Author - David Williams,Agriculture Victoria, Tatura
Mites are causing concern for some growers this season. 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.
Spider mites include two-spotted mite (TSM), European red mite (ERM), and Bryobia mite. TSM is generally the most common mite and overwinters as hibernating females amongst webbing around the lower trunk of the tree. Overwintering females appear as tiny spider-like orange-red coloured mites, which is why they are often called ‘red spider’ mites. In spring as the temperatures warm up, and there are leaves on the trees, the overwintering TSM start to feed on the lower leaves, change colour to the summer form (pale straw) and develop the two dark spots that give them their common name. At this stage they start to lay spherical, pearl coloured eggs usually close to the veins on the undersides of the leaves. As populations start to build up the mites move upwards and outwards along the scaffold limbs of the trees. Predatory mites were introduced and distributed throughout Victorian orchards for biological control of TSM. Populations of TSM resistant to many miticides are widespread and growers need to carefully plan their mite control program to maximise the benefits of biological control before resorting to applying pesticides that may disrupt the biological control agents (especially predatory mites and Stethorus beetles).
ERM is a dark red mite about the same size as TSM. It has white tubercles, with strong hairs (setae) protruding from the tubercles, on its back. ERM overwinters as dark red, onion-shaped eggs that have a spine arising from the central top. These eggs are laid around the bud scales on twigs and shoots. The white tubercles on the backs of the adult mites, and the spines on the eggs, are easily seen with the aid of a hand lens. The overwintering eggs hatch around the green-tip stage of tree development in spring and the nymphs move to the developing leaves. This pattern of egg laying means that ERM infestations are more widespread throughout the tree than are TSM infestations. Where ERM and TSM appear on the same leaves ERM will usually inhabit the upper surface of the leaf and TSM the lower surface. ERM does not produce much webbing. ERM appeared in orchards on the Mornington Peninsula in the 1980s and spread to Gippsland, Yarra Valley, Bacchus Marsh, North-East, and Harcourt. It has been found in small pockets in the Goulburn Valley since about 2000. Predatory mites were introduced to Victoria to control ERM in the 1980s and the comments above regarding biological control of TSM hold equally for ERM.
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 (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.
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. A survey of growers, in all Victorian production areas, conducted in 1999 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/ leaf, or the percentage of leaves infested by mites (with an adjustment for the % 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 (which is a fancy way of saying the number of days leaves were infested multiplied by the average percentage of leaves infested). 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.
To calculate the leaf-infested days between two sample dates (D1 and D2) the formula is:
(number of days between D1 and D2) times (% leaves infested on D1 plus % leaves infested on D2)/2
A worked example is given in the table below 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 % 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 what to do.
David Williams
Principal Research Scientist- Invertebrate Sciences | Biosciences Research
Department of Economic Development, Jobs, Transport and Resource
