- Author: Ian Grettenberger
- Author: Addie Abrams
Two of the worst pests plaguing lettuce growers in the Salinas Valley area are aphids, specifically lettuce-currant aphids (Nasovonia ribisnigri), and western flower thrips (Frankliniella occidentalis). Lettuce-currant aphid is an invasive pest that sets up shop in the heart of the lettuce plant and will render the crop unsellable when it reaches high enough numbers. Thrips can both cause cosmetic damage to lettuce crops and are also responsible for the spread of Salinas impatiens necrotic spot virus (INSV), the fatal lettuce disease that has driven large losses since the 2020 growing season.
While effective tools exist to control both aphids and thrips, they are almost exclusively chemical. Chemical sprays are increasingly under pressure due to changes in the regulatory framework in California as well as the development of pest resistance and discoveries of key chemistries in area watersheds1,2. The UC Davis FiVE lab biological control research program addresses a growing interest in developing alternative tools for managing both pests that do not rely on chemical applications. Biological control provides an opportunity for the management of thrips and aphids that do not rely on chemical tools.
Biological control is defined as the use of natural enemies to control a target pest. Three general categories of biological control could possibly be used as management practices for lettuce pests in the Salinas Valley area:
• Conservation biological control refers to the establishment and maintenance of resources and conditions favorable
• Inundative biological control involves the release of a beneficial insect species in large numbers with the expectation that the beneficials that are released will only provide control for a short amount of time before eventually dying out. Such releases would need to be repeated at regular intervals for the duration of the growing cycle for a crop.
• Augmentative biological control refers to the use of releases of smaller numbers of beneficials to areas where a smaller population of the species already exists, but not in numbers great enough to provide adequate control of the targeted pest species. The goal of augmentative releases is to bolster already-existent populations of beneficial species so they achieve great enough numbers to provide control of the pest or pests of interest.
Conservation biological control in the Salinas Valley
Syrphid flies
Aphid pests of lettuce have been effectively managed in some lettuce production systems through the planting of sweet alyssum adjacent to and interspersed within crop fields3. Sweet alyssum is a favorite of the Syrphid fly (Diptera: Syrphidae), the primary biological control agent used to control aphid pests in lettuce. Syrphids, also called hoverflies or flower flies, are a family of black and yellow pigmented flies which resemble bees and stinging wasps. The coloration is a protective camouflage; Syrphid flies are harmless to humans. Syrphid adults are frequently seen visiting flowers for their nectar and pollen, which the insect consumes both as an energy source and to support their reproduction.
In exchange the female Syrphid flies will lay eggs in lettuce plants with lettuce aphid infestations, the primary food source for their young. Once the eggs hatch, the syrphid maggots, which are predatory on slow, soft-bodied insects, will feed on the aphids and suppress their population. Syrphid larvae are known to be voracious; some California species have been shown to consume upwards of 100 aphids per day4!
Syrphids are the intended beneficiaries of most conservation biological control in central coast lettuce fields, but other beneficial species take advantage of these resources as well.
Other predatory species love sweet alyssum
Many other biological control agents are supported by insectary plantings5. Ladybird beetles often inhabit lettuce fields and may provide some control of lettuce aphid infestations. Common lacewings (family Chrysopidae) are also found in lettuce fields and insectary plantings. Lacewings, which are only predatory in their immature or larval life stage, can provide biological control services against lettuce aphids and western flower thrips. Minute pirate bug (Orius sp.) and aphid midges (Aphidoletes aphidimyza) have also been observed in and collected from insectary plantings in lettuce fields, but it is not known the extent to which they can suppress populations of lettuce aphid or Western flower thrips.
UC Davis Fi-VE Bug IPM Lab biological control research programs
Including insectary plantings to attract naturally occurring predators has historically been the only efficient way to get beneficial species into crop fields. Newly developed technology using drones as a dispersal tool may provide another option for growers interested in using biological control as part of their pest management programs for aphids and thrips. This technology drastically reduces the time and labor required to conduct large releases of laboratory-reared beneficial insects, making the approach more feasible for growers.
