Species of Euseius are the most common phytoseiids on both citrus and avocado. Euseius hibisci is known from Santa Barbara County in California to the sate of Oaxaca in southern Mexico .Euseius hibisci is common and abundant in avocado orchards year round, is an important generalist predator and feeds on pollen and leaf exudates in the absence of prey . The most studied member of this genus is probably Euseius tularensis Congdon and not nearly as much is known about E. hibisci with regards to pesticide exposure. In fact, E. tularensis was ‘discovered’ and described as a new species different from E. hibisci based on finding several populations of the former that showed a high tolerance to pesticides . Several studies have indicated the relevance of E. hibisci as effective biocontrol agents of spider mites and thrips on some crops and although E. hibisci is not a specialized predator, it potentially aids in enhancing the control of many different pest mites and thrips . Assessment of acaricides and insecticides on non-target organisms is an essential component of any IPM program and is of particular interest to California avocado growers. This study evaluates the non-target impacts of the four pesticides currently recommended for avocado thrips management on Euseius hibisci. Avocado trees for all of these studies were located at the Agricultural Operations Facility at the University of California, Riverside.

All avocado leaves used in these studies were fully expanded,cut flower bucket mature leaves but not ‘hardened’ off based on leaf flexibility and color . Leaves selected for bioassays were hand flagged with color-coded flagging tape the day prior to pesticide application. The four pesticides used in these studies were abamectin , fenpropathrin , spinetoram and sabadilla . All pesticides were applied at their maximum per ha label use rate using a dilution rate of 2,805 L/ha . A water only control was also applied. Pesticides were mixed the morning of the application and administered using a hand sprayer . Each flagged leaf was located, the flag was marked with a number and the leaf sprayed one time each on the axial and abaxial sides with a light spray that ensured the whole leaf area was covered. The leaves remained on the avocado trees to weather naturally in the field until they were picked on the day laboratory bioassays were conducted. An initial pesticide application and bioassay were conducted beginning November 9, 2009 to determine a number of factors for the subsequent spring pesticide application and bioassay. Late April early May is typically when growers would be treating for avocado thrips in California avocados . Therefore pesticides for the field trial were applied May 4, 2010 and based on the preliminary study, bioassays were conducted, 1, 3, 7, 14, 21, 28, 49, 70 90, 111 and 132 d post pesticide application. From each treatment group five replicate leaves were randomly selected on the morning of each bioassay. If the flagging tape tied to a leaf did not bear a hand written number , it was not included in the study.

The leaf petioles were placed into 2.4 ml glass jars filled with deionized water such that the leaf surfaces were not in contact with the water at any time, and transported to the laboratory. A 2.5 cm hole was punched out of the center of each leaf and placed abaxial side up on a wet, white felt covered sponge in a plastic rectangular Tupperware container with no lid. Each of the five replicate leaf discs was randomly arranged in the tray with a disc near each corner and one disc in the center. The discs were surrounded with strips of wet white felt to provide a wet border around each disc to keep the mites on the disc. Four to five strands of CelluCotton were placed in the middle of each leaf disc and topped with 1/16 of a clear plastic cover slip . A very small amount of ice plant pollen was carefully placed on the piece of cover slip with a fine tipped paintbrush ensuring no pollen came into contact with the previously treated leaf surface. Once each of the Tupperware trays was set up with the five replicated leaves, a minimum of 20 mature female E. hibisci was added to each disc. This was conducted systematically by dipping the tip of the paintbrush into DI water, lightly tapping the brush to remove excess water; then while viewing under a microscope, mature females were selected by lightly and carefully touching the tip of the brush to their dorsal surface, removing the mite from the colony tray and placing it onto the leaf disc such that the mites would grab the disc and pull themselves away from the paintbrush tip. The trays were then placed in an environmental chamber at 24°C, 50% RH, and long day light conditions . The trays were checked daily for 5 days noting the number of live and dead mites and any mites drowned in the wet felt. Mite mortality was recorded and assessed by lack of movement upon light probing with a fine tipped paintbrush. Drowned mites were recorded as mites stuck in the wet felt, either moving or not moving, and were not ‘rescued’ from the felt if found alive struggling in the felt .

