Enhancing System Sustainability by Mitigating the Impact of Three Major Constraints to Efficient Cowpea Production and Use: Pests, Pollination and Nodulation

Objective 1. Objective 1. Identify cowpea cultivars for resistance to pests and diseases and pollinator activity and nodulation efficiency.

Results from NC A&T: From visual observations, honey bees, carpenter bees and bumble bees and wasps were noticeable pollinators on the research plots.

Overall, during the five weeks sampling period, more honey bees were observed followed by bumble bees (Fig. 1).

During the first three weeks of visual sampling, there was a steady increase in the total number of pollinators observed followed by a decline (Fig. 2). This increase can be attributed to the presence of more honey bees observed during this time compared to the other pollinators (Fig. 2). Later in the season, the population of honey bees declined but carpenter bee and wasp populations were on the increase (Fig. 2).

More honey bees were observed during the first week and our data during this sampling period show most of the honey bees (21, 22 and 25) were recorded on Early Scarlet, Dixielee and CT pinkeye purple Hull cowpeas respectively  (Fig. 3).

In the second week honey bees were still the major pollinator observed in the presence of some bumble bees and a few carpenter bees (Fig. 4). The distribution of honey bees varied among the twenty-four cowpea varieties with >30 honey bees observed on Mississippi Silver, whippoorwill Steel Black and whippoorwill; >40 observed on Perking black and Rouge et noir; >60 on CT Pinkeye purple Hull and Dixielee (Fig. 4).

In week three more honey bees and bumble bees were observed on most of the cowpea varieties except for Red Bisbee which had no bumble bees. Most of the pollinators were observed on CT Pinkeye purple Hull, Dixielee, Penny Rile and Whippoorwill, Whippoorwill Steel Black (Fig. 5).

In week four and five, the population of bumble bees observed had declined and that of carpenter bees and wasps had increased with more pollinators observed on CT Pinkeye purple Hull, Penny Rile and Whippoorwill and Iron and Clay, Red Bisbee and Tohono O'odham recording the least pollinators (Figs. 6 and 7). Results from sticky cards indicate that the number of pollinators was highest during the third week of sampling, similar to results from visual observations.

The number of brown marmorated stink bug (BMSB) observed on the twenty-four cowpea varieties was different among the varieties with the least BMSB observed on Carrapichio, California Black Eye5, Cream 40, Iron and Clay, Lady, Running Conch, Purple Hull Bigboy and Tohono O’odham (Fig. 8). More BMSB were observed on CT Pinkeye purple Hull, Whippoorwill and Dixielee (Fig. 8) the same varieties on which more pollinators were observed.

During the five weeks of visual observations, the number of brown marmorated stink bug varied over the weeks and among the varieties (Figs. 9-13).

A number of cowpea cultivars were efficient at producing a large number of nodules with indigenous rhizobia (Mixed Iron and Clay, Rouge et noir, Carrapichio and Whippoorwill) and low mean number of nodule were recorded in Black Crowder, Running Conch, CT Pinkeye Purple Hull and Early Scarlet (Fig. 14).

Our results indicate that the chlorophyll content was not different among the different cowpea cultivar (Table 1). Results from yield data indicate that most of the cowpea varieties produced more pods except Iron and Clay and Tohono O’odham which were vegetative throughout the season. The least fresh pod weight was recorded for Cream 40, Lady and Penny Rile (Table 2). From the fresh seeds, Big boy had the most (51%)damaged fresh seeds and Whippoorwill had the least (27%) damaged dry seeds (Table 2).

Table 2. Fresh and dry seed yield and percentage damage of 24 cowpea varieties

Results from FVSU:

From sweep net trap, wasps were more prominent on the research plots (Figure 15). Numbers of wasp were significantly higher in first week (Figure 16). In addition, more wasps were recorded on SaDandy cowpea variety in first week while in second week, higher numbers of wasp were found on Big Boy cowpea variety (Figure 17 and 18).

With respect to bees, high numbers of sweat bees were found on all cowpea varieties in the first week of trapping. Honey bees were next to sweat bees in the numbers trapped. Some bumble bees were trapped in second week (Figure 17 and 18). C.T. Dimpled Brown Crowder variety was found to be more susceptible to insect pest attack particularly leafhopper and aphid during first and second week (Figure 19).

During third and fourth week, sweet bees were found consistently attracted to C.T. Dimpled Brown Crowder, California Blackeye and Cream 40 (Figure 20 and 21). Leafhopper and aphids were consistently recorded on each variety of cowpea during third and fourth weeks of sampling (Figure 22). More pollinators were found during fifth and sixth week. In addition, bumble bees and sweet bee numbers were significantly high on sixth week (Figure 23). SaDandy variety of cowpea was found to be more attractive to wasp. Mississippi silver variety was least susceptible to attack by insect pests (Figure 24).

Figure 15. Total number of pollinators collected from cowpea farm in first and second week.

Figure 16. Mean pollinators per variety per week.

Figure 17. Mean (%) insect pollinators per cowpea variety during first week.

Figure 18. Mean (%) insect pollinators per cowpea variety during second week.

Figure 19. Percentage mean of insect pest (%) on cowpea during first two weeks.

Figure 20. Average (%) of pollinator per cowpea variety in third week.

Figure 21. Average number (%) of pollinator per cowpea variety in fourth week.

Figure 22. Percentage mean of insect pest (%) that attacked cowpea during third and four weeks.

Figure 23. Mean percentage of insect pollinators on variety of cowpea during fifth and sixth weeks.

Figure 24. Percentage mean of insect pest attack on each variety of cowpea during fifth and sixth weeks.