As part of a research program funded by the California Department of Pesticide Regulation (CA DPR) and in collaboration with Daniel Hasegawa at USDA-ARS and with Parabug, we are studying the release of biological control agents using drones for the management of aphid and thrips pests of lettuce crops. Our three experimental programs are as follows:
Experiments run by former Monterey County IPM Advisor Alejandro Del Pozo-Valdivia found that a single inundative release of green lacewing eggs (Chrysoperla rufilabris) in lettuce fields reduced aphid pressure six weeks after release6. Our experiment builds on Alejandro's work, examining whether repeated releases of green lacewing eggs throughout the lettuce growing cycle reduce aphid numbers. Additionally, the experiment includes two treatments aimed at suppressing western flower thrips: inundative releases of a species of predatory mite (Amblyseius cucumeris), and a combined release of both predatory mites and green lacewing eggs.
Augmentative releases to bolster non-syrphid predatory species in insectary strips and intercropped alyssum
Other native predators of aphids and thrips are present in the insectary plantings growers use to attract syrphids, but their numbers are too low to provide suppression of thrips and aphids in adjacent crops. These species are reared by commercial insectaries, but using them in an inundative release could prove too costly for growers. Experiments in this program examine the use of smaller releases of these predatory species early in the growing cycle over insectary plantings. The goal is to determine whether the presence of floral resources allows the predators to stick around and build up enough in population to control aphids and thrips in the crop field. Experiments will be conducted with aphid midge (Aphidoletes aphidimyza), an aphid predator, and minute pirate bug (Orius insidiosus), a predator of western flower thrips.
Augmentative releases to manage thrips in non-crop areas
Western flower thrips plague not just vegetable crop fields but also the vegetation surrounding crop areas. In this experiment, we will examine whether releases of cucumeris mites and minute pirate bugs over field edges planted with ice plant will establish these predators in the vegetation and provide long-term suppression of western flower thrips.
Citations
- Deng, X. Study 321: Surface water monitoring for pesticides in agricultural areas in the Central Coast and southern California (2022)
- Gao, Y., Lei, Z. & Reitz, S. R. Western flower thrips resistance to insecticides: detection, mechanisms and management strategies. Pest Manag. Sci. 68, 1111–1121 (2012).
- Brennan, E. B. Agronomic aspects of strip intercropping lettuce with alyssum for biological control of aphids. Biol. Control 65, 302–311 (2013).
- Hopper, J. V., Nelson, E. H., Daane, K. M. & Mills, N. J. Growth, development and consumption by four syrphid species associated with the lettuce aphid, Nasonovia ribisnigri, in California. Biol. Control 58, 271–276 (2011).
- Bugg, R. L., Colfer, R. G., Chaney, W. E., Smith, H. A. & Cannon, J. Flower Flies (Syrphidae) and Other Biological Control Agents for Aphids in Vegetable Crops. (University of California, Agriculture and Natural Resources, 2008). doi:10.3733/ucanr.8285.
- Del Pozo-Valdivia, A. I., Morgan, E. & Bennett, C. In-Field Evaluation of Drone-Released Lacewings for Aphid Control in California Organic Lettuce. J. Econ. Entomol. 114, 1882–1888 (2021).
- Author: Richard Smith
Richard Smith, Joji Muramoto, Tim Hartz and Michael Cahn
UCCE Emeritus Farm Advisor, Extension Specialist, Emeritus, Extension Specialist and Irrigation and Water Resources Farm Advisor.
The winter of 2023 had the highest rainfall years in the last 25 years. The high rainfall resulted in flooding onto farmland along the main branch of the Salinas River in both January and March. The flood waters disrupted planting schedules as well as inundated established plantings resulting in a disruption to the beginning of the vegetable production season.
The river also deposited a layer of sediments in flooded fields (Photo 1). The sediments came from several sources: river sediments from as far away as San Luis Obispo County; sediments from side channels; and soil sediments scoured from upstream farms. Several growers and industry personnel have asked what is the composition of these sediments? In April after the flooding had subsided, we collected samples at river crossings from San Lucas to Salinas. The layer of sediment left by the flood waters tended to curled up as it dried out and were easy to collect. Any field soil was brushed from the bottom of the sediments and they were sent to the UC Davis Analytical Laboratory for analysis.
Tables 1 and 2 have analysis of the sediments collected. The data in the table is arranged with sites from south to north; the two side channels, Arroyo Seco and Monroe Canyon are listed separately. Monroe Canyon is the drainage that comes from the west side of Hwy 101 just south of the intersection of Hwy 101 and Central Avenue north of King City; it cuts through a large section of the Monterey shale formation that contains elevated levels of cadmium.
