On the Role of Saffron Bulb Infection to Soil Born Fungi on Biology, Behavior and Damage of Bulb Mite Rhizoglyphus robini Claparede (Acari: Astigmata)

Saffron is one the most expensive crops and like other agricultural products, attacked by pest such as bulb Mite Rhizoglyphus robini Claparede (Acari: Astigmata). The bulb mite is one of the most important soil pest attacking plants with bulbs, corms and also tubers. In Iran it has been reported by Rahimi and Kamali (1993) for the first time on saffron corms from Gonabad and Qaen cities. Also it has been recorded that the bulb mite feeding on soil born fungi (Diaz et al., 2000; Nesvorna et al., 2012). On the other hand, there are many saprophytic fungi into the soil of saffron fields. Subsequently it may question whether the mite is primary or secondary pest on saffron corms. Despite many literatures on biology and ecology of Rhizoglyphus mites, there is not sufficient evidence on understanding the biology, behavior and colonization of R. robini regarding its damage to saffron corms when arriving after establishment of the soil born fungi.
To investigate the impact of soil-borne fungi on the biology of the bulb mite, we obtained a cohort of even-aged eggs from the mites in our stock culture. These eggs were then transferred to individual experimental units and monitored until they reached adulthood. Daily observations were made and recorded.
To assess mite fecundity, we selected thirty young ovipositing females and divided them into two groups: one group was exposed to fungal infection while the other group was not. Each saffron corm was placed in a 50mm Petri dish lined with wet filter paper. A starved mated female was added to each dish, and every three days for a period of 21 days, the number of eggs laid per dish was counted and then removed. The number of eggs per day per female was calculated based on these counts.
To study the attraction of bulb mites to the fungus, saffron corm sections with and without fungal infection were placed in a 50mm Petri dish. Four sections, with equal distances from each other and from the center, were arranged. Female R. robini mites were introduced into the dish, and after a four-hour period, the number of mites on each section was recorded.
For evaluating the population dynamics of the mite, we used four types of saffron corms: healthy corms, corms infected by the fungus, mechanically injured corms, and corms both infected and injured. Each experimental unit consisted of three saffron corms of the same size placed in an 80mm Petri dish. Five adult mites were added to each unit. Continuous observations were made daily to track the initial penetration and colonization of the mites in each treatment. The numbers of all motile stages of mites in each experimental unit were recorded using a stereomicroscope. These observations continued until the corms were completely destroyed by the feeding mites.
After culturing of sections of infected corms and mite body, the fungi, Penicillium spp., Aspergillus niger Vantieghem, Embelisia sp. and Fusarium oxysporum Schlecht were isolated and identified mutually in both samples. As the fungus F. oxysporum was the most abundant species, then it was used in the experiments. The fungus significantly affected the generation time (from egg stage to egg produced by adult) of the mite (Table. 1). Also mite fecundity was significantly higher on infested corms with the fungus than on non-infested ones (t = 10.79, d.f.= 27.31, P<0.001)(Figure 1). An obvious attraction of the females was observed toward fungal infected sections and significantly more mites were recorded on them than non-infected ones (W = 400, P<0.001)(Figure. 2).These findings are supported by some other studies (Czajkowska, 1995; Kasuga and Honda 2006; Ofek et al., 2013). Higher fecundity and faster development when mites were fed on the fungus on the infected corms are probably due to availability of a special nutrient source (mycelium). The ability of the bulb mite to digest fungi has been attributed to chitinase-producing symbiotic bacteria (Zindel et al., 2013). Based on the evidence provided by this study and previous ones (Okabe and Amano, 1990; Ofek et al., 2013), the mite R. robini was attracted more to fungal infected corms, it might because of metabolites and alcoholic secretions of the fungi. These findings demonstrate the suitability of saffron corms infected with soil fungi for development and population increase of the Robine mite.
The result on mite penetration and population dynamics on four types of treated corms indicated that the mite on infected corms penetrated within two weeks and thereafter population increased exponentially until the end of 5th week. In comparison on healthy corms and even injured ones the mite showed almost no increase during first three weeks and it was not able to penetrate and develop a stable colony on these corms. Also on infected and injured treatment similar population dynamics was observed as on infected ones (Figure. 3). These observations implying that the mite for penetrating into healthy corms encounters some difficulty and considerable time is needed to establish and colonized on such environment. Okabe and Amano (1991) has been found similar results and suggested that earlier penetrations of mites result in a faster population growth and colonization.
For many years the saffron bulb mite has been considered as a primary pest and historically control strategies has relied on the use of chemical miticides (for disinfection and etc.,) and some non-chemical methods. Subsequently the role of soil born fungi has receiving limited attention in this regards. According to the results of the present study, this acarine pest relies on the soil born fungi to penetrate and establish on the saffron corms. In other words, a close relationship exists between fungal infection and damage by R. robini on saffron bulbs. It suggest that for improving management strategies in regards of this pest, we should consider the role of saprophytic fungi as a main cause which provides condition for the bulb mite colonization and occurring damage. Further researches is proposed using appropriate methods to suppress soil born fungi and subsequent the bulb mite damage on saffron.