Yield data showed that the highest dry pod weight was for Collossus variety of cowpea and the least for White Acre as well as Top Pick cream cowpea varieties (Table 3). Furthermore, Big Boy had the most (45.1 ± 9.9) damaged seeds and Red Ripper had the least (3.3 ± 0.5) damaged seeds.

Table 3. Dry seed yield and damage as well as undamaged seeds of 20 cowpea varieties

Objective 2. Evaluate the effect of two best cowpea cultivars with pest and disease resistance, high nodulation and high pollinator attractiveness (from objective #1) on overall yield of vegetable crops in two cropping systems with/without cowpea (mono and intercropping).

Intercropping is the growing of two or more crops in close proximity to enhance biological interaction. Intercropping cowpea with pollinator-dependent crops (squash, okra and watermelon[SOW]) resulted in increased abundance and diversity of pollinators and increased crop yield. 

Visual observation and direct count of pollinators recorded in 2018 and 2019 shows that different types of pollinators were observed on each cowpea variety (planted on both sides of the main crop [SOW] and as monocrop) during entire 5-week sampling periods. This include bumble bees, carpenter bees, honeybees, and sweat bees. A prominent number of wasps, butterflies, and moths were also recorded similar to what was observed in objective one. In both years, the total number of pollinators recorded on each cowpea cultivars were higher compared to the number of BMSB (Figs. 26 and 27). In both years, the total number of pollinators recorded was different among cowpea cultivars with PPH and PPH+PPH recording the highest. On the contrary, in both years all the cowpea varieties recorded low numbers of BMSB compared to pollinators. Overall, there were fewer BMSB observed in 2019 compared to 2018 (Figs. 26 and 27).

Fig. 26. Mean number of pollinators and BMSB observed and recorded on each cowpea variety in 2018

Fig. 27. Mean number of pollinators and BMSB observed and recorded on each cowpea variety in 2019

In 2018 and in 2019, the number of pollinators observed and recorded on all three main crops combined (squash, watermelon, and okra [SOW]) was different from the number that was recorded on the control (SOW without cowpea) (Figs. 28 and 29).  

Fig. 28. Mean number of pollinators observed and recorded on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2018

Fig. 29. Mean number of pollinators observed and recorded on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2019

Results from sticky trap shows that in 2018 and 2019 the number of pollinators recorded was different among the various cowpea varieties (Figs. 30 and 32). In 2019, PPH attracted more pollinators than WHIP as shown in the numbers of pollinators recorded in PPH, PPH+PPH, and PPH+WHIP compared with the numbers on WHIP and WHIP+WHIP (Fig. 32). In both years, the number of pollinators observed on all the three main crops combined (SOW) was higher compared to the control (SOW without cowpea) [SOW] (Figs. 31 and 33). About >50% more pollinators were recorded on the main crops intercropped with cowpea than on main crops without cowpeas.

Fig. 30. Mean number of pollinators recorded on sticky trap on each cowpea variety in 2018

Fig. 31. Mean number of pollinators on sticky trap on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2018.

Fig. 32. Mean number of pollinators recorded on sticky trap on each cowpea variety in 2019

Fig. 33. Mean number of pollinators on sticky trap on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2019

In both 2018 and 2019, the white traps caught the highest number of pollinators on both cowpea (Figs. 34 and 36) and main crops (Figs. 35 and 37). In 2018 the white trap on cowpea recorded the highest number of pollinators on WHIP+WHIP (Fig. 34). While in 2019 the number of pollinators captured in the white traps on cowpea were similar for all the treatments (Fig. 36). Overall, in both years the number of pollinators on cowpea was higher than was recorded on the main crops with more pollinators recorded on the intercropped plots compared to the control.

Fig. 34. Mean number of pollinators recorded in pan traps on each cowpea variety in 2018.

Fig. 35. Mean number of pollinators on pan traps on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2018.

Fig. 36. Mean number of pollinators recorded in pan traps on each cowpea variety in 2019.

Fig. 37. Mean number of pollinators on pan traps on all three main crops (combined SOW) intercropped with cowpeas and control (SOW) without cowpea in 2018

Pollinators from six insect families (Apidae, crabonidae, halictidae, tachinidae pyralidae and vespidae) were recorded on sticky traps however, pollinators in the tachinidae and halictidae families were the most abundant (Table 4). Total pollinator from sticky trap (Table 4) show cowpea intercrop recorded the highest number of pollinators compared to the control (main crop without cowpea).

Table 4: Mean number of different pollinator families captured on sticky trap per 5m for each of cowpea intercrop with three main crop (PPH+Squash+PPH), (WHIP+Watermelon+WHP) and (PPH+WHIP) and control without cowpea intercrop.

Anthocoridae, berytidae, dolichopodidae and Geocoridae, were the most abundant natural enemies recorded and their abundance (Table 5). Total natural enemy abundance recorded from sticky trap show cowpea intercrop recorded the highest number compared to the control (main crop without cowpea).

Table 5. Mean number of different beneficial families captured on sticky trap per 5m for each of cowpea intercrop with three main crop (PPH+Squash+PPH), (WHIP+Watermelon+WHP) and (PPH+WHIP) and control without cowpea intercrop.

The result shows there were more pollinators recorded on the main crops with cowpea intercrop compared to the control without cowpea intercrop in both years (Table 6). At harvest, almost all main crops intercropped with cowpea recorded the highest average yield compared to the control without cowpea intercropped in both years (Table 6).

Table 6. Total number of pollinators (mean) recorded from all treatments during the five-week period and crop yield (kg) in 2018 and 2019