The San Lucas, Arroyo Seco and Monroe Canyon samples are coarser indicating that they were transported by rapid water movement, while the rest of the samples are dominated by silts and clays, indicating that they were transported by slower moving water. In general, there is a good correlation between the clay content of the sediments and nutrient and organic matter content. Higher nutrients in the silt and clay sediments include total nitrogen, calcium, magnesium, sulfate, zinc and iron. The sediments are generally fertile which may indicate that they are at least partially composed of soil eroded from farmed fields farther upstream. Sediments that are low in phosphorus likely originated from non-farmed or vineyard areas.
The elevated cadmium levels measured in sediments from the Arroyo Seco and Monroe Canyon indicate that these side channels carried sediments from the Monterey shale formation which has naturally high levels of cadmium into the Salinas River. Presumably these sediments originating in the Monterey shale formation are transported to areas further downstream by flood waters.
Photo 1. Sediments deposited in a field along the Salinas River
Table 1. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.
Table 2. Analysis of river sediment samples from locations from San Lucas to Salinas and two side channel locations.
- Author: Shimat Villanassery Joseph
Springtail (Protaphorura fimata) (Figure 1) is a serious pest of lettuce in the northern part of Salinas Valley of California. The direct seeded young lettuce seedlings in fields with high densities of springtail show retarded or stunted growth and do not emerge in a synchronous pattern (Figure 2). Springtails are reported to feed on soil fungi, decaying plant materials and live roots.
Springtails attack the germinating seeds of the lettuce, but it is not certain if irregular or inconsistent plant stand is due to the persistent feeding by springtail on both germinating and developing seedling stages of lettuce. Springtail continue to occur in the soil beyond 30 days after planting. Knowing the most vulnerable stage(s) of lettuce to springtail feeding will help in determining the best timing for control measure intervention to achieve a uniform lettuce stand.
Lettuce fields are heavily irrigated at least once before and up to three weeks after planting the seeds for uniform seed germination and seedling establishment. However, the behavioral response of springtail to feeding injury on lettuce under high soil moisture condition has not yet been studied in the central coast of California.
Similarly, the temperature has a profound impact on lettuce plant development as well as the growth and activities of springtail. Springtail has been found causing crop losses during February to May in the Salinas Valley and beyond June, springtail related problems are not widely reported. Perhaps slower lettuce seed germination and subsequent development during cooler seasons (January to May) is the pre-disposing factor as seedlings are exposed to springtails for an extended time frame than during the rest of the year. The relationship between temperature and springtail feeding of germinating lettuce seeds has not been investigated.
A study was conducted to determine the effect of germinating stages of lettuce seeds (up to 7 days after planting), soil temperature and moisture to springtail feeding injury.
Germinating seeds or one day old lettuce seedlings were the most vulnerable stage to springtail feeding, resulting in reduction in seedling growth. Thus, it appears that once the roots are established in the soil, lettuce is less susceptible to springtail feeding injury. Because the germinating phase of the plants is more likely to be injured, springtail monitoring activity should start prior to planting the seeds to determine the presence of springtail in the soil. Previous studies showed that, beet or potato slice baits attract springtail if placed in the top layer of the soil; thus, these baits could be used for monitoring springtail activity in the soil. If the soil is not moist, the baits may not capture springtail and springtail activity may go undetected.
When the experiments were conducted with germination phase in the temperatures as low as 41°F, springtail feeding was still evident. This suggests that although the seed germination and seedling development is progressing slowly in the cooler temperatures, springtail can be still active in feeding if there is sufficient moisture in the soil. Also, this suggests that lettuce seedlings might require prolonged protection from springtail with additional insecticide sprays until the seedlings are established in the cooler temperatures especially in spring and early summer (January to May). In the later part of summer and fall, the temperatures are higher than 60°F even at nights, which allows the seeds to germinate and develop quickly and not providing springtail to persistently feed and cause economic damage. In these circumstances, an at-plant application of insecticide is likely to provide adequate springtail control and multiple applications may not be required.
High moisture content in the soil will favor springtail feeding on the germinating lettuce seeds. In the Salinas Valley, before the lettuce seeds are planted, fields are pre-irrigated to aid land preparation and bed shaping. It has been observed that the springtail density increased from the sub-surface of soil when the field was recently irrigated or after a rain event. This cultural practice which maintains high moisture levels for seed germination on the sub-surface profiles of the soil might be favoring the faster buildup of springtail populations. Springtail captures in bait traps were greater immediately after irrigation.
