Question of the Week
What can you tell me about organic control of the cherry fruit fly? Is there a role for gypsum applications?
Answer: The major insect pest of cherry is the cherry fruit fly (Rhagoletis cingulata and R. fausta). Failure to adequately control this pest can cause severe crop loss due to the presence of fruit fly larva (maggots) in the cherries at harvest. Both federal (USDA) regulations and consumers demand a zero tolerance for maggots in fruit at harvest.
Spinosad, derived from a naturally-occurring bacteria, is quite effective at controlling the fruit fly, and it is approved for use as an organic pesticide in cherry production. It has been used in combination with bait as well as applied as a foliar insecticide. The bait-plus-spinosad product (GF-120NF) is an attractive substance that is lethal to flies that feed on it while "grazing" on the tree. This bait is "squirted" and spattered on the trees weekly at 20 fluid ounces per acre diluted in about 1.5 to 2 gallons of water per acre (it should NOT be applied with an air-blast sprayer). EntrustTM is a spinosad-based product that kills flies both by contact and residue, and, unlike GF-120NF, it can be applied by air-blast sprayer. Sprays of Entrust every seven to 10 days beginning with fruit fly emergence have provided excellent fruit fly control.
Regarding gypsum as a possible fruit fly control, there seems to be no research references on this topic. However, the fact that the cherry fruit fly maggot falls from the fruit, burrows into the soil, pupates in the soil, and then re-emerges from the soil as an adult could lead one to think that some sort of manipulation or treatment of the soil could result in significant mortality to the fruit fly. That might be the case, but it doesn’t seem to me that gypsum would be the best candidate for this. Gypsum (calcium sulfate) is pH neutral, so it probably would not alter the pH enough to be deadly to the pupae. Besides, if some soil amendment did alter the soil pH enough to affect the pupae, it might also be deleterious to the trees. And applying enough gypsum to the soil surface so that it might present some sort of abrasive barrier (like diatomaceous earth) to the entrance and exit of the fruit fly from the soil seems unlikely due to the inevitable weathering of the gypsum by wind and rain.
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