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 25^th/26^th). 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.

Fig 1. Adult and nymph bagrada bug on a still-green shortpod mustard plant after grasses have completely dried.

Fig 2. Shortpod mustard growing in the shoulder.

Fig 3. Bagrada bugs feeding on shortpod mustard flowers.

Fig 4. A stand of perennial pepperweed in the spring.

Fig 5. A pair of bagrada bugs on perennial pepperweed flowers.

Fig 6. Bagrada bug damage on perennial pepperweed. New lesions still appear green in color and with distinct lines or “fans” from the “lacerate and flush” feeding by bagrada bugs. Lesions turn white as they age and the leaf tissue becomes chlorotic.

Fig 7. Older bagrada bug damage on pepperweed. Lesions have developed into tattered holes.

Fig 8. A typical roadside scene in the Salinas Valley (photo taken July 26, 2016). Some of the shortpod has completely senesced, but most plants still have green leaves, some flowers, and many pods.

Fig 9. Shortpod mustard growing in the roadside near Gonzales, CA. Plants further from the road have almost completely senesced. Bagrada bugs were not found at this time point. Photo taken March 26, 2016.

Fig 10. Shortpod mustard growing in the roadside near Gonzales, CA. Bagrada bugs were found on these plants. Photo taken July 26, 2016.

Fig 11. The same site near Gonzales, CA, further from the road. Plants are almost completely senesced. Bagrada bugs were found on these plants. Photo taken July 26, 2016.

If you have an interest to receive state funding for upgrading your irrigation system to improve water use efficiency, then you may want to consider applying for SWEEP funding.   Upcoming workshops on SWEEP will be in Salinas on July 14, and in Watsonville on July 15th.   Please see the announcement below:

FUNDING OPPORTUNITY FOR IMPROVING FARM

WATER AND ENERGY USE EFFICIENCY!

UP TO $200K PER FARM

The California Department of Food and Agriculture (CDFA) is accepting applications for its 2016 State Water Efficiency and Enhancement Program (SWEEP) Round II. SWEEP is a competitive grants program supporting growers and agricultural operations in California to reduce their Greenhouse Gas (GHG) emissions and save water by investing in water irrigation system efficiency. Projects must reduce GHG emissions and save water. Applicants must provide supporting documentation directly related to actual, on-farm water consumption and GHG emissions during the prior growing season to be eligible for funding. Grant submission deadline is August 5, 2016, and grant-funded work must be completed by November 2017.

THE RESOURCE CONSERVATION DISTRICTS OF

MONTEREY, SANTA CRUZ and LOMA PRIETA

INVITE YOU

TO ATTEND ONE OF THE FOLLOWING

TECHNICAL ASSISTANCE WORKSHOPS TO PREPARE YOUR 2016 SWEEP PROPOSAL

These workshops will give you an overview of the program, and provide hands-on assistance with project requirements, application process and materials. You will have an opportunity to meet representatives from various companies that sell the types of irrigation supplies and equipment you can acquire with your SWEEP grant. For more details visit the program's website at https://www.cdfa.ca.gov/oefi/sweep/, or contact Ben Burgoa at ben.burgoa@rcdmonterey.org (805-717-3261) for assistance in Monterey County, or Sacha Lozano at slozano@rcdsantacruz.org (831-224-0293) in Santa Cruz and Santa Clara Counties. Be ready! Please bring information about your irrigation system and pump characteristics to assess your farm water consumption and GHG emissions.

The short answer is not very much (unless you are fertigating).   Nitrate-N concentrations in most agricultural wells on the Central Coast range from 2 to 50 ppm.   Some wells do have background levels of NO₃-N as high as 70 ppm but these concentrations are uncommon.

We used an electrical conductivity (EC) meter to measure the change in salinity of well water after adding different concentrations of nitrate salts.   The well water tested in our study was highly suitable for vegetable production:  the EC was 0.5 dS/m and the background concentration of nitrate was 3 ppm NO₃-N.   We added sodium- and calcium-nitrate salts to the water to achieve concentrations of nitrate ranging from 3 to 42 ppm N. 

Figure 1 shows the effect of nitrate concentration on salinity of the water.  The salt content of the water did increase with increasing concentrations of NO₃-N, but increases in salinity were modest.  For each increase of 10 ppm NO₃-N, the EC of the water was raised by 0.07 dS/m.     At the highest NO₃-N concentration evaluated (42 ppm N) the corresponding EC of the water averaged 0.77 dS/m, or a boost in salinity of 0.27 dS/m.    Even at the highest nitrate concentration, the water that we tested was still very suitable for producing cool season vegetables and strawberries without causing yield loss.  

Figure 1.   Nitrate concentration effects on salinity of water.

A high concentration of nitrates in irrigation water can also supply a significant portion of the nitrogen needs of a crop.  Irrigation water with an NO₃-N concentration of 42 ppm would supply approximately 9.5 lbs of N/acre for each inch of water applied, or about 77 lbs of N/acre for a lettuce crop if 8 inches of water were applied during the season.  

Some growers have observed that their high nitrate wells are problematic for producing good vegetable crops, and have remarked that high nitrate water tends to be salty.  This study demonstrated that nitrate is probably not the cause of the salinity, unless the nitrate concentrations are very high (> 100 ppm N).   

Well water can have a high salt content for a variety of reasons.  Underground aquifers derived from marine sediments can contain salts that increase the salinity of groundwater.   Near the coast, wells can become salty if seawater intrudes into ground water.  Another source of salts is from irrigation water.   Approximately 1 ton of salts are contained in an acre-foot of water that has an EC of 1.15 dS/m.  As crops transpire moisture, salts accumulate in the soil, and will eventually leach deeper in the soil profile as more irrigation water is applied.   Over time, leached salts could eventually reach the ground water, and increase the salinity level of the water.  

