There are several species of mites that infest pome and/or stone fruit trees in Victoria and they have different life cycles and habits that growers need to understand in order to efficiently manage their impact on crops. Bryobia mites are brownish, flattened mites with very long front legs.
Bryobia mites (cont from last month)
Bryobia mites are brownish, flattened mites with very long front legs.
They overwinter as red eggs laid in similar locations as those of ERM but, in contrast to ERM, the eggs are spherical instead of onion shaped and do not have spines.
Bryobia mites feed on both surfaces of the leaves but during the brighter parts of the day they move off the leaves and can be found on nearby twigs. This is something important to remember when scouting for mites reveals silvering of the upper leaf surface but no mites can be seen on the leaves.
Bryobia mites do not usually produce webbing on fruit trees.
Eriophyid mites
Eriophyid mites (apple rust mite, pear leaf blister mite, and peach silver mite) are tiny (about the size of a TSM leg) worm-like mites that are useful prey to maintain predator mite populations in readiness for when TSM or ERM become active.
The eriophyid mites rarely cause significant damage unless their populations build up to extreme levels due to poor choice of pesticides resulting in decimation of predator populations.
Predator mites
Growers who have disrupted their predator mite populations by choosing to use pesticides that are toxic to the predators—for control of other pests such as woolly apple aphid or weevils—can re-establish predator populations by purchasing suitable predators from commercial suppliers.
These suppliers will also provide information on the impact of pesticides on the predators, so that growers can adjust their spray programs against other pests while still protecting the predator mites.
Pest monitoring
A survey in 1999 of growers in all Victorian production areas demonstrated that more than 92% of growers used pest monitoring to inform their spraying decisions and 57% used consultants for advice on pest management based on their monitoring results.
More than 84% were prepared to balance pest control with the value of maintaining predator populations by selecting pesticides on the basis of compatibility with predators.
Prior to 1990, spraying for mites was based either on direct counts of the number of mites per leaf, or the percentage of leaves infested by mites (with an adjustment for the percentage of leaves with predatory mites present).
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 (cont next month)
See this article in Tree Fruit Jan 2018
