Soil mites are considered one of the major components of soil arthropod communities, although they are not well studied. Soil-inhabiting mites play an important role in litter/organic matter decomposition in three different ways: (1) direct litter decomposition, (2) organic matter fragmentation or dispersal of microbes, and (3) feeding on microorganisms. Soil mites indirectly affect the structure of soil by increasing aeration and water holding capacity. They also decrease bulk density and decay organic matter by controlling microorganisms. Thus, soil mites play an important role in the production of good crop yield.
Muhammad A. Qayyoum, a visiting scientist from Pakistan, joins an ongoing study on soil improvement with Dr. Raul Villanueva, UK field crop entomologist at the University of Kentucky Research and Education Center (UKREC) at Princeton. This multiyear study is directed by Dr. E. Ritchey. The study evaluates nitrogen amendments with three different rates each of poultry litter, composted swine manure, and bio-solid and comparing them with four levels of commercial fertilizer (urea) and an untreated control. However, Qayyoum evaluates this study from the point of view of an acarologist (studies mites and ticks).
The goal of this study is to determine the population densities and species composition of soil mites in corn and wheat fields under the treatments described above.
Collection of Mites & Identification
All treatments were evaluated on replicated experimental plots in wheat and corn fields near Princeton. Mite identification is done by collecting soil samples and placing them in Berlese funnels for 24 hours to extract the mites on ethanol. Later, these mites are sorted under the dissecting microscope and placed on permanent slides to be identified at the species level. The samples are being collected on 15-day intervals starting on June 1, 2016, and continuing until September 30, 2016. To simplify this article, the results are summarized by nutrient source (poultry litter, composted swine manure, bio-solids and urea), averaged across the three rates of each source, and compared to the untreated control.
Preliminary Results & Conclusions
So far, a total 192 soil samples have been evaluated. The population density of soil-inhabiting mites recovered from wheat fields immediately after harvest was lower than that of corn fields planted this year.
However, in both studies the mite populations follow a similar trend – the highest densities found in poultry litter amended soil and the lowest in the untreated control (Figure 1). The composted swine manure, biosolids, and commercial fertilizer amendments gave intermediate populations. Briefly, we found 3 orders, 5 families, and 9 genera in both fields. In terms of densities and species richness in both crop fields, Oribatid mites dominated the soil mite communities, followed by Mesostigmata and Prostigmata mites,
Figure 1: Population density percentages for soil mites (oribatids, mesostigmatids and prostigmatids) on soils treated with poultry litter, composted swine manure, biosolids, and commercial fertilizer, as well as an untreated control. The total number of mites found in each treatment is shown in the white circle.
Oribatids are the most abundant mites an in all treatments, comprising more than 68% of all mites found. Oribatid mites (Figure 2a) live in the top layer of the soil, leaf litter, or other debris. They can also be found on mosses, lichens, and other low growing plants. Oribatids eat fungi, algae, dead plant matter, dead springtails (tiny insects that live in the soil), and live nematodes (tiny worms). These mites are extremely important; they “recycle” old material on the ground. Mesostigmatids (Figure 2b) are free-living mites and predators; they prey on insect and mite pests, and can be used for biological control (whether natural or human instigated). Prostigmatids (Figure 2b and 2c) are parasitic in nature and all of the major plant parasitic mites belong to the Prostigmata. Many Prostigmatid species found on soil samples are fungivores and include some pests of cultivated mushrooms, such as Tyrophagus spp. The two spotted spider mite (Tetranychus urticae) (Figure 2c) is an example of a prostigmatid mite; however this mite species does not live in the soil, but feeds on the foliage of many important cultivated plants.
Figure 2. (a) Oribatida, (b) Mesostigmata feeding on Prostigmata mite and, (c) the common two spotted spider mite (Photos courtesy of Dr. Ronald Ochoa, ARS-USDA)
All the soil amendments evaluated in this study have had a great impact on soil nutrient availability to the crops, and Dr. Ritchey’s data has shown increased corn and wheat yields, relative to the untreated control. However, these amendments also greatly benefit the soil micro fauna. The difference between the untreated control and the rest of the treatments is of particular importance. Poultry litter amended soil had 10 times more soil mites; composted swine manure and biosolids amendment resulted in 5 times more; and the commercial fertilizer amendment almost 3 times more soil mites. The mechanism behind these differences is still unknown. Further studies will continue to evaluate the effects of biosolids, poultry litter and composted swine manure on arthropod density and diversity.
By Muhammad Qayyoum, Visiting Scholar at the UKREC-Princeton, Department of Entomology, University of Kentucky and Ph.D. Student at the University of Agriculture, Faisalabad, Pakistan; and Edwin Ritchey, UK Extension Soils Specialist, and Raul T. Villanueva, UK Extension Entomologist
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