Effects of Organic Insecticides on the Bagrada Bug

Bagrada-hilaris2

The invasive stinkbug known as the bagrada bug has been aggressively moving north through California. First discovered in LA County in 2008, it has now been identified as far north as Yolo County. The FarmsReach Conversations have been active with concerns, questions and suggestions for how to deal with these persistent pests. See what others are saying and chime in!

Shimat Joseph, PhD, IPM Advisor for UCCE Monterey County, has published findings on the devastating effects bagrada bugs have on brassicas and offers some possible solutions for pest management. Read on to learn more about dealing with this pest and possible ways to prevent [further] damage.


Written by Shimat Villanassery Joseph

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Figure 1. Bagrada bug

Bagrada bug, Bagrada hilaris (Fig. 1) continues to be a major pest of brassica crops in the Salinas Valley and Hollister causing severe crop losses for both organic and conventional growers alike. Organic growers are struggling because there are very limited options at disposal to suppress the bagrada bug population in the field. Conventional growers on the other hand, are relying heavily on pyrethroid and neonicotinoid insecticides and are resorting to a higher number of applications than normal during early stages of crop development (cotyledon to four true leaves stage).

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Figure 2. Direct feeding plant injury

This tactic (multiple applications) benefit the young seedlings as insecticide residues protect the plants from bagrada bugs feeding, especially on the growing point or apical meristematic tissue. Feeding injury to meristematic tissue would cause “blind” head (no head) and multiple shoots on heading brassica crops such as broccoli, cauliflower or cabbage. With a couple of insecticide applications at the early stages of brassica crop growth, conventional growers are facing 5-30% loss from bagrada bug feeding injury. Some growers finding noticeably high mortality of cotyledons forcing them to skip thinning operations to maintain a decent crop stand.

FIgure 3.

Figure 3. Young plants eaten by bagrada bug

Bagrada bugs are taking a huge toll on leafy brassica crops such as Chinese cabbage (Pak Choi or Bak Choi), Arugula, Mizuna, Totsoi, and Kale. These crops become unmarketable from direct feeding injury on the leaves (Fig. 2). These crops are like “magnets” for bagrada bugs. Bagrada bugs can precisely detect the seeds planted in the soil and most of the seedlings emerge with bagrada bug feeding injury. Sometimes, severe feeding at young stages cause plants die upon emergence (Fig. 3).

Two field trials were conducted in Hollister seeking organic insecticide options for bagrada bug management. The insecticide products chosen were Surround (Kaolin clay), Pyganic, Aza-direct, Entrust and M-pede. Surfact 50 was added when the Pyganic and Aza-direct were used alone. These organically approved insecticides were combined with other insecticides based on certain assumptions.

Figure 4.

Figure 4. Experiment field layout

For example, when insecticide Surround is sprayed, it forms a thin particle film on the leaf surface without affecting light transmission or photosynthesis. As bagrada bugs crawl on the Surround treated surface, the particles could stick to their body and possibly cause irritation, and force them to crawl off from the treated surface. This phenomenon, if it occurs, could reduce the incidence of bagrada bug feeding injury. Moreover, it is possible that combining insecticides such as Pyganic and Aza-direct with Surround may increase insecticide exposure as bagrada bugs groom to remove the clay particles stuck on their body using their legs or wings.

The field where the trials were conducted had enormous bagrada bug pressure. Bagrada bugs were everywhere that one would easily kill “thousands of bugs” just by walking over the beds. All stages of bagrada bug were present in the field at the time of insecticide applications. The applications were targeted to protect the plants from feeding injury and not particularly to kill the bugs. The first trial was conducted in Mizuna field and the second trial was conducted in Arugula field. In both the trials, the products were applied four times at three-day intervals until harvest between 7 AM and 9 AM.

Figure 5.

Figure 5. Leaves of sampled plants

The water volume used was 40 gal per acre. The products were applied using the pneumatic sprayer or back pack sprayer. The details of the products, active ingredients and the rates used for the trials are shown in the Tables 1 and 2 (see below). The design of experiments was Completely Randomized Block Design with four replications (Fig. 4). Plant samples were collected twice a week and were evaluated for bagrada bug feeding injury on the true leaves (Fig. 5).