In conclusion, nitrates in irrigation water are unlikely to be a significant cause of salinity of ground water.   If your well water has a high concentration of nitrates and also has a high EC (> 1 dS/m), a water suitability test may reveal other salts contributing to the salinity of the water.   Nitrates in the water can also be a significant source of nitrogen for your crops. 

     For several years the leafy greens industry has been engaged with regulatory agencies regarding the issue of cadmium (Cd) concentration in fresh produce; Cd, a heavy metal element, poses human health risks if ingested in significant quantity. The 2016 production season marks the first time that California spinach growers may be required to alter their production practices to minimize the amount of Cd taken up by this crop. Spinach is the focus of concern because it takes up much more Cd than other common vegetable crops (Fig. 1).

Fig. 1. Effect of total soil Cd concentration on tissue Cd concentration in spinach, romaine and broccoli (fresh weight basis); data from a 2013 survey of Salinas Valley fields.

     The graph makes clear that soil Cd content is a major factor determining crop Cd uptake. We have a general understanding about where the highest Cd soils are located, but flooding events over centuries have moved and mixed soil sediments. Therefore, soil testing is a critical step in evaluating the field-specific risk. The soil test procedure for which we have the most data is total soil Cd measured by nitric acid/hydrogen peroxide extraction. Soil samples should be taken from the top foot, which is the effective rooting depth of spinach.    

In fig. 1 it is clear that individual fields differ substantially from the general trend line relating total soil Cd to tissue Cd because other factors also have influence. Those factors include:

• Variety – preliminary field evaluations suggests that tissue Cd concentration may differ among varieties by up to 25%.
• Soil pH – Cd bioavailability declines with increasing soil pH.
• Irrigation water chloride (Cl) concentration – increasing Cl concentration makes Cd more bioavailable.
• Soil zinc (Zn) level – Zn and Cd compete for plant uptake, so soil with higher Zn availability tend to have less Cd uptake.

     Individual spinach shippers may impose soil Cd limits on their growers, or require remediation practices to be deployed if fields above a certain soil Cd level are used; given the uncertainties in the relationship between spinach Cd concentration to total soil Cd, different shippers may set different rules for their growers to follow.

     Where growers are required to use remediation practices to reduce spinach Cd uptake, our research suggests that high-rate Zn application is the most practical approach. Zn and Cd are closely related ions, and plants are not able to distinguish well between these ions. Increasing the ratio of plant-available soil Zn to Cd suppresses Cd uptake. However, to significantly decrease Cd uptake much higher levels of Zn application are needed than would typically be used to remedy a soil Zn deficiency. Across numerous field and pot trials we have observed that applying 25-50 PPM elemental Zn on a soil dry weight basis (equivalent to approximately 100-200 lb elemental Zn per acre foot of soil) suppressed crop Cd uptake by roughly 30-60%. However, the effectiveness of Zn application was affected by the following factors: 

Zinc form:

• Zinc sulfate and zinc chelate were more effective in reducing Cd uptake than zinc oxide. Zinc sulfate was more economical than zinc chelate.
• Granular forms of zinc sulfate were less effective in reducing Cd uptake than powdered forms, or zinc solutions. We presume that Zn from the granules did not disperse thoroughly through the soil.

Zinc incorporation:

• To be maximally effective, Zn must be distributed throughout the primary rooting zone of spinach (the top 8-12 inches of soil). Zn distributed only in the top few inches of soil was only marginally effective.
• Disking incorporates Zn to a depth of approximately 6 inches and is therefore more effective than mulching, which incorporates Zn only to about 3 inches. Where practical, incorporation of Zn even deeper than 6 inches would be ideal.

Zinc foliar applications:

• Foliar applications of zinc sulfate or zinc chelate were not effective at reducing Cd uptake in spinach.

Zinc longevity:

• When Zn is applied to soil it slowly becomes less plant-available over time, as chemical compounds of low solubility are formed. However, high-rate Zn application will have measurable effects over several years at least. In a field trial, in the first crop following application of 100 pounds of elemental Zn (280 lbs of zinc sulfate) per acre we observed a 40% reduction in Cd uptake by spinach; after two years the Zn-treated area still showed a 15% reduction in crop Cd uptake. In pot trials, when 80 PPM elemental Zn was added to high-Cd soils, the reduction in Cd uptake of four successive spinach crops was 66%, 66%, 51% and 49%. Based on these observations, it is clear that Zn applications can be effective across years, but additional Zn application may be needed to maintain maximum efficacy over the long term.

     Other potential remediation practices include raising soil pH, or applying organic amendments. These practices are aimed at making soil Cd less plant-available. Increasing pH does this by precipitating Cd salts, while organic amendments like compost may tie up Cd on its complex cation exchange sites. Our preliminary data suggest that both practices can have a positive effect, but the effects are less substantial than those we observed with high-rate Zn application.

     In summary, regulatory pressure is forcing the spinach industry to adopt practices to limit spinach Cd concentration before we have developed a comprehensive understanding of how best to do that. Based on our current data we believe that fields with total soil Cd below 1 PPM present relatively low risk, while those above 2 PPM will be more difficult to remediate reliably. Remediation practices can be effective in fields between 1-2 PPM total soil Cd, but results in individual fields will vary, based on the complicating factors we have outlined.