Clearly, this study demonstrates that early lettuce seed development stages are the most vulnerable to springtail feeding injury. Moisture has a profound effect on springtail feeding on germinating lettuce seeds. This study also suggests that springtail can attack the germinating lettuce seeds at all growing temperatures in the Salinas Valley, although the seed germination and subsequent seedling development at cooler temperatures would increase the vulnerability of lettuce seeds to springtail feeding. This information provides insights not only on the timing of protection but the extent of protection under various temperature ranges also in managing springtail in the Salinas Valley. Plants growing the cooler temperature need prolonged protection for springtail if adequate moisture is present in the top soil of the bed. In the warmer temperatures, seed development would occur rather quickly which suggests that prolonged protection against springtail is not necessary. These results warrant the need for more field studies on protecting lettuce seeds from springtail in the cooler temperatures especially during spring and early summer lettuce plantings in the Salinas Valley.
If interested in the details of the study, please read the published article:
- Author: Shimat Villanassery Joseph
Cabbage maggot (Delia radicum) (Fig. 1) is a serious and destructive pest of brassicas in the Salinas Valley of California. Brassica crops damaged by cabbage maggot are broccoli, cauliflower, cabbage, and Brussels sprouts. Cabbage maggot flies lay eggs in the soil around the base of a plant. Legless, white maggots feed on the taproot and affect plant development. After feeding for about 3 weeks, the maggot pupates in the surrounding soil for 2-4 weeks before emerging into an adult fly. The symptoms of cabbage maggot feeding in the root are yellowing, stunting, and slow growth.
Research showed that infestation by cabbage maggots in direct-seeded broccoli could be severe throughout the growing period, except the first 30 days after seed was planted. Typically, insecticide targeting cabbage maggot is applied immediately after planting seeds and before sprinkler is turned on. Efficacy studies with at-planting application of insecticide did not provide adequate cabbage maggot control. This suggested that insecticide applied at planting might be early relative to cabbage maggot incidence and thus, delaying application might be more effective.
In 2014 and 2015, replicated experiments were done in a commercial planting of baby turnip. The treatments were one chlorpyrifos application at planting and 2 weeks after planting seeds. A tractor-mounted sprayer was used to apply insecticide. Samples were collected and were transported to UCCE entomology laboratory where roots were evaluated for damage by cabbage maggot.
Results suggested that delayed application of effective insecticide suppresses cabbage maggot (Fig. 2). In a previous study, Joseph and Martinez (2014) showed cabbage maggot flies did not lay many eggs at the base of brassica plants until 3 weeks after plant emergence (Fig. 3), despite adult cabbage maggots in the field during early stages of plant development. Also, cabbage maggot infestation tend to be continuous after 3 week stage depending on local pest pressure and crop disturbances (e.g., harvest) in the surrounding fields (Joseph and Martinez 2014).
Delaying insecticide application would increase the likelihood of intercepting cabbage maggot larvae seeking roots. In the Salinas Valley of California, use of organophosphate insecticides including chlorpyrifos is regulated. This stringent regulation is forcing growers to seek alternate insecticides for cabbage maggot control. Previous study showed that clothianidin, thiamethoxam, and spinetoram as well as pyrethroid insecticides such as zeta-cypermethrin, fenpropathrin, bifenthrin, lambda-cyhalothrin, and pyrethrins were effective against cabbage maggot larvae, and efficacy was comparable to chlorpyrifos (Joseph and Zarate 2015). However, alternate insecticides are likely to be less persistent because they break down quickly (e.g., spinetoram) or become immobile in soil under field conditions because they bind to organic matter in contact (e.g., pyrethroid insecticides). Thus, as fewer effective older chemistries (e.g., organophosphate insecticides) are used against cabbage maggot because of use restrictions, delayed application of insecticide might be more critical.
For more details on this study, please read the published paper. http://cemonterey.ucanr.edu/files/248875.pdf
References
Joseph, S. V. 2014. Efficacy of at-planting and basal applications of insecticides on cabbage maggot in seeded-broccoli. Monterey County Crop Report. January/February 2010-2013. http://cemonterey.ucanr.edu/newsletters/i__b_ Monterey_County_Crop_Notes__b___i_50471.pdf
Joseph, S. V.,and J. Martinez. 2014. Incidence of cabbage maggot (Diptera: Anthomyiidae) infestation and plant damage in seeded brassica fields in California's Central Coast. Crop Prot. 62: 72-78.
Joseph, S. V., and J. Zarate. 2015. Comparing efficacy of insecticides against cabbage maggot (Diptera: Anthomyiidae) in the laboratory. Crop Prot. 77: 148-156.
