Residue Decline and Efficacy Trials

Figure 1.  Male spotted wing drosophila.  Note the prominent black spots near the ends of the wings.
Figure 1. Male spotted wing drosophila. Note the prominent black spots near the ends of the wings.
Figure 2.  Female spotted wing drosophila.  The image next to the fly is the modified section at the end of the abdomen which is
Figure 2. Female spotted wing drosophila. The image next to the fly is the modified section at the end of the abdomen which is
Figure. 3. Excerpt from maximum residue chart of insecticides registered for use in OR and WA blueberries for management of D. s
Figure. 3. Excerpt from maximum residue chart of insecticides registered for use in OR and WA blueberries for management of D. s
Figure 4.  Airblast sprayer applying an insecticide to a blueberry field.
Figure 4. Airblast sprayer applying an insecticide to a blueberry field.
Figure 5.  A species of parasitic wasp native to Japan, Asobara japonica, which is being studied to determine its ability to par
Figure 5. A species of parasitic wasp native to Japan, Asobara japonica, which is being studied to determine its ability to par

Spotted wing drosophila (Drosophila suzukii), SWD (Figs. 1 and 2), is an invasive vinegar fly from Japan, which poses a serious threat to the eastern and western small and stone fruit crops in the United States.  This pest was first found in the US in raspberries and strawberries growing in California, and by 2009 it had made its way to Oregon.  Since then, SWD has wreaked havoc on many growers’ crops, causing drastic yield losses of up to 40% in blueberries, 50% in caneberries and 33% in cherries in Oregon, Washington and California (Bolda 2010).  Due to its potential to cause such serious damage, many growers begin treating fruit with insecticides as soon as fruit starts to ripen, and they continue to make regular applications up until all the fruit has been harvested.  Since insecticide applications have risen due to the threat posed by this pest, there is also an increased risk that fruit could contain excessive pesticide residue, which hasn’t had a chance to be sufficiently reduced by photodegradation, rainfall, or other factors that lessen the pesticide load.  This is of particular concern to growers who sell fruit to overseas markets because many countries differ in the levels of certain pesticides allowed to remain on the fruit.  For example, referring to the table below, the US allows for a residue level of 0.25ppm spinosad to be detected on fruit when it is harvested and sent to market, while most of the other countries allow for higher levels of this chemical to be present (Fig. 3).  However, Taiwan does not allow for the detection of this chemical.  One important tool which can help growers determine how far in advance they need to apply a pesticide in order for residues to decline to acceptable levels are residue decline curves, which are developed by sampling for residues following a chemical application over the course of time.  After residue levels are analyzed, they are plotted against time, and then a curve can be developed, which is then used to forecast where residue levels will be at a given time.  The length of time a pesticide is effective against a pest is another very important factor to be considered.  Many of the insecticides currently being used against SWD only provide up to one week of protection, which is why they must be applied regularly (Fig. 4).  Increasing the amount of pesticides applied also increases the pressure on the pest to develop resistance, particularly when there are a very limited number of effective pesticides available for growers to use.  Because growers need to know not only how long an insecticide will combat a certain pest, but also how long it will take to degrade to acceptable levels, it is essential to have both residue decline and efficacy information that can be readily available in an easy to comprehend format.  The purpose of this study was to evaluate both the efficacy and decline of a couple of different spinosyn insecticides, which included Success (spinosad) and Delegate (spinetoram) in the control of SWD in blueberries.  Spinosyns are compounds that are naturally derived from fermentation of a specific species of bacteria.  These compounds exert insecticidal activity against susceptible insects, and are therefore fairly specific in targeting the pest while leaving other organisms unharmed (Meihua et. al 2007).  Spinosad is an OMRI certified organic insecticide, while spinetoram is an insecticide that is based upon naturally derived spinosyns that have been chemically modified for increased efficacy and longevity.

Three different trials were conducted at the OSU North Willamette Research and Extension Center, and also at Pan-American Berry Growers in Salem, OR.  Berry and leaf samples were collected at the different sites at regular intervals following applications of the insecticides mentioned above.  The samples were analyzed for residues, and in the laboratory, experiments were conducted to determine how effective the insecticides were at killing the flies. 

We found that both spinosad and spinetoram residues on leaves declined rapidly over a seven-day period, and fly mortality tended to follow a similar trend.  Neither of the insecticides were extremely effective in killing the flies, although we did find that spinetoram remained effective for a longer period of time than spinosad.  We also learned that when spinetoram is used as part of a management program in conjunction with other insecticides, it can help prolong protection against SWD, while providing growers with another pesticide that kills the flies using a different mode of action than many of the commonly used insecticides.  This is very beneficial in reducing the likelihood of SWD developing resistance to the few insecticides that are available to growers.  In addition, residues of both insecticides had declined to acceptable levels after the required amount of time had passed before they would be harvested and sold to consumers. 

Currently, there are only a few synthetic insecticides, and even fewer organic compounds that are effective against SWD.  While insecticides are one tool which many growers are heavily relying upon to control this pest, a large amount of very exciting research is being conducted to explore other methods for controlling SWD.  Such alternatives include harvesting the fruit early and at regular intervals to reduce the amount of very ripe fruit in the field (SWD have a high affinity for ripe fruit), pruning the plants, and installing plastic weed mats to create a drier and much warmer environment, which SWD do not tolerate well.  Scientists are also studying the effects of other animals that eat SWD (a.k.a. biological control agents), which include animals that are already present in the field such as ladybugs, spiders and tiny wasps called parasitoids, as well as natural enemies that are present in the country that SWD originate from (Fig. 5).  While pesticides are tools which are important to many growers, most agree that it is important to try to limit the use of such compounds, and whenever possible to use other methods for controlling pests.  This benefits not only the consumers, but also the environment, and the growers as well.

References

Bolda M., R.E. Goodhue, and F.G. Zalom. 2010.  Spotted wing drosophila: potential economic impact of a newly established pest.  Agricultural and Resource Economics Update 13: 5-8.

Qiao, Meihua; Daniel E. Snyder; Jeffery Meyer; Alan G. Zimmerman; Meihau Qiao; Sonya J. Gissendanner; Larry R. Cruthers; Robyn L. Slone; Davide R. Young.  2007.  Preliminary Studies on the effectiveness of the novel pulicide, spinosad, for the treatment and control of fleas on dogs. Veterinary Parasitology: 345–351. doi:10.1016/j.vetpar.2007.09.011.