Plots of UC 92 were vacuumed in 2019 and monitored with water traps in 2021 to check the difference in L. hesperus densities. In 2019, plots in the sprayed block had on average 0.5 L. hesperus while plots in the unsprayed block had on average 1.5 L. hesperus. In 2021, water traps collected every 3 days for 2 weeks after the insecticide treatment indicated that in UC 92 plots there were an average of 0.125 L. hesperus in the sprayed section and an average of 0.375 L. hesperus in the unsprayed section. These data indicate that there was a difference in L. hesperus densities between the two treatment blocks. High tolerance of damage by L. hesperus was defined by dividing the yield of untreated plots by the yield of insecticide treated plots . If the quotient was high, the variety was highly defended against L. hesperus. The most desirable varieties for breeding are those with high tolerance to L. hesperus and high yield. It should be noted that the term “tolerance” is used here as a place holder for the true mechanism of resilience. It is possible that some varieties may be resistant rather than tolerant. A resistant variety can avoid damage while a tolerant variety can recover from damage. Sixteen commercial cultivars from the United States were included in this study . On average, square pot these elite lines yielded 65% as much in conditions unprotected by insecticide as compared to conditions with protection of insecticide. Pat and Kingstone had the highest L. hesperus tolerance while UC Lee , UC 92 , and Fordhook had the lowest.
UC Haskell performed the best when both yield and L. hesperus tolerance are accounted for.Among the 56 heirloom and international cultivars, the average yield in unprotected conditions compared to protected conditions was 73%. Noir De Kisenyi – G26196 , a baby, black-and-white-seeded, bush type from Rwanda, and Hopi 50 , a baby, white-seeded, vine type from the United States, were the least susceptible to L. hesperus but did not compete well on overall yield. The American heirloom variety, Jackson Wonder, had the best combination of high L. hesperus tolerance and an average yield of 2338.4 g/plot when treated with insecticide and 1945.4 g/plot when unprotected by insecticide. Many of the international and heirloom varieties included in this study have already been incorporated into the UC Davis Lima Bean Breeding Program. These results will offer additional guidance in parent selection and the choice of varieties used in future research on insect defense mechanisms.When comparing total number of L. hesperus per plot and yield per plot, yield did not appear to be affected by the density of L. hesperus, as shown by the nearly horizontal trend line . In this group of varieties, UC 92 appeared to be an outlier with low yields but also a low number of L. hesperus. UC 92 is the only variety in this study from the Andean gene pool of Lima bean. While the yields of Andean and Mesoamerican varieties are comparable, Andean lines tend to have much larger seeds. As a result, the loss of a single seed due to L. hesperus herbivory would constitute a loss of a higher percentage of yield than when a single small seed is lost .It should also be noted that data from this experimental design may over represent resistance or tolerance traits. On a typical California farm, a single variety will be planted across several if not tens of acres. While L. hesperus adults are highly mobile, they have less choice in an actual farm setting than in this experimental field.
Nymphs were not considered in this study because they were crushed by the strong pressure of the vacuumand difficult to identify with the available expertise. Nymphs are much less mobile than adults and therefore would have less ability to choose between varieties. It is interesting to note the strong difference in the average number of L. hesperus per plot between the two highest yielding varieties, UC Haskell and UC Beija Flor. The large difference in average L. hesperus might be attributable to sampling bias caused by the denser canopy of the indeterminate UC Haskell compared to the determinate UC Beija Flor. It could also be that these two varieties have different mechanisms of resistance or tolerance to L. hesperus. UC Haskell and UC Beija Flor share as a parent UC Cariblanco N, but their resistance probably comes from their other respective parents. UC Beija Flor is an F10 progeny from a cross of UC Cariblanco N and CIAT accession G25165 . This accession was included in the diversity panel yield comparison under insecticide treated and untreated conditions described above. It has an average sprayed yield of 1650.8 g per plot and an average unsprayed yield of 1106.4 g per plot. This puts its average yield with insecticide protection above average but its estimated tolerance of L. hesperus below average. UC Haskell is an F10 progeny from a cross of UC Cariblanco N and an accession introduced from CIAT accession “P&T 4255” . This accession has been less well studied at it is unknown what degree or mechanism of resilience it may offer to insect herbivory. These results open several avenues of research to pursue in the future. From further analysis of these and other data it may be possible to distinguish resistance and tolerance traits.
Another area of future exploration should be the differences between the defense phenotypes of Lima beans in the Andean and Mesoamerican gene pools. Unlike the Mesoamerican gene pool, the range of Andean Lima beans falls completely outside the native range of the genus L. hesperus and therefore may have fewer defense adaptations to it and closely related species . In figure 12, the only Andean variety, UC 92, supports very low L. hesperus densities. However, as shown in Figure 11, it is very sensitive to L. hesperus with nearly a 50% drop in yield in the unsprayed plots compared to the sprayed plots. This combination of data may indicate that the variety has some resistance traits but very little tolerance to L. hesperus herbivory. Future studies should include additional Andean lines for a more robust comparison of defense phenotypes between the two gene pools.There was a very significant effect of variety on levels of cyanide in flowers, but the effect was not significant in pods . Large-seeded Lima beans, UC 92 and UC Lee had significantly less cyanide in their flowers than UC Haskell. Henderson Bush and UC Beija Flor had intermediate levels of cyanide in flowers that were not significantly different from UC Haskell, UC 92, or UC Lee . Cyanide was not detectable in the mature seeds. The presence of L. hesperus had no effect on the level of cyanide in flowers but it did affect the level of cyanide in pods . Pods collected from plants with L. hesperus had higher levels of cyanide . This may be due to increased enzymatic activity increasing the cyanogenic capacity rather than potential as this was found in Lima bean leaves . There was no significant difference in cyanide levels when considering the interactions between treatment and variety. The duration of the L. hesperus presence, as measured by “Time” in the study, did not affect the level of cyanide inflowers or pods. Nor was there a significant difference in cyanide levels when considering the interaction of treatment and time, square plastic plant pots or time and variety.From the greenhouse experiment of cyanogenic capacity with and without the presence of L. hesperus, insects from each of the cages were counted four weeks after flowering and three weeks after the L. hesperus had been introduced. For thorough counts, all plants were completely deconstructed with each leaf carefully checked for nymphs and the surface of the potting soil carefully searched. It is interesting to note that the two varieties known to be resistant or tolerant of L. hesperus, UC Haskell and UC Beija Flor, have very different numbers of surviving adults and new nymphs. It is possible that cyanogenesis may be responsible for this difference. In quantified studies of greenhouse grown samples without L. hesperus, UC Beija Flor was estimated to have a cyanogenic capacity of 153.4 nM/30 minutes of volatile HCN released from floral bud samples and 224.1 nM/30 minutes of volatile HCN released from immature pod samples .
In the same study, UC Haskell only had a cyanogenic capacity of 126.86 nM/30 minutes of volatile HCN released from floral bud samples and 63.4 nM nM/30 minutes of volatile HCN released from immature pod samples. Given that survival of adult and nymph L. hesperus is more strongly correlated with cyanide in immature pods than in flowers this difference in cyanogenic capacity could be contributing to the higher number of insects surviving on UC Haskell as compared to UC Beija Flor. It should be noted however that this was a no-choice experiment and in the field collected vacuum samples, in which there were many varieties to choose from, more L. hesperus were captured on UC Beija Flor than UC Haskell. This could be due to sampling bias due to the denser canopy of the indeterminate UC Haskell or it could indicate that a mechanism other than cyanogenesis is contributing to L. hesperus preference and survival.This study has several limitations. One is that no wild or weedy types were included for comparison with the domesticated forms. While growing these accessions can be challenging in confined greenhouse space, including them in a similar analysis could shed light on how the trait of cyanogenesis has been affected by domestication. Another limitation of the study was the use of discrete semiquantitative measurements using Fiegl Anger paper. While this method of measuring cyanogenic capacity is well established, safe, and high throughput, it does not provide a true quantitative measurement, or the continuous sampling provided by the enclosed detection systems used for studies of volatilized cyanide .This study found that the survival and reproduction of L. hesperus was negatively correlated with the cyanogenic capacity of their host plant. This indicates that selecting plants with higher cyanogenic capacity in their flowers and young pods may be an effective way to control L. hesperus. The evidence does not support the hypothesis that cyanogenic capacity was induced by the presence of L. hesperus. For consumer safety, future research should determine if there is a relationship between the cyanogenic glucoside content of flowers, immature pods, and the mature seeds they grow into as a correlation has been found between the cyanogenic content of mature seeds and the cotyledons from the seedlings those seeds grow into .Farmland covers more than 35% of Earth’s ice-free terrestrial area, and agriculture is expanding and intensifying in many regions to meet the growing demands of human populations . This trend threatens biodiversity and the ecosystem services on which agriculture depends, including crop pollination . Indeed, recent reviews have highlighted how multiple anthropogenic pressures lead to a decline in wild pollinators such as bees, flies, beetles, and butterflies . However, practices to enhance wild pollinators in agroecosystems are still in development , and considerable uncertainty remains regarding their effects on crop yield and farmers’ profits. Here we review recent research on the topic, including the impacts of certain practices on wild pollinators, crop pollination, yield, and profits . We focus on practices that enhance the carrying capacity of habitats for wild insect assemblages that may then provide crop pollination services; practices to conserve or manage a particular pollinator species are outside our scope although they have received attention elsewhere . We offer general science-based advice to land managers and policy makers and highlight knowledge gaps. Throughout, we emphasize the need to consider population-level processes, rather than just short-term behavioral responses of pollinators to floral resources.Plant–pollinator interactions are typically very general, with many pollinators being rewarded with pollen, nectar, or other resources from several plant species , and with most angiosperms being pollinated by multiple insect species . Humans benefit from this generalized nature of pollination systems, as exotic crops brought far from their ancestral ranges can find effective pollinators within native insect assemblages .