Agriculture Victoria research conducted in the Goulburn Valley developed a more accurate and reliable threshold based on the cumulative number of leaf-infested days.
Pest monitoring (continued from last issue)
Agriculture Victoria research conducted in the Goulburn Valley developed a more accurate and reliable threshold based on the cumulative number of leaf-infested days (that is, the number of days leaves were infested, multiplied by the average percentage of leaves infested).
Cumulative leaf-infested days
The research compared the cumulative leaf-infested days (CLIDs) with yield losses in the current and subsequent growing seasons, and concluded that for WBC pears the threshold value—above which some yield losses would occur—was 1500 CLIDs.
For Packhams the threshold was around 2000 CLIDs, and most apples had thresholds greater than 2500 CLIDs.
This allowed for quicker, more cost-effective monitoring of mite populations by using simple presence/absence assessments of leaves, and the graphical output allowed growers and consultants to use the trend line to predict when the mite population would exceed the threshold well in advance of the event.
Calculate the leaf-infested days
To calculate the leaf-infested days between two sample dates (D1 and D2) the formula is:
(number of days between D1 and D2) times (percent of leaves infested on D1, plus percent of leaves infested on D2) divided by 2.
A worked example is given in Table 1 for weekly samples.
To get good mite control:
•Plan your potential spray program around pesticides that have minimal impact on biological control agents.
•Use biological control agents to reduce reliance on pesticides. Pesticides should be used to support predator activity rather than rely on pesticides and hope that predators come to your rescue when you really need them after resistance has developed.
•Understand what mites are present and how their behaviours influence your monitoring techniques.
•Monitor mite populations from green tip through to leaf fall in autumn.
•Where two or more species are present record the percentage of leaves infested for each species separately.
•Calculate CLIDs for each species and also calculate the combined species CLIDs.
•Decide which species is more important or more difficult to control and choose a miticide that will give optimum control of that species and sufficient control of the secondary species.
•Pay careful attention to the slope of the CLIDs graphs, forecast weather, predator activity, and any critical crop management stages (especially harvest windows) and predict when you may need to spray early enough to keep the mite populations below the threshold CLIDs.
•The threshold holds for the entire season, not individual spray dates.
•Remember that you do not have to wipe them out, you just need to keep them below the threshold.
•Seek professional advice if you are unsure of what to do.
See this article in Tree Fruit Feb 2018
