ORCHARD MITE MANAGEMENT AND UPDATE ON PLUM CURCULIO IN UTAH

Diane G. Alston

Department of Biology, 5305 Old Main Hill, Utah State University

Logan, UT 84322-5305; email: dianea@biology.usu.edu

ORCHARD MITE MANAGEMENT

Background:

Integrated mite management in tree fruit orchards in northern Utah relies on a balance of phytophagous (spider and rust mites) and predaceous mite (Typhlodromus, Zetzellia) densities, and on synchrony of their dispersal (i.e., movement) from ground vegetation or outside sources into trees. In the winter and spring, spider mites and Typhlodromus predaceous mites are found primarily in crevices on tree trunks and on ground vegetation while rust mites and Zetzellia predaceous mites are exclusively in the trees. A number of factors determine whether or not phytophagous mites will increase to economically damaging levels in fruit trees during the summer. In addition to the number of mites and ratio of prey to predator mites on ground vegetation and trees, orchard pest and ground cover management practices (e.g., types of pesticides used, types of plants in ground cover, and ground cover mowing and herbicide practices) and weather conditions (hot, dry weather is often associated with rapid increases in mite densities) primarily determine how many and when mites disperse into fruit trees during the summer.

Northern Utah fruit growers have expressed concern in recent years for economically damaging spider mite populations and a lack of mite biological control. It has been noted that apple rust mites, which play an important role in integrated mite management by serving as early season prey and helping to coax predaceous mites into trees before spider mite densities build to problem levels, have been few to absent. Low predaceous mite populations seem to follow low rust mite populations. Low numbers of apple rust mites and predaceous mites have been observed across many orchards in multiple locations. Therefore, it seems that whatever factor(s) are responsible for their decline must also be widespread. As few as 3-4 years ago, I observed adequate populations of apple rust mites and predaceous mites in some of the orchards where they now seem to be nearly absent.

This study was conducted to update our survey for the presence and abundance of the major tree fruit mite species (phytophagous and predaceous) in northern Utah apple and tart cherry orchards, and to assess the status of biological control of spider mites.

Objectives:

  1. To determine densities of phytophagous (plant-feeding) and predaceous mites on ground vegetation, tree trunks and tree leaves in apple and tart cherry orchards during the season.
  2. To use information on mite abundance and predator:prey ratios in space and time in orchards to speculate on the current status of biological control of spider mites.

Brief Methods:

Three apple and five tart cherry orchards were surveyed at approximately monthly intervals from May through late August (surveys were initiated in March at two sites, in April at one site, and in July at one site). Ten pairs of trees were randomly selected in each orchard and marked with flagging as sample trees. On each survey date, three types of mite samples were collected from each of the 10 pairs of sample trees: 1) 10 leaves from one tree, 2) sticky trunk band removed and replaced on the other tree, and 3) broadleaf "weed" vegetation collected on the orchard floor under the trees. Ground vegetation samples were placed on Berlese funnels (funnel with light bulb over the top to drive the living mites off the vegetation, down the funnel, and into a vial of alcohol underneath) for 2 days, and then the dry vegetation was weighed. The number of phytophagous (spider mites and rust mites) and predaceous (Typhlodromus spp. and Zetzellia spp.) mites was counted in each type of sample.

Results:

In five of the eight orchards surveyed, spider mite (mostly twospotted, some McDaniel, and a few European red mite) densities reached potentially economically damaging levels on tree leaves (apple: Santaquin and Payson; tart cherry: Santaquin, Payson and Kaysville) using an economic threshold of 10 spider mites (mobile stages + eggs) per leaf (Fig. 1).


Figure 1. Spider mite densities (number per leaf) in 8 orchards in 2000.

 

Rust mites were observed in low numbers in only the Kaysville-apple orchard and none were found in the other orchards. Predaceous mites (mostly Typhlodromus, a few Zetzellia) were found in all orchards by July or August, but numbers were low except in the Kaysville-apple orchard.

Ratios of the number of predatory mites to the number of plant-feeding mites (prey) on leaves were low in most orchards until August, except for the Kaysville apple orchard (Figs. 2 and 3). Desirable ratios of predators to prey are at least 1:10 (0.1) and preferably 1:5 (0.2) or higher by July when spider mite densities can increase rapidly in trees under hot, dry weather conditions. The generally low ratios in July indicate that there were not adequate numbers of predaceous mites to keep spider mites below economically damaging levels in most orchards.

Figure 2. Ratio of predatory mites to prey mites (spider mites) on tree leaves in 3 apple orchards.


Figure 3. Ratio of predator to prey mites (spider mites) in 5 tart cherry orchards.

 

Correlation analyses were conducted to determine relationships between mite densities on ground vegetation, tree trunks and tree leaves within orchards. There were good relationships between the number of spider mites on ground vegetation and on tree trunks in June and July and between the ground vegetation and tree leaves in July and August. There were very strong relationships between the number of spider mites collected on sticky trunk bands and those on tree leaves in July and August. And the number of predaceous mites on ground vegetation throughout the season was a strong indicator of the number found in trees.

Conclusions and Summary:

In summary, this one study does not tell the entire story of why effective biological control of spider mites is not occurring in more northern Utah orchards. It does substantiate a link between a lack of rust mites and predaceous mites, although we do not have data from enough orchards in the past to substantiate that rust mites and predaceous mites were present and contributing to effective biological control at that time. Why are rust mites and predaceous mites not more prevalent than they are? I do not have a definitive answer. Because the lack of their presence is so widespread, it may be due to changes in climate, horticultural practices, or pesticide use that are difficult to document.

A strong relationship between densities of spider mites on tree trunks and those moving into trees suggests that sticky trunk band traps might be a useful monitoring tool for growers if they were willing to check them every week or two during "mite season". The study also substantiates that spider and predaceous mites begin their growing season on ground vegetation in orchards. Although it is difficult to directly predict numbers of mites expected in trees in July and August based on numbers found on floor vegetation earlier in the summer, it reconfirms that good ground cover and orchard management practices that minimize build-up of spider mites and conserve predaceous mites are important to a successful integrated mite management program.

Suggestions for further studies in 2001 include demonstration of sticky trunk bands to detect when spider mites begin to increase and move into trees and release of insectary-reared predaceous Typhlodromus onto orchard ground vegetation early in the season to suppress spider mites before they move into trees. Inoculation of rust mites into trees may also provide some benefit. A source of rust mites would need to be determined. Another suggestion is to conduct a study to determine if some pesticides, such as Carzol and Sevin, that we know are toxic to predaceous mites are having a negative effect on mite biological control when they are applied early in the season for control of Campylomma, white apple leafhopper, and leafrollers and for fruit thinning.

 

UPDATE ON PLUM CURCULIO

Surveys conducted from 1998-2000 have delimited the distribution of plum curculio to an approximately 50 sq. mile strip along the highway from Honeyville to Willard, centered on Brigham City in Box Elder County. Surveys for the insect in Cache and Weber Counties were negative. The majority (82%) of sites infested with plum curculio are home yards. The remainder of infested sites is neglected orchards (11%) and roadside trees, primarily American plum (7%). The most common fruit host infested in the field is sweet cherry (57% of infestations).

In the laboratory, it was found that plum curculio would attack a native host, wild hawthorn. If the insect should spread to native hosts, this could create a more difficult situation for population suppression.

Efforts to suppress the insect are focused on the residential areas of Brigham City where the majority of infested sites occur. USU Extension and Utah Department of Agriculture are working together on outreach education for homeowners. Host tree removal and insecticide treatments are the primary treatments recommended. Additional alternative controls appropriate for home yards are also being explored.