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- Author: Ian Grettenberger
- Author: Larry Godfrey
- Author: Richard Smith
- Author: Shimat Villanassery Joseph
Bagrada bug (Bagrada hilaris) is an invasive stink bug that was first observed in the Salinas Valley in October-November 2013. We started monitoring bagrada bug populations in non-crop habitat up and down the Valley starting in January 2015 and have continued to do so since then. We have seen bagrada bug populations beginning to develop on the weeds in spring and summer months. Weeds are clearly a key factor for bagrada bug populations in our region. While cruciferous crops are available year-round in the valley, stands of weeds are typically where populations really build up during early- and mid-summer (Fig 1).
In the Salinas Valley, shortpod mustard (Hirschfeldia incana, or summer mustard; Figs. 2 and 3) and perennial pepperweed (Lepidium latifolium; Figs. 4 and 5) appear to be the two most important weeds for buildup of bagrada bug populations. We have found bugs (in extremely high numbers) on perennial wall rocket (Diplotaxis tenuifolia), a weed commonly found along Hwy 101 between Chualar and Soledad, but this was very late in the year when temperatures were cooling and after the time bagrada bugs are typically problematic. These three weeds are non-native and invasive, just like the bagrada bug.
For our survey, we have been surveying a number of sites that cover the length of the Salinas Valley (Salinas, Chualar, Gonzales, Soledad, Greenfield, King City, San Lucas, San Ardo, and Watsonville). These sites contain a stand of at least one of these weed species, although we focused on pepperweed and/or shortpod mustard at the majority of the sites. Each month, we search for damage and bagrada bugs on the weeds at each site for twenty-minutes or until insects are counted on five plants of each species. When leaves are actively growing and not already damaged and worn out, damage is often easier to detect than insects (Figs. 6 and 7). This is similar to scouting in fields, where checking plants for fresh damage is the recommended method of scouting (Palumbo 2015). Unfortunately, fresh damage can be hard to find on older weeds because old damage obscures fresh damage, leaves are extremely tough, and bugs feed on stems or seed pods, so we primarily rely on detection of actual bugs.
When we surveyed near the end of June this year, we did not find high populations of bagrada bugs or damage, although they were present at some sites. We were left wondering what would happen with bagrada bug populations this year. After our July sample date, how 2016 populations compare with 2015 is still not clear, although we have started finding small populations of bagrada bug, which suggests we won't be having a bagrada bug-free year. If anything, there may be a slight delay in bagrada bug movement into fields, although this is fairly speculative. The spring rainfall this year was definitely greater than in 2015, although what effect of rainfall had on winter and spring bagrada bug populations is not clear. In 2016, we added a number of new sites, so some sites have already been surveyed during one period of population peaks, while others have only been surveyed since the beginning of this year.
At the three sites we surveyed in 2015 with perennial pepperweed, we did not see populations jump until our survey time point in late August. This year, we have not seen populations increase appreciably, although we will have to see what we find at the end of August to compare to 2015. At some of the sites with shortpod mustard that we surveyed in 2015, we had already found a fair number of bagrada bugs by this time last year, including some large populations of nymphs (up to 3 to 5 adults per plant and 18 to 57 nymphs per plant). This year, we haven't seen the same populations yet at those sites.
Next, we have the sites that have only been surveyed in 2016. While the initial sites were chosen based on prior issues with bagrada bugs in nearby fields, the new sites were not necessarily chosen based on the same conditions. We chose these new sites to improve the coverage of our survey. Each site also contains at least one of the three weed species we have focused on (shortpod mustard, perennial pepperweed, or perennial wall-rocket). A few sites were chosen knowing that bagrada bugs were present in 2015. A few of these sites have plenty of weeds available for bagrada bugs, but as of yet, we haven't seen any bugs. Some sites do have developing bagrada bug populations. We found what appears to be growing populations of bagrada bugs on both shortpod mustard and perennial pepperweed at one site near San Ardo on our last survey date (July 25th/26th). In the foothills near Gonzales, we found 1 to 26 bagrada adults and 0 to 22 nymphs on shortpod mustard plants. In addition, we were able to easily find bagrada bugs (0 to 8 adults, 0 to 3 nymphs) on shortpod mustard plants growing along US-101 between Greenfield and King City (Fig. 8). As many of you that have driven this stretch of road may know, there are a lot of weeds there. This means that a handful of bagrada bugs on each plant can quickly add up to large populations when summed over thousands of plants. As the weeds senesce, bagrada bugs will likely concentrate on still-green plants, although this patch can only support the bagrada bugs for so long with plants continuing to senesce.
