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.

Fig. 1. Cabbage maggot larvae feeding on broccoli roots

Fig. 2. (A) Number of cabbage infested turnip roots, and (B) severity of feeding injury

Fig. 3. Number of cabbage maggot eggs collected from the crown area of the plant following weeks after planting

Cabbage maggot (Delia radicum) is a serious insect pest of Brassica crops such as broccoli and cauliflower in the Central Coast of California. These crops are grown throughout the year; as a result cabbage maggot problems persist year long.Cabbage maggot eggs are primarily laid in the soil around the crown area of the plant. A single female fly can lay 300 eggs under laboratory conditions. The eggs hatch within 2-3 days and the maggots feed on the taproot for up to three weeks and can destroy the root system of the plant. The maggots pupate in the soil surrounding the root system and emerge into flies within 2-4 weeks. Severe cabbage maggot feeding injury to the roots cause yellowing, stunting even plant death.

Control of cabbage maggot on Brassica crops primarily involves the use of soil applied organophosphate insecticides such as chlorpyrifos and diazinon. However, the persistent use of organophosphate insecticides has resulted in high concentrations of the insecticide residues in the water bodies posing risks to non-target organisms and public health through contaminated water. Currently, use of organophosphate insecticides is strictly regulated by California Department of Pesticide Regulation. There is therefore an urgent need to determine the efficacy of alternate insecticides for cabbage maggot control.

The efficacy of 29 insecticides was determined against cabbage maggot through a laboratory bioassay by exposing field collected maggots to insecticide treated soil immediately after application. Three parameters were used to evaluate efficacy (1) proportion of maggots on the soil surface after 24 h, (2) proportion of change in weight of turnip bait, and (3) dead maggots after 72 h. Based on the assays, 11 insecticides performed better and they were Mustang, Torac, Danitol, Belay, Capture, Warrior II, Lorsban, Mocap, Durivo, Pyganic and Vydate in the order of highest to lowest efficacy. Eight insecticides were selected based on superior efficacy to determine the length of residual activity on cabbage maggot larvae. The persistence of insecticide activity was greater with Capture, Torac and Belay than with other insecticides tested.

The mode of exposure of insecticides in this study was entirely by contact (through skin) and other modes of exposure such as ingestion (through mouth) or through respiratory holes (spiracles) were not investigated. Some of the insecticides tested in the study were insect growth regulators (IGRs) (Dimilin, Rimon, Trigard, and Aza-direct), which normally interfere with the growth and development of the insect and they showed a low efficacy against cabbage maggot larvae. Entrust (spinosad) showed a moderate efficacy possibly because the primary mode of exposure to Entrust is by ingestion. The diamide insecticides (Beleaf, Coragen and Verimark) have systemic activity as they move within the plant and likely away from the site of application. It is possible that the soil applied diamide insecticides are absorbed by the roots and translocated to the above ground plant parts with little effect on the feeding larvae in the tap roots.

This study was conducted under controlled conditions in the laboratory and the results may not be entirely consistent in field conditions. The Brassica fields in the California's Central Coast are profusely sprinkler irrigated up to three weeks after sowing to ensure uniform germination and proper establishment of plants. It is likely that applied insecticides are partially or completely leached out of the root zone area without providing anticipated maggot control. In this study, insecticides were drenched into the cup and none of the applied insecticide solution leached out. Therefore, it is likely that the insecticides were more effective in the laboratory assay than they would be in the field. Certain insecticides such as pyrethroids tend to bind to the soil organic matter. The organic matter in the California's Central Coast soils can be up to 4%, which could reduce the availability of soil applied pyrethroid insecticide to the root zone where cabbage maggot larvae typically colonize. In situations with poor insecticide spray coverage, invading cabbage maggot larvae are possibly exposed to no or sub-lethal doses of the soil applied insecticide and may be able to penetrate the soil and infest the roots. The air temperature in the field at the time of insecticide application may influence the efficacy of the applied insecticide. The efficacy of pyganic decreased as the temperature increased against onion maggot. This suggests that application of pyrethroid insecticides should be avoided during warmer periods of day.