In trial 1, the bagrada bug feeding injury was numerically lower on Mizuna plants that received higher rates of Surround (alone) and Surround combined with Pyganic or Aza-direct than on untreated plants (Fig. 6). When the percentage change in bagrada bug injury on true leaves was calculated (taller the histogram, better the product performance), the plants treated with higher rates of Surround, and Surround combined with Pyganic or Aza-direct had a greater reduction of bagrada bug feeding than untreated plants (Fig. 7). In trial 2, none of the treatments showed any indication of suppression when compared with untreated plants (Fig. 8).

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Table 1. The products, active ingredients, and rates in Trial 1 in Mizuna.

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Table 2. The products, active ingredients, and rates in Trial 2 in Arugula.

Basically, these studies did not provide definite answers to the questions posed or problem, but provided some trends. It appears that combining Pyganic or Aza-direct with Surround may have some value, rather than applying it alone. The Surround rate 20 lb per 40 gal of water is the maximum rate for this product. Because Surround could clog the spray tanks, it requires rigorous agitation before application. Also, because Surround easily comes off from the leaf surface with sprinkler irrigation, reapplications are warranted if irrigated at closer intervals, especially during the early stages of the crop. The rate of M-pede used in the study was 2% of the water volume. Typically, 2% of M-pede is considered a high rate and increasing the rate (> 2%) may cause phytotoxicity (burning of leaves).

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Figure 6. Trial 1: Effect of organic treatments on bagrada bug feeding injury on Mizuna leaves (refer Table 1).

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Figure 7. Trial 1: Percent change in bagrada bug feeding injury after application on various treatments (refer Table 1) in Mizuna.

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Figure 8. Trial 2: Effect of organic treatments on bagrada bug feeding injury on Arugula leaves (refer Table 2).

Then, can we manage the bagrada bug?

A few thoughts:

  1. Perhaps, we should approach this problem differently. Bagrada bug is a landscape scale pest that could reproduce and build-up huge populations if the food is available in plenty, and warm and dry conditions persist. So far, we learned that their population build-up starts in late July to December, in the Salinas Valley and Hollister. The warmer conditions favor rapid reproduction and several overlapping generations of bagrada bug can develop in short periods.
  2. We observed that their population pressure varies across landscapes and is a serious problem where the control options are limited. For example, bagrada bug problems are less severe when the management is aggressive, such as conventional fields where growers have effective products that could suppress or knock down their populations at least in the crops grown. Organic growers, on the other hand, have limited options to fight bagrada bug and its population rapidly grows into uncontrollable size.
  3. We also noticed that initially these bugs invade the plants in the edge of the field from the surrounding fields or risk zones (e.g. ditches).
  4. These facts suggest that this is a landscape level problem rather than a field level problem. Any approach to bagrada bug management probably should include the management of various kinds of hosts that function as a reservoir (e.g. brassica weeds) and aid to sustain their populations (brassica crops).
  5. Cultural management: Avoid planting brassica crops in a back-to-back pattern or staggered pattern. This will provide opportunity for bagrada bug to utilize the continuous supply of food to reach uncontrollable population size in a short period of time. If somehow, the growers could disrupt the ecology of bagrada bug by not growing brassica crops in succession for a period, and instead rotate with non-brassica hosts, their population might crash and reach to a controllable level.
  6. Weed management: Aggressive weed management, especially brassicaceous weeds, along with tight cultural management would disrupt the food supply and prevent escalation of population size.
  7. Bagrada bugs have demonstrated the ability to survive on non-brassica hosts especially solanaceous crops such as tomato, potato or pepper, but would rarely reach to the levels we are seeing in brassica fields.

For further reading on the bagrada bug check out these resources:


For more information on this study, or the bagrada bug in general, contact Shimat: svjoseph@ucanr.edu/831-229-8985.

If you have questions or comments about the bagrada bug for our farmer community, please join the conversation. We love to hear from you!

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