As we see it, the issue is when these growing populations of bagrada bugs run out of food in the patches of weeds, and then go in search of new food sources. At this point, they can start finding their way into fields of cruciferous crop. This time of year, much of the shortpod mustard in the Valley is starting to set seed and will soon completely dry up if it has not already. In many locations, the vast majority of plants are almost dry. In the same localized area, some shortpod mustard plants will be completely senesced, while others are still green and flowering (see Figs. 9-11). Differences in growing conditions at a fairly small scale can therefore be very important. If all of the plants dry up, bagrada bugs will be forced to disperse, possibly triggering infestations in crop fields. If green plants remain, they may retain bagrada bugs, shifting the risk of infestation. Perennial pepperweed can persist late into the fall, but this is dependent on availability of water, damage by bagrada bugs, and disease pressure (pepperweed is often afflicted by white rust; Koike et al. 2011). The timing and severity of bagrada bug infestations in fields seems to be closely tied to what is happening on the weeds, so a better understanding of population dynamics on weeds will be needed to better predict an influx of bagrada bugs.
The next step will be to figure out how to incorporate bagrada bug populations in weeds into management and scouting plans. It is already common for PCAs to check weeds for bagrada bugs and we believe this to be a useful tactic. At this point, we suggest checking weeds on the edges of fields and any large patches of weeds within ~ 0.5 miles that seem to be likely sources of bagrada bugs. Consider which sides of crop fields often get infested the most or earliest and search for weeds on that side of the field. The dispersal ability of bagrada bugs has not been well characterized, but cases in which fields are colonized even when no cruciferous weeds or only bare ground is nearby show that bagrada bugs can move significant distances. While often seen walking, bagrada bugs readily fly, especially when surface temperatures are above 100° F based on preliminary observations. When perennial pepperweed is green and leaves are growing, new or recent damage should be readily apparent. Damage on “old growth” shortpod mustard is not always readily apparent, so finding bagrada bugs themselves is necessary. Adults are often found feeding on flowers, buds, or seed pods, so these are the best plant structures to scan. We believe bagrada bugs on weeds are mainly a threat to crop fields once the quality of these plants starts to decline, so pay attention to weed phenology and damage severity on weeds from both bagrada bugs and disease.
Management of weeds is another option to limit the population growth potential of bagrada bugs. Sanitation of both weeds and crops has been a recommended cultural management tactic in the pest's Old World range (Palumbo et al. 2016). By removing nearby weeds, you may be able to prevent an economically significant infestations. If weeds are managed, the timing will be important. For shortpod mustard, if it is possible to successfully manage weeds when they are still small, this may prevent build up of bagrada populations from the very beginning. However, resurgence of the weeds may happen with sufficient soil moisture early in the year. A more efficient tactic may be to manage shortpod mustard once it has bolted and is flowering later in the season (~ late April to May). This will help prevent resurgence of weeds but will still intercept bagrada bug populations before they have a chance to build to a significant degree. Managing weeds later and once bagrada bug populations have developed could push insects into crop fields. The timing of weed management could also be tied to the susceptibility of nearby crops, which could help if bagrada bugs are already present on the weeds. Adult bagrada bugs moving into a 40 day-old broccoli field because the weeds they were on were mowed or disced would likely not cause economic damage. However, managing weeds near a newly planted field could make matters worse and create a pest problem. Even if nearby weeds don't harbor bagrada bugs early in the season, they may serve as an intermediate bridge between the crop fields and weeds that are further away. Unfortunately, it is likely impossible to manage weeds far enough out from fields to eliminate the threat of bagrada bugs. The landscape context is important, but exercising control over the entire landscape is not possible. Perennial pepperweed is a trickier weed to deal with given its phenology and ability to rapidly re-sprout. These weed species are not about to disappear from the landscape in the Salinas Valley, so they will continue to play an important role for developing bagrada bug populations.
References
Koike, S. T., M. J. Sullivan, C. Southwick, C. Feng, and J. C. Correll. 2011. Characterization of white rust of perennial pepperweed caused by Albugo candida in California. Plant Disease 95:876.
Palumbo, J. C. 2015. Association between Bagrada hilaris density and feeding damage in broccoli- implications for pest management. Plant Health Progress 16:158–162.
Palumbo, J., T. Perring, J. Millar, and D. A. Reed. 2016. Biology, ecology, and management of an invasive stink bug, Bagrada hilaris, in North America. Annual Review of Entomology 61:453–473.