RAMP Grant: Functional Ecology
Mites as an Ecological Sustainability Indicator in Orchards
Tree Stress- Many different factors can contribute to tree stress in tart cherry orchards. Stress is often expressed in reduced yield, color, Brix, firmness and fruit size as well as tree vigor, leaf chlorosis, premature leafdrop, increased blind areas on branches, lowered winter hardiness and reduced return bloom and growth the following season. Although pests like plum curculio (Conotrachelus nenuphar) and cherry fruit flies (Rhagoletis fausta, Rhagoletis cingulata and indifferens) certainly can affect fruit yield, quality and marketability; these pests have little effect on tree stress. Incidentally, fruit flies usually feed on leaf exudates, aphid honeydew and bird droppings while plum curculio feed sporadically on leaves and frequently on fruit. Neither species is thought to stress the tree other than fruit yield and quality. Recently, however, it was discovered that early season plum curculio feeding on the stems of developing cherry fruit actually can cause significant yield reduction, yet this injury has little effect on overall tree stress.
Leaf feeding mites cause tree stress- Two-spotted spider mite (Tetranychus urticae), European red mite (Panonychus ulmi), Bryobia spp., Czenspinskia lordi and rust mite (Aculus fockeui & other spp.) can have a dramatic impact on cherry leaf cells; their feeding injury either causes water and nutrient stress or leaf cells are killed. Thus tree photosynthesis decreases and stress in the tree increases with increasing pest mite populations. Left unmanaged in the Midwest and East, the combination of mite injury and cherry leaf spot (Blumeriella jaapii) can cause significant defoliation by mid-summer, which in turn causes soft fruit, poor color, and reduced soluble solids. In the West, cherry leaf spot is not a problem in more arid climates, but mites are even more severe. Early defoliation from either or both stressors also delays winter-acclimation of fruit buds and wood to cold temperatures in the fall, dramatically increasing tree mortality during severe winters, and reduces fruit bud survival and fruit set the following year (Howell and Stackhouse 1973).
Natural control of tree feeding mites easily disrupted by orchard operations- Certainly, various orchard operation like mowing, herbicides, insecticides or fungicides can disrupt regulation of plant feeding mites by killing predator mites like the phytoseids, Neoseiulus fallacies or Galendromus occidentalis or Stigmaeid spp. like Agistemus fleshnerii and Zetzellia mali. Insecticides and even fungicides are quite often more toxic to predators then to plant feeding mites because the plant feeding mites have natural defenses against harsh chemistries plants produce. Many plants including cherries produce an array of secondary defense chemistries that are quite toxic to insects and mites in general. That is why all insects and mites do not survive on all plants because plant secondary compounds kill or repel most plant feeding insects and mites. Yet plant feeding mites have evolved an array of means to escape being adversely affected by plant secondary chemistries, and these defenses to plant secondary chemistries also provide some protection against some pesticides used in orchards.
OP-alternatives and new fungicides can disrupt mites too- Mite induced cherry tree stress has steadily increased since the mid-1990s when use of organophosphate (OP)-alternatives like carbamates, synthetic pyrethroids and some of the newly registered chemistries began to occur more frequently in cherry orchards. OPs have been used in cherry since the early 1960's. In fact, in the early days, OPs where marketed as miticides, but pest mites very quickly developed resistance to the OPs and companies stopped claiming control. After nearly 35 years of OP use in cherry orchards, both predaceous mites along with pest mites have evolved resistance, and the predators have been able to regulate leaf feeding mites below levels that stress the tree even in the presence of OP sprays. Today, the synthetic pyrethroids, neonicotinoids, strobilurins and other new chemistries used in cherry orchards can disrupt predator regulation of pest mites and lead to greater populations of pest mites and more stress on cherry trees.
Mites are easily impacted by orchard practices and environmental conditions- Mites are viewed by B. Croft (OR State University) and M. Whalon (MI State University) as indicator species of the overall 'health' of the fruit tree ecosystem (collectively we have studied mite and fruit tree dynamics for over 50 yrs. Collectively, we have published well over 200 scientifically refereed journals on mites in orchards. Our hypothesis is that if plant feeding mites are not being regulated by predators, than the system has been overly disrupted by pesticide, horticulture management and/or nutrient inputs, and steps are required to re-establish a regulating ratio between predators and plant-feeding prey.
Mites are one of the most sensitive species to changes in the tree ecosystem because they undergo between 8-25 generations in a growing season and they live in very intimate association with the leaf surface. Both pest and predator mites also exploit the ground cover for habitat or prey, and may even overwinter in the ground cover. Mite populations therefore readily respond to disruption in orchards. For instance, clean cultivation or overuse of herbicides can flare pest mites by reducing habitat and prey in the ground cover where predators overwinter. Introduction of a new fungicide or insecticide with delayed insect growth regulator effects may not affect the adult mites present at the time the spray is applied, but it could kill or delay development of the predator's eggs while having no or little effect on the pest mite eggs. Some insecticides like synthetic pyrethroids exhibit negative selectivity in that they kill more predators than pest mites. Therefore, a single, early-season orchard spray of the synthetic pyrethroid, Asana, can kill predators for months, even the entire growing season. But Asana does not just kill mite predators, it kills many of the other insect predators that feed on mites like Stethorus punctum, a ladybird beetle, Nabids or assassin bugs, Chrysopids or lacewings and Syrphids or hover flies. This allows pest mites to multiply unheeded causing leaf bronzing, early leafdrop and their damage puts the tree in a stress situation where it might not survive a severe winter, and if it does, the stress results in reduce fruit load, color and soluble solids in next season's crop.
In another more complex instance, trees might be stressed by drought or poor irrigation. Since mites do well in hot, dry weather, their population begins to rapidly increase. But there is another factor at work, secondary plant chemistry. As the tree's metabolic systems are more and more impaired by drought, the trees begin to allocate more and more of their resources toward primary plant functions necessary for survival. Therefore, secondary chemistry is down regulated, plant mites have and easier time feeding on leaves without the plants natural defenses and the result is a mite population explosion.
Mites are good indicators of the ecological state of an orchard (Figure 1)- . Since mite populations respond so quickly to plant stress and to pesticides, they are an ideal indicator of the ecological state of orchards (see Fruit Crop Ecology and Management, MSU Bulleting E-2759 or http://www.msue.msu.edu/ipm/Pubs_eco.htm). The premise is simple: if plant feeding mites are not regulated by predators, the orchard system is not functioning properly. Since mites reproduce exponentially and because they are in such intimate association with the tree, their species composition and population responses can indicate the health of individual tree and the whole orchard. On the other hand, plant feeding mite predators are particularly vulnerable to new pesticides and reduced predators translate into higher pest mite numbers. Therefore, as growers move to adopt OP alternatives mandated by FQPA or other pesticide changes, mites could be used as a means to determine whether or not these changes lead to a more disrupted or more sustainable orchard ecosystem.
Therefore, we believe that the ratio between pest mites and their predators coupled with the species present and time of the year provide easily obtained insight into the Functional Ecology of an orchard system. We term this functional ecology because orchards in balance are productive and sustainable = functional, and orchards that are out of balance result in stressed trees, hence stressed people and their operation's economics. The whole system is out of alignment; it is ecologically dysfunctional. Therefore, knowing the state of mite predator prey ratios and population dynamics is a very useful indicator of orchard ecology, overall health and therefore, sustainability.