Biological Compatibility Of Fungal Biocontrol Agents With Common Chemical Acaricides And Fungal Chemical Interaction Against Spider Mite Eggs

Entomopathogenic hyphomycetes are well known fungal biocontrol agents that play more and more important roles in microbial control of insect pests infesting crops or forests. To search for alternative measures for spider mite control, this study sought to test short-and long-term compatibility of common chemical acaricides with Beauveria bassiana and Paecilomyces fumosoroseus, and to evaluate quantitatively the interaction of a B. bassiana emulsifiable formulation with low application rates of chlorpyrifos, the most compatible acaricide, against the eggs of the carmine spider mite, Tetranychus cinnabarinus. The results are summarized below.Short-term compatibility of common chemical acaricides with two fungal agents. Ten commercial acaricides widely applied to mite control in China were assayed for their compatibility with the strains B. bassiana 2860 and P. fumosoroseus based on 24-h germination rates of their conidia on the plates of Sabouraud dextrose agar or in liquid medium (2% sucrose plus 0.5% peptone) containing a concentration labeled for field spray and its 5- and 10-fold dilution of each chemical. As a consequence, the 24-h viability varied significantly among the tested acaricides at the same level or among the concentrations of each in the plates or in the liquid medium. The liquid germination method resulted in less variation in conidial viability among the three replicates of each concentration and apparently was superior to the plate method. The responses of the two fungal agents to the acaricides also differed. The acaricides azocyclotin, liuyangmycin, dicofol and avermectin were found greatly inhibiting conidial germination of both fungi even at low concentration. However, several acaricides, including, hexythiazox and amitraz, were compatible quite well with the fungal agents and can be incorporated into fungal formulations for prompt spray in the field.Long-term compatibility of selected acaricides with B. bassiana. Threeacaricides, pyridaben, propargite and chlorpyrifos, with desired short-termcompatibility were separately incorporated into an oil-based, emulsifiable formulationof B. bassiana 2860 at the three concentrations as above and then stored at 4°C orambient temperature in dark for up to 12 months. Conidial viability determined bysamples at intervals during the storage differed largely over storage time among thethree acaricides. At the ambient temperature, no viable conidia were observed in thepropargite-inclusive fungal formulation, even at the low concentration, after one-daystorage. The formulations containing high, medium and low concentrations ofpyridaben were able to be stored at the ambient temperature for no more than 45, 90and 105 days, respectively. When the high or medium concentration of chlorpyrifoswas included in the fungal formulation, 100% viability loss was found on day 3 or day7. At the low concentration of this chemical, however, the fungal formulation did notsuffer from visible viability loss during a 6.5-month storage at the ambient temperature.While stored at 4°C, the propargite-inclusive fungal formulation lost its viability at thehigh concentration only after one day and maintained an acceptable viability for only1-3 weeks at the medium or low concentration. A viability of 94% was observed in thefungal formulation containing the low concentration of pyridaben after 12-monthstorage at 4°C. Safe periods of storage at 4°C for the chlorpyrifos-inclusive fungalformulation were 7 months at the high chemical concentration and over 12 months atthe low and medium concentrations because a viability of -90% was observed at theend of each period. The results indicate that the acaricide chlorpyrifos has the bestlong-term compatibility with B. bassiana conidia but still does not meet requirementsfor integration of it into the fungal formulation for normal storage.Lethal effect of fungal / chemical interaction on spider mite eggs. Fourindependent bioassays (Assays 1-4) were performed with a standard spray towermethod to quantify interaction of three conidial concentrations of B. bassiana 2860(low, medium and high: 58-67, 154-224 and 815-1036 conidia/mm2) with threeapplication rates of chlorpyrifos, equivalent to 5, 10 and 20% of its labeled rate,against the eggs of T. cinnabarinus on fava bean leaves in Petri dishes. Assay 1 includedthe fungal sprays only whereas the concentrations of 24, 48 and 96 ^ig/mL chlorpyrifoswere included in the fungal sprays of Assays 2-4, respectively. Each spray for atreatment consisted of four replicates and each replicated included 23-48 fresh miteeggs laid on a fava bean leaf. After exposure to the sprays, all Pertri dishes weremaintained at the regime of 25°C and 12L:12D and hatch rates of the mite eggssprayed were monitored daily until no more eggs hatched for three consecutive days.As a consequence, most of the mite eggs hatched on days 4-7 after spray and the hatchrates decrease with the increasing concentrations of fungal or chemical sprays or both.Mite egg mortalities attributed to the fungal infection (i.e., percentages of mycotizedeggs) in Assays 1-4 were estimated as 82.7, 90.6, 95.6 and 97.9% at the high conidialconcentration, 29.1, 50.6, 79.1 and 87.2% at the medium concentration, and 9.5, 19.6,58.6 and 60.5% at the low concentration, respectively. Based on the fitted linearrelationship between the egg mortalities and the egg mortalities, the LC50 with 95%confidence intervals of the B. bassiana formulation against the mite eggs wereestimated as 340 (276-433), 219 (186-259), 63 (35-91) and 77 (38-114) conidia/mm2 atthe dosages of 0, 24, 48 and 96 |ig/mL chlorpyrifos for interaction in Assays 1-4,respectively. In conclusion, inclusion of >48 ng/mL chlorpyrifos into the fungal spraysgreatly enhanced ovicidal activity of the fungal formulation against the spider mitepest with an LC50 less than 100 conidia/mm2. The acaricide applications rate includedis equivalent to only about 10% of its labeled rate for field application. The resultssuggest a high potential for the fungal / chemical interaction to be exploited for spider mite control.The results presented as above determined short- and long-term compatibility of the selected chemical acaricides with concerned fungal bicontrol agents. The discovery of the most compatible chlorpyrifos and its synergistic interaction with B. bassiana to kill spider mite eggs would help to facilitate incorporation of fungal formulations into mite pest management systems where application of chemical acaricides can be greatly reduced or minimized.