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Scorpions are easily recognized arachnids closely related to spiders, mites and ticks. Scorpions have become one of the most important arthropod pests in relation to public health and economic concerns, as they are responsible for a large majority of annual deaths caused by arthropods, more so than any other venomous group of animals with the exception of snakes and bees. Scorpions also affect the quality of human life in a more subtle way by causing fear and trepidation when they invade homes and businesses. Subsequently, they are a primary target of pest management professionals in regions where they are common. Most scorpions are found in temperate, arid regions, but some species can be found in tropical rainforests or both. In the United States, members of the genus Centruroides are the most common scorpions. Among the Centruroides group, the Arizona bark scorpion (Centruroides sculpturatus) stands out as the most medically important scorpion in the United States. Of the 185,402 scorpion envenomation incidents reported in the United States between 2005 to 2015, 68.2% occurred in Arizona, 10.3% in Texas and 4.2% in Nevada. Despite the medical importance of scorpions in these states, there is a lack of information on how Arizona bark scorpions interact with pesticide residues and, subsequently, how to best manage them.

Scorpion Habitats

Arizona bark scorpions are common in Arizona, but they also inhabit western New Mexico, southern Utah, southern Nevada and the states of Sonora and Chihuahua in Mexico. Because of their size and ability to move, they can easily hide in their natural environment and conceal themselves from threats.

Arizona bark scorpions are not strong diggers and in the wild they lie under rocks, within crevices and on trees. They are also commonly found indoors, as one of the most common structure-invading scorpions, where they are discovered in sinks, tubs, shoes, drawers or dark cabinets. Scorpions are nocturnal animals that feed on insects by ambushing them. It is during this time, when searching for food or returning to their harborages, that they are most likely to encounter insecticide residues. Among many other objectives, we became interested in how scorpions interact with insecticide residues and the importance that these interactions might have on their control. Two assays were designed to simulate the possible interactions that occur in real-world scenarios between scorpions and insecticide deposits. These interactions were recorded and analyzed with video tracking software and direct observations. In the first study, scorpions were exposed to arenas where one half of the arena floor was treated with an insecticide, while in the second assay, scorpions were exposed to a single insecticide-treated shelter. Our results enhance our understanding of the behavior of scorpions exposed to various insecticides and give us a better understanding of methods for their management.

Study Methods

Experiment #1:

Responses to treated areas

The goal of this study was to evaluate the responses of scorpions upon encountering insecticide-treated areas. The scorpions were released onto 12" x 12" square concrete pavers or ceramic tiles in which half of each square was treated with one of the three treatments (Fig. 1). To keep the scorpions secure in the arena, clear PVC pipe (10" diameter x 2" height) was used to prevent them from climbing out of the arena (Fig. 1). We tested three insecticides/combinations: Demon Max insecticide (25.3 % cypermethrin; used concentration of AI: 0.02%), Demand CS insecticide (9.7% lambda-cyhalothrin; used concentration of AI 0.03%) and Demand CS (0.03%) tank mixed with Demon Max (0.02%). The activity of scorpions was recorded under dark conditions for 20 minutes with an infrared camera and two infrared illuminators positioned above the arena. EthoVision XT version 11.5 software was used to capture and analyze video images and to track the scorpions' movements, allowing us to visualize the response they had to treatments. The arena-treated zones were established within the software (Fig. 1) and the time the scorpion spent on the treated zone was used to characterize responses. Mortality post-recording was evaluated every day for 10 days to correlate time of insecticide exposure and mortality.

Figure 1. View of example experimental arena used for the evaluation of scorpion responses to insecticides. Left: scorpion about to encounter an insecticide-treated area. Middle: tracks of scorpion activity recorded during the 20-minute bioassay. Right: an arena containing an insecticide-treated harborage.

Figure 2. Localization of scorpions on arenas (concrete or tile) whose halves were treated with insecticides. Mortality post-assay was recorded by day 10.

