- Author: Shimat Villanassery Joseph
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.
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