosts that continue to feed by consuming plant tissues were found to be more affected compared to haemolymph feeders (Harvey 2005). Parasitoid species that feed by completely consuming host tissues were known to be affected by low quality plant food taken by host larvae. This was confirmed for L. javana and P. syleptae which development and fecundity were affected cowpea varieties. Moreover, sex-ratio of L. javana was male-biased when parasitized larval were fed using sprouts from the cowpea varieties Tawa and Vitoco, suggesting a small host size at oviposition. Similar observation has been reported in L. javana by Aboubascar Souna (2021). Many studies pointed out the influence of host size at oviposition on parasitoids sex allocation (Jones, 1982, King, 1987 and Dicke, 1999). When ovipositing, parasitoids deposit female offspring in larger hosts and male offspring in the smaller ones (van Nieuwenhove and Ovruski, 2011; Ueno, 2015). This may also be true in the current study for the egg-larval parasitoid P. syleptae, which may deposit male offspring in small eggs of M. vitrata. On the other hand, many factors were found to influence sex ratio in koinobiont parasitoid species (Aguirre-Gil et al. 2020). Thus, sex-ratio was female-biased in the parasitoid Diachasmimorpha longicaudata Ashmead when superparasitism increased on its host Anastrepha ludens Loew (van Nieuwenhove and Ovruski 2011). Similar observation was reported by Mackauer and Chow (2016) in Dendroceus carpenter (Curfis) when parasitizing its host Aphidius eri Haliday. Host diet was also found to affect sex-ratio in Doryctobracon areolatus Szépligeti when offered its host Anastrepha fraterculus Wiedmann (Ongaratto et al. 2019). Higher male proportion was observed when A. fraterculus was fed using wheat flour mixed with soybean meal.

Larval mortality remains high when unparasitized or L. javana-parasitized larvae, or larvae hatching from eggs parasitized by P. syleptae were reared using sprouts from the different cowpea varieties. Larval mortality ranges between 30–60%, 28–70% and 14–75% in M. vitrata, L. javana and P. syleptae, respectively at 26 ᵒC. At similar temperature, Wetro and al. (2014) reported between 13–25% of M. vitrata in the varieties Kpodji, Tawa and TVX 3236 compared to artificial diet with mortalities rates up to 40%. On artificial diet diet, Dannon et al. (2010) observed similar findings in M. vitrata at 26ᵒ when larvae were reared using artificial diet. Mortality should be reduced by changing regularly towel paper during rearing using sprouts from different cowpea varieties.

In summary, the biology of M. vitrata and of its two parasitoids L. javana and P. sylepate was affected by cowpea varieties included in the current study. Sprouts from cowpea varieties Kpodji and Tawa were found to be more suitable for M. vitrata, L. javana and P. syleptae rearing.

Acknowledgements

This work was carried out with the support of the CGIAR Research Program on Grain Legumes and Dryland Cereals (GLDC) through the International Institute of Tropical Agriculture (IITA), the Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ), United States Agency for International Development (USAID) and Bill & Melinda Gates Foundation (BMGF) . We also grateful to Mathias Azokpota, Basile Dato of IITA-Benin for their technical assistance during this study.

Conflict of interest:

The authors declare that they have no conflict of interest.

Ethical standards

The study did not include humans as subject; therefore, ethical standards were not applicable.

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Suitability of cowpea sprouts to optimize rearing of biocontrol agents Phanerotoma syleptae and Liragathis javana

Status:

Journal Publication

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Abstract

Figures

INTRODUCTION

MATERIAL AND METHODS

Experimental site

Insect material

Selection of cowpea seed

Cowpea seed germination process

Mass rearing of Maruca vitrata

Mass rearing of the parasitoids on several varieties of cowpea germinated seed in lab

Phanerotoma syleptae

Liragathis javana

Experimental set-up

Effects of cowpea varieties on some biological parameters of M. vitrata

Effects of cowpea varieties on some biological parameters of P. syleptae

Effects of cowpea varieties on some biological parameters of L. javana

Statistical Analysis

RESULTS

Effects of cowpea varieties on some biological parameters of M. vitrata

Mortality

Cumulative daily fecundity

Sex-ratio

Development time

Effect of cowpea varieties on some biological parameters of Liragathis javana

Mortality

Cumulative daily fecundity

Sex-ratio

Development time

Effect of cowpea varieties on some biological parameters of Phanerotoma syleptae

Mortality

Cumulative daily fecundity

Sex ratio

Development time

DISCUSSION

Declarations

References

Status:

Journal Publication

Version 1