Figure 3. Typical posture of scorpions upon contact with insecticide-treated surfaces. A shows a scorpion that is not exposed to insecticides, where it typically lies fully flat on the surface, dragging its body or maybe raised a few millimeters off the surface. Their legs are also extremely angled at the joints and are often angled backwards. B shows a scorpion that begins to stilt. Scorpions during this phase of the exposure will often hold poses where the body is raised off of and parallel to the surface. When full agitation occurs, the legs are almost fully extended and straight like stilts, and they appear to be almost walking on their "tiptoes" (C). Adoption of this posture potentially reduces their exposure to insecticide deposits.

Figure 4. Scorpions interacting with insecticide-treated harborages and mortality post-assay. Colored bars indicate percentage found in the shelter and the line indicates mortality after 10 days.

Experiment #2:

Responses to insecticide-treated harborages

The goal of this study was to determine whether a scorpion would occupy an insecticide-treated harborage and to simulate crack and crevice treatments commonly recommended in integrated pest management (IPM) programs. The arena was similar to that described in Experiment 1. However, the only treated surface in the arena was a harborage consisting of a 3" x 5" cellulose tent, which was placed in the center of the area (Fig. 1). Above the arenas were four fluorescent tube lights that provided lighting on the scorpions for 12 hours to simulate daytime and to stimulate the scorpions into a sheltering behavior. After 12 hours, the position of the scorpions was recorded as either under the tent, touching the tent or outside the tent. Scorpions were then kept for 10 days for post-mortality readings.

Results

Experiment #1:

Responses to treated areas

On concrete, scorpions spent similar amounts of time in areas treated with Demon Max, Demand CS, and Demand CS + Demon Max +, when compared with untreated arenas (control) (Fig. 2). Post-assay mortality of scorpions on concrete showed 0% mortality with Demon Max, 70% for Demand CS, and 40% Demand CS + Demon Max (Fig. 2). In assays on tile, the scorpions tended to avoid the insecticide-treated halves (Fig. 2) and often adopted "stilting" posture when contacting insecticide deposits (Fig. 3). While Demon Max did not kill any scorpions post-assay, a high mortality (90%) was observed in Demand CS-treated arenas. However, this mortality was reduced to 70% in arenas that were treated with the mixture Demand CS + Demon Max (Fig. 2).

Experiment #2:

Responses to insecticide-treated harborages

After 12 hours in the arena under bright light, all the scorpions in the untreated control group sheltered in the tents, while most of the scorpions avoided sheltering in harborages treated with insecticides (Fig. 4). A low proportion of scorpions was observed hiding in shelters treated with Demon Max (20%) or Demand CS + Demon Max (30%), and none of the scorpions sought harborage in shelters treated with Demand CS. Despite the low sheltering rate of scorpions in arenas with Demand CS or Demand CS + Demon Max, scorpions from these treatments had significant mortality (=80%) 10 days after exposure. This indicated that the scorpions had spent at least enough time in the harborage to receive a lethal exposure during the 12 hours before abandoning the shelter. Minimal mortality (10%) was recorded in assays with Demon Max (Fig. 4).

Implications

Scorpions spend most of their time in harborages and only leave them temporarily to hunt prey during the night. It is at this time that scorpions might encounter insecticide deposits. While it is generally assumed that insecticides do work well on scorpions, little is known about how these insecticide residues influence their behavior, which could ultimately determine the efficacy of the treatments. Scorpions, like many arthropods, avoid prolonged exposures to insecticides by moving away from the treated areas. If scorpions avoid insecticides and move to insecticide-free areas, it could reduce the efficacy of treatments.

Since scorpions spent more time on the insecticide-treated side of concrete than on tile, it would follow that the scorpions would have more opportunity to pick up a larger amount of insecticide from this substrate, which could lead to a faster death. However, this was not the case in our study. While insecticides such as Demon Max killed scorpions at a similar proportion on both concrete and tile, Demand CS and the mixture Demand CS + Demon Max enhanced their killing effect when presented on tile. These differences in scorpion responses and mortalities between concrete and tile could be explained by the absorption rate of insecticides on each substrate. In porous materials (concrete, sand, stones), some of the insecticide is absorbed into the substrate, making the deposit less detectable by scorpions (eventually reducing the lethal effect) than non-porous substrates, such as tile, which would have