Data were analyzed by day in two ways: % of mites found trapped in the felt and % dead mites, excluding those trapped in the felt . Because high numbers of mites were found drowned in the felt after exposure to spinetoram in the preliminary trial, an additional study was conducted beginning at the same time as the May 4, 2010 field trial by confining mites to the leaf disc and not allowing them to suicide in the moist felt. This was done using Munger cell bioassays . Briefly, Munger cells were constructed using a 3-layer Plexiglas “sandwich”; the middle cell layer was drilled with 3.2-cm diameter bit to provide a circular test arena . The upper and lower parts of the Plexiglas sandwich were solid and between the lower base and test arena a piece of filter paper was placed under the leaf to allow moisture exchange and to extend the life of the leaf during the bioassay. Airflow through the test arena was provided through two holes drilled through the center cell layer directly opposite one another,flower display buckets with fine-mesh screening melted onto the interior of the test arena to prevent mite escape. The Plexiglas sandwich was held together with four binder clips positioned such that the passive airflow was not obstructed. The lid of the Munger cell had a 0.5 cm hole that could be plugged and unplugged with a small cork. The mites were transferred into the Munger cell via this hole and the cork remained in place at all times except when probing a mite to evaluate mortality. The control leaves were sprayed with water only. The Munger cells were placed in the same environmental chamber as the open-faced Tupperware containers described above at 24°C, 50% RH and long day light conditions . The bioassay was conducted 1, 3, 10, 14, 21 and 28 d post pesticide application and mortality readings were taken daily for 5 d after each bioassay was set up. Two of the pesticides registered for avocado thrips management, abamectin and spinetoram, exhibit translaminar activity. To determine if photodecomposition might affect the impact of these chemicals on Euseius hibisci, twice as many control, abamectin and spinetoram leaves were included in the May 4, 2010 field trial and were bioassayed on each date. Half of these field-weathered leaves were randomly selected for exposure to intense ultraviolet light for 120 min after they were picked and before the bioassays were conducted. These leaves were placed perpendicularly into 1-dram vials filled with DI water to prevent desiccation and placed in a hood with a 15-watt UV bulb with a 250-320 nm range . After 120 min UV exposure, the leaves were removed and leaf discs were set up in trays as described above with a minimum of 20 mature female E. hibisci mites. Data for each treatment were analyzed by day by calculating: % of mites found trapped in the felt and % dead mites, excluding those trapped in the felt .To determine how/if E. hibisci females would respond if provided both spinetoram treated and free spaces on a leaf, an additional study was run in which half of each bioassay leaf was treated with spinetoram. The leaves were selected, flagged and half of each leaf was randomly selected for the field trial on August 23, 2010; the left or right side of the leaf beyond the midrib was randomly selected for treatment on both the axial and abaxial surface, alternating which side was treated.

Depending on which side of the leaf was treated, a paper towel cut into approximately 12 x 16 cm rectangles was covered with clear plastic wrap and was paper clipped to the opposite leaf side protecting it from the pesticide spray. Prior to treatment, the leaves were held such that the leaf mid-rib was parallel to the ground with the covered portion above the bare side such that when the pesticide was sprayed on the leaf, run-off fell to the ground and did not contact or accumulate on the covered side of the leaf. The paper towel and plastic wrap covering remained on the leaves for one hour after treatment and were then carefully removed so as to not tear the leaf or drag any remaining wetness across the untreated side of the leaf. The control leaves were covered in exactly the same fashion, but were treated with water. The half-leaf trial bioassay was conducted on 1, 3, 7, 10 and 14 d post pesticide application. As described previously, the leaves used in the bioassays were selected, transported to the laboratory, and the whole leaf was placed abaxial side up on wet white felt covered sponge. The whole leaf was rimmed with strips of wet felt and at least 20 mature female E. hibisci were hand transferred to each leaf. To account for possible positioning bias, the leaves were treated on the left or right side and the mites were deposited either on the treated or untreated sides so that all combinations were accounted for, with three replicate leaves for each combination. We wanted to determine if the mites could detect the spinetoram on the leaf surface; therefore on each of the bioassay days, the bioassay was checked every 20 min for the first hour then 3, 5, and 10 h post setup, and once every 24 hours for five days. Mites were scored as being dead, alive or stuck in the felt and which side of the leaf they were found was recorded on at each of the observation time intervals. Data were analyzed by ANOVA with repeated measures using SAS 9.2 with the following factors: treatment , bioassay day , treatment side , initial mite placement location and observation time . Bioassays were conducted, 1, 3, 7, 14, 21, 28, 49, 70 90, 111 and 132 d post pesticide application. Mite mortality and mite repellency were recorded separately because it was unknown if the mites that were found dead in the wet felt were dead due to pesticide exposure or from drowning alone. Data were recorded for all bioassay days for 5 days post bioassay setup and data from the day 4 count were selected because there was little further activity post 5 days and day 4 counts best represented the data overall. High levels of mite repellency were seen only with the spinetoram treatment and >20% of the mites in the spinetoram treatment were found drowned in the wet felt surrounding the leaf discs through 14 days post-treatment. Mite mortality was the highest in the fenpropathrin treatment and this pesticide showed the most persistent impact . Mite mortality with the spinetoram treatment appeared to increase and then decrease but this was an artifact of mortality being calculated by excluding mites that drowned. Only moderate mite mortality was observed with the abamectin and sabadilla treatments and this dropped to below 10% on 14 and 21 days post-treatment, respectively .Data from the day 4 count are described .