Other field conditions that influence efficacy of insecticides are cabbage maggot incidence and frequency of invading cabbage maggot flies on Brassica crop in the Central Coast of California. The earliest peak of cabbage maggot infestation occur a month after sowing broccoli seeds and infestations can be continuous until harvest. Also, insecticides applied at sowing as a banded spray on the seed lines did not provide adequate cabbage maggot control based on the insecticide efficacy trials conducted in commercial broccoli fields. These findings suggest that delaying the insecticide application by 2-3 weeks after sowing is more likely to maximize maggot control. Because the cabbage maggot infestation can last several weeks, insecticides with extended persistence of efficacy would increase the value for cabbage maggot control. Overall, results show that Capture, Torac and Belay which performed effectively against cabbage maggot for a month after application. This indicates that insecticides used before the first peak of infestation may protect the younger stages of the Brassica plants allowing them to establish and tolerate milder cabbage maggot infestations thereafter.

In conclusion, 11 insecticides with high efficacy were identified for future investigation. Future studies will focus on determining the effects of application timing and delivery methods compatible with cabbage maggot incidence in both directly sown and transplanted Brassica crops in the Central Coast of California.

If you are interested in reading the details of this study, please click the link below to access the published article.

http://cemonterey.ucanr.edu/files/218667.pdf

Cabbage maggot (Delia radicum) is one of the most destructive pests of cruciferous crops in the Salinas Valley. Cabbage maggot flies lay eggs in the soil around the base of the plant. A single female can lay about 300 eggs under laboratory conditions. Legless,8-mm long white-maggots feed on the taproot and affect normal plant development. After about 3 weeks of feeding, the maggot pupates in the surrounding soil and remains at this stage for 2-4 weeks before emerging into an adult fly. The most common above-ground feeding symptoms of cabbage maggot are yellowing, stunting and slow growth.

Because the winter weather in the Salinas Valley is mild and rarely goes below freezing point, not all cabbage maggot pupae go into a resting stage, often called as diapause. This means our unique environment enables cabbage maggot flies to remain active even in winter months, producing multiple overlapping generations throughout the year. In this post organophosphate era with stringent restrictions for chlorpyrifos and diazinon use and less persistent insecticides being available for cabbage maggot management, knowledge of field-level incidence of cabbage maggot infestation is critical to determine precise timing for insecticide applications in brassicas. We studied the temporal incidence of cabbage maggot relative to seeded broccoli and turnip in the Salinas Valley.

Cage studies showed that severe injury from cabbage maggot did not appear during the first 14 days after plant emergence but was greater during 15-28 days after plant emergence. Similarly, survey in broccoli fields indicates that cabbage maggot flies did not lay a high number of eggs at the base of the plant until three weeks after plant emergence, despite presence of adult cabbage maggot in the field during the early stages of plant development (Figures below). On turnip, notable injury from cabbage maggot did not appear until five weeks after plant emergence. This is important information because typically insecticides targeting cabbage maggot were applied mostly at planting. Researchers showed that cabbage maggot infestation could be suppressed by using organophosphate insecticides, particularly chlorpyrifos, for more than a month after planting because product residues persisted for an extended period. However, most of us are not using these insecticides and a consistent cabbage maggot control using organophosphate insecticides was never attained in the Salinas Valley. I'm working on insecticides to determine their effectiveness against cabbage maggot and will share that information as soon as it is available.

It is unclear why increased cabbage maggot oviposition did not occur during the early stages of plant development. It is possible that the invading cabbage maggot flies cannot distinguish the young seedlings at a certain size relative to the surrounding area of bare soil. Cabbage maggot populations and crop injury from this pest tend to be more abundant in the border than the interior zone of the field; this invasion pattern continues throughout the growing period. In conclusion, our data suggest that the important season periods in the central coast vegetable production area to consider targeting cabbage maggot control are three to four weeks after planting the seeds. I will continue to monitor if other maggot species (seedcorn maggot or onion maggot) which could attack when brassica plants are at younger stages especially during spring or early summer.

For those of you, who are interested to read more on it, please find the published article after clicking the link (below) and feel free to contact me (Shimat Joseph) at svjoseph@ucanr.edu or 831 759 7359.

 http://cemonterey.ucanr.edu/files/190245.pdf