Chemical control: Determination of insecticide resistance levels in mealybug populations

To examine if insecticide resistance is the reason for increasing obscure mealybug (Pseudococcus viburni) populations in the pome and stone fruit orchards mealybug colonies need to be reared successfully in sufficient numbers to use in bioassays with different insecticides used for their control.

Mealybugs from apples collected from an old neglected orchard on the Elgin Experimental farm were used to start a colony susceptible to insecticides. A colony was also started from apples with mealybugs from the Molteno farm in Elgin in 2014 to test for resistance. Mealybugs were reared on butternuts as rearing medium (Mudavanhu 2009) inside a Perspex cage.

A bioassay method where mealybugs were established on small sprouted potatoes was developed and tested.

First instar crawlers were allowed to develop until the 3^rd to 4^th instar stage. Five replicates were used for each concentration of the insecticide tested and water was used as control. Each insecticide was tested at 5 different concentrations (1/4x, ½x, 1x, 2X and 5x the concentration recommended for mealybug control). Each potato with 10 mealybugs was dipped in the insecticide concentrations or water for 5 seconds. Mortality was evaluated 24h and 48 hours after treatment. Chlorpyrifos was tested with all five concentrations and the control on both the susceptible and Molteno mealybug colony simultaneously.

The susceptible mealybugs were very sensitive to chlorpyrifos (Dursban), with most mealybugs dead after 24 hours at all concentrations. The Molteno colony was less sensitive to chlorpyrifos as more survived within the first 24 hours at ¼, ½ and with the field concentration (Fig. 1). LC_{50 and} LC₉₀ could not be determined for the susceptible colony from this experiment as they were too sensitive to the chlorpyrifos treatment.

There was no statistical difference between the susceptible and Molteno mealybug colonies for the contact effect of acetamiprid (Maintain 200 SP) at all the concentrations (Table 3 and 4). However, a probit regression analysis indicated that at the time of testing the Molteno colony had a slightly higher LD ₅₀ than the susceptible colony at 24 hours indicating that they are slightly less affected by the treatment. However at 48 hours the probit regression lines were the same and therefore the acetamiprid has the same LD₅₀ (0.734 field dose) and LD ₉₀ (7.292 field dose) for both mealybug colonies.

This bioassay method only test the contact effect of an insecticide and therefore the systemic effect of acetamiprid must still be investigated.