Standard advice is to use different chemical controls from time to time to avoid buildup of resistance in the target insect population.
Malathion is a very old organophosphate (OP), which is still worth considering IMNSHO.
Permethrin is a synthetic pyrethroid similar to natural pyrethrin or pyrethrum (PyGanic, Pyrenone). These are not long-lived insecticides. Their potency wanes rapidly after application. They may be effective for a day or two. As such, they can be applied at dusk on the first warm evening after petal fall for control of curculio which are migrating into your orchard without severely impacting daytime polinators. (Obviously, this has already occurred in your location.) You should be able to control a large portion of your infestation (but probably not all) with one application. Upcoming treatments for other insects will have to take up the slack.
Carbamates are broad spectrum insecticides with a similar mode of action to the OPs but less toxic to humans. Several carbamates are systemic, meaning they can be absorbed by the plants. Carbaryl … has plant growth regulator activity and can be used to thin apples. Many carbamates are highly toxic to predatory mites and should be applied carefully (McCamant).
Carbaryl (Sevin) applications made within 30 days after petal fall have a pronounced thinning effect on most apple varieties. Sevin does not thin pears. It is a broad-spectrum material that acts as a stomach as well as contact poison and controls various fruit insects, including organophosphate-resistant pests such as white apple leafhopper. It is one of the low-hazard insecticides and can be used up to one day before harvest on most fruit crops … in home orchards. Sevin is very toxic to bees and, therefore, should not be used just before bloom. It is also toxic to certain predator mite species (Cooley et al.).
This is from the label for a brand of carbaryl, concerning thinning apples:
SPECIFIC DIRECTIONS
Apples Only, for Fruit Thinning
OBSERVE BEE CAUTION
Apply 1 to 3 quarts per acre of PROKOZ SEVIN SL between 80% petal fall and 16 mm fruit size. Use the higher rates on hard to thin varieties at the early timing which is the 80% petal fall to 6 mm fruit size. Use the lower rates on easy to thin varieties and at the later thinning period which is the 10 to 16 mm fruit size. The rate to use per acre will depend on varieties, tree size, row spacing, weather conditions at the time of and following applications. Consult with the local fruit thinning experts in your area for the proper rate to use under your conditions.
The optimum spray gallonage will depend on the tree size, planting density, row spacing and amount of foliage. Use sufficient spray volume to insure adequate coverage (100 to 400 gallons/acre). Avoid spray to the point of runoff. Reduce spray coverage to the lower portion of the tree since overthinning may occur here.
Factors such as climatic temperature, high humidity, frost, tree age, variety, nutrition, previous crop, pruning and bloom may influence fruit thinning results with the product. Exercise caution to avoid possible overthinning. For the most effective results, apply under good drying conditions and when daytime temperatures (°F) will be 70 to low 80’s for the following one to three days. Application with daytime temperatures in excess of 80° F may result in overthinning.
PROKOZ SEVIN SL may be mixed with other fruit thinners, however, use caution to avoid overthinning and other adverse effects. Consult with local fruit thinning experts in your area for recommendations. Refer to the other product labels for specific use directions.
Consult with local fruit thinning experts in your area for advice on the proper use of this product on your varieties under your growing conditions.
CAUTION: The use of PROKOZ SEVIN SL may result in fruit deformity under certain environmental conditions. Before using on any variety of apples, the user must weigh the risk versus benefits when using this product, particularly when using between 80% petal fall and 6 mm fruit size. Red Delicious are more sensitive to this phenomenon and in particular, the varieties Bisbee, Red Chief and Vallee Spur are very susceptible to conditions causing fruit deformity. Precipitation and temperatures below 65° F increases the possibility of fruit deformity. The use with summer spray oils and wetting agents may increase the risk of fruit deformity and injury.
