Karp et al. estimated 4–12 birds/ha of species that are confirmed or suspected CBB predators. Flocks of migratory birds on coffee farms are estimated at 19/ha and 24/ha , but these values are also short of our estimates of necessary densities for suppressing larger CBB outbreaks. One caveat to our conclusions is that our calculations were based on CBB accounting for 5%–10% of a bird’s daily diet . This assumption meant birds would only eat a set maximum of 7 CBB per day. Sherry et al. reported up to 116 CBB in the stomach contents of a single warbler, suggesting under certain circumstances in the field, birds eat more CBB. Generalist insectivores, particularly Neotropical migrants, have flexible foraging preferences and would likely feed opportunistically on CBB in response to dramatic dispersal peaks. Therefore, birds might be expected to increase feeding rates as CBB disperser abundances increase, though it may depend on the relative abundances of other prey. Better data on CBB consumption rates by birds under different circumstances would improve our estimates of the circumstances under which birds can control CBB populations.
A second caveat is that bird densities used in the model may not represent the potential for CBB control because bird densities depend on the structure of the agricultural landscape, pot with drainage holes which the current model does not consider. On coffee farms, birds are more abundant when native tree cover is highest and natural forests are close by . Across tropical and temperate regions, the propensity for birds to forage on farms, and thus exert pressure on agricultural pests, is correlated with the physical complexity and diversity of the agroecosystem . For example, birds make more frequent foraging trips to apple orchards with high native tree coverage . In alfalfa fields, edge habitat complexity supports greater avian richness leading to lower pest abundances . Under some circumstances, the density of birds foraging in certain areas may be higher than average densities would imply, leading to greater control potential than our models suggest. More generally, our CBB population model is density independent and assumes environmental conditions and sufficient resources to allow CBB populations to increase without restriction. As a result, our model is limited, as it does not consider localized effects of weather and temperature fluctuations on CBB developmental time , nor characteristics of coffee farms that influence both CBB infestation and bird density. We assumed maximal capacity for CBB population growth and used estimates of bird densities from the literature that only included birds known to consume CBB, perhaps underestimating the potential for avian control.
Models are an important tool for estimating population dynamics, but as with any species, the growth potential for CBB and availability of its predators, is context dependent. Our study echoes Kendall et al.’s conclusion that, even though errors in model construction are common, these seldom change qualitative conclusions. From our population matrix, CBB daily growth rate converged on λdaily = 1.042 around day 124, with an observed rate of population change across the entire coffee-growing season of 705 . Our λdaily is higher than Mariño et al.’s reported lambda of 1.32 over 50– 56 days, which corresponds to λdaily ≈ 1.006 . Part of this discrepancy may come from the fact that Marino et al. combined vital rates across life stages with different time steps. Nonetheless, both models are consistent in predicting rapidly growing populations. Observed CBB population growth rates are similar to ours: Baker, Barrera, & Rivas, calculated a 1.067 growth rate in wild populations and RuizCardenas and Baker reported 1.047 in CBB reared in laboratory settings. In their sensitivity analysis, Mariño et al. reported that adult female survival, and transitions from larva to pupa and pupa to juvenile had high sensitivity in contributing to population growth rate, with adult survival the highest . We found a similar peak sensitivity value for female adult survival in our matrix , supporting the idea that CBB population growth is most sensitive to adult survival rate. Interestingly, dispersal survival from our matrix was estimated to have low impact on population growth , even though this life stage is when CBB are vulnerable to bird predation.
Thus, our analysis superficially suggests that population control once CBB are established should focus on reducing adult survival rather than on trapping dispersing females , if the same impact on numbers could be achieved. However, dispersing females are much more accessible to control methods like spraying fungal bio-insecticide than are adult females, which are inside the coffee cherries, so despite the tremendous difference in sensitivity values, management of an established population is likely to be more cost effective by continuing to focus on dispersing females . Population models specific to CBB have been criticized for not being representative of wild populations, since more generations are estimated through modeling than are observed in field studies . We analyzed CBB population growth using a deterministic model, with an even distribution of dispersal and a fixed predation pressure. While CBB dispersal is continuous, there can be dramatic intraseasonal peaks in numbers that were not captured by our model . In addition, reported longevity of female CBB varies widely from 55 to 380 days, though some studies looked at CBB reared on artificial diet . Refinements of survival in natural settings would, therefore, improve models of CBB population growth, and the potential for control by birds. If field data on CBB vital rate stochasticity become available, and bird densities opportunistically increase during CBB peak numbers, it could affect our conclusions about the capacity of birds to control larger CBB outbreaks. Based on our analyses, there is a population density of CBB above which their capacity to produce more adults exceeds the ability of birds to control their numbers, at least to limit the population size by 50%. This positive density-dependent relationship between population growth and density is an Allee effect , and escape from predation is one mechanism for this phenomenon . In general, predator-driven Allee effects can occur when predators are the main driver of prey dynamics and when predators are generalists as are insectivorous Neotropical migrants . Additionally, large pot with drainage predators can exert strong pressure when prey availability is not temporally or spatially limited—a potential limiting factor in the coffee system, since CBB are only available to birds during dispersal. The degree to which birds exert an Allee effect on CBB might depend on the starting population size of the pest. Variation in starting population size is likely dependent on how recently CBB have colonized in an area, timing of trapping , the size of the farm , and the extent to which farmers used control measures the previous year . We found that only under very low initial population sizes of CBB could birds be expected to suppress pest numbers by 50%. We note that earlier, stronger CBB suppression by birds would lead to lower infestation numbers later in the coffee season, but this might require selective foraging by birds, depending on relative abundances of other prey species. In conclusion, our models suggest that birds can control CBB under some circumstances, depending on the relative size of the starting CBB population and existing local bird density. To put this idea into practice it is important to remember that managing farms for bird habitat does not always result in pest reduction. Birds may not prey on the pest of interest or birds might cause pest numbers in increase by preying on insect predators that normally regulates the pest population . Aside from predators, pest species are also impacted by the agricultural environment directly . In fact, on coffee farms where bird densities are higher in shade, CBB infestations are also higher , possibly because CBB native range is in humid, shade forests of Africa .
It is important that future modeling include such habitat-specific factors to understand Our research helps quantify the densities under which birds have the potential to control CBB populations. Putting these numbers into practice will require understanding how manag Raspberries are an important crop for California, where it is among the top 20 commodities with an average annual value of $448 million from 2015 to 2017 . Maintaining a high yielding, disease-free crop has been difficult due to low availability of farm workers to harvest and maintain the fresh market raspberry canopies. As a method to adjust to their limited field workers, some growers in California began experimenting with new cultural practices, such as pruning leaves near the base of the canes. In Ventura County, the common pruning practice is to remove mature and senescent leaves using the twine that is part of the trellis. The influence of these experimental cultural practices on incidence and severity of cane Botrytis is not known. Managing B. cinerea cane and fruit infections during this early period can be important for reducing severe epidemics during the rest of the crop. The first chapter focuses on our studies that evaluate the influence of pruning leaves on incidence and severity of cane Botrytis. In the previous decades, soil borne diseases were commonly managed with the use of chemical fumigation, but the widening restrictions on the use of fumigants in the San Joaquin Valley of California has posed a challenge for growers. The California processing tomato industry averaged $1.1 billion in value from 2013 to 2017, and accounted for 93% of the production in the United States in 2017 . Southern blight is a soil borne disease of processing tomato that has long been an economic concern in the San Joaquin Valley, and recently caused a widespread epidemic in both the San Joaquin and Sacramento Valleys . The threat of southern blight has caused reductions in acres planted with processing tomato in the southern San Joaquin Valley . The objectives of the second chapter were to: evaluate susceptibility of commercial processing tomato cultivars to southern blight; and evaluate grafting and increased height of the graft union with the resistant rootstock Maxifort for southern blight management in processing tomato.Raspberries are an important crop for California, where it is among the top 20 commodities with an average annual value of $448 million from 2015 to 2017 . This represented 82% to 88% of the domestic raspberry production. The four California counties where raspberry is produced are Ventura, Santa Cruz, Santa Barbara, and Monterey. Specifically in Ventura and Santa Cruz counties, raspberries are among the top commodities . On the West Coast of the United States, raspberry is typically produced in two stages from a single planting that is grown for a maximum of two years. In the primocane stage or first year cycle, harvest generally begins four months after planting of bare root transplants and continues for approximately three to five months. After harvest, the primocane growth is pruned near the last fruiting lateral or is mown at the soil line. The growth that follows this pruning begins the floricane stage or second cycle, which has a harvest period that generally begins three to four months after pruning and can last approximately four months. In Ventura County, a crop can be planted during four periods throughout the year: early spring, late spring, mid-summer, or late summer. The ascomycete fungal pathogen Botrytis cinerea Pers.:Fr causes multiple diseases of raspberry. The first is grey mold or Botrytis fruit rot, a widespread and damaging disease of fruits and flowers of many hosts including raspberry . The second is cane Botrytis, a disease of the raspberry cane that was first reported in England in 1931 . Cane Botrytis is reportedly more severe on red raspberry canes compared to other Rubus spp. . Cane botrytis is known to more commonly affect the floricane stage in the U.S. Pacific Northwest , but in coastal California and in eastern Canada B. cinerea causes disease primarily to the primocane stage of raspberry . Managing B. cinerea cane and fruit infections during this early period can be important for reducing severe epidemics during the rest of the crop. For example, a study evaluating B. cinerea conidia dispersal in raspberries showed locally-produced inoculum was important for controlling fruit infections . New raspberry plantings are often established next to older plantings where mummified fruits, damaged canes, and dead plant debris potentially colonized by B. cinerea sclerotia are prevalent . These infected debris from older plantings are an important source for primary spore inoculum . Resting mycelia and sclerotia can also be found on weeds .