Spotted wing drosophila management: A challenge for stone fruit this year

Wendy McFadden Smith, Horticulture IPM Specialist, OMAFRA, Hannah Fraser, Entomologist – Horticulture, OMAFRA, Jean-Philippe Parent,  AAFC 

Spotted wing Drosophila (SWD) populations usually start to soar in late-July, after the majority of the tart cherry crop is harvested. However, numbers rose quickly this year, reaching levels much higher than normal by the time cherries started to colour. Eight cherry blocks in Niagara were monitored for SWD using apple cider vinegar traps (ACV). Each week the traps were emptied and refilled and the number of SWD counted. We have used ACV as a bait in our monitoring programs since 2011, and while it is cost effective and easy to obtain work with, it is not very selective or effective at attracting SWD at low population densities. For this reason, we have recommended protective sprays for ripening fruit when SWD is present in a region (SWD + ripening fruit = spray).

The flies are attracted to the fruit when they lose their green ground colour and continue to be attracted to them through harvest. SWD eggs are laid under the skin of ripening fruit. Each egg has a pair of thread-like “breathing tubes” that protrude from the oviposition puncture and are visible on the fruit surface with a good 16X magnifying lens.

While SWD have commonly been reported in other areas to be a serious pest of tart and sweet cherries, the other tender fruit are generally not at risk. Infestations of plums and over-ripe peaches have been reported in BC and when SWD populations are high. It is advisable to use products that have activity against both SWD and other pre-harvest insect pests when making your spray choices for all stone fruit. Do not assume that pre-harvest products used to target cherry fruit fly or oriental fruit moth will be effective against SWD. Note: Table 6-12 in Publication 360 lists insecticides that are registered for other insect pests of stone fruit that may have activity vs SWD. Group 3 insecticides do not work as well and do not last as long at temperatures over 26°C.

The following table (extracted from a previous ONFruit posting) shows the products that are registered for SWD in stone fruit. If a product is labeled for stone fruit, it is usable on all stone fruit. Some products are labeled only on specific crops. The higher the number for relative activity, the more effective the product.

¹ Re-entry interval; ² Pick-your-own re-entry interval

Because SWD can complete a generation in 8-10 days at 25°C, it is necessary to maintain excellent coverage with effective insecticides. This requires shorter spray intervals, especially with heavy rains. It is also important to make sure your sprayer is well calibrated and the spray reaches the tops of the trees as well as the centres. You must spray every row rather than alternate rows. Based on experience in Michigan, SWD control under the high pressure conditions of this year required a 7-day spray interval under dry conditions, with another application required after rain. If SWD is your primary target, it is recommended to rotate among insecticide groups for resistance management. This will work if you are using mating disruption to manage OFM. However, if you rely on insecticides to manage OFM, the strategy for managing resistance is to use the same group within generations and rotate to a different chemistry between generations.  It is more critical to continue to follow this strategy if you are spraying for both OFM and SWD.

It is important to know the rainfastness of the insecticides used as there is considerable variability among insecticides. The intensity of the rainfall is also important: 5 cm of rain in an hour is going to remove a lot more residue than the same amount over a 2 week period. The tables below (taken from a Hort Matters article) provide information on relative rainfastness of the insecticides registered for SWD management.

Rainfastness Rating Chart: General Characteristics for Insecticide Chemical Classes

* H – highly rainfast (≤30% residue wash-off), M – moderately rainfast (≤50% residue wash-off), L – low rainfast (≤70% residue wash-off), S-systemic residues remain within plant tissue

Insecticide persistence, plant penetration, and rainfastness rating.

The bottom line: This year SWD pressure is extremely high so we may see damage where we normally do not. Use the most effective products for both SWD and the usual preharvest pests. Shorter spray intervals will be required and sprays will have to be reapplied if there is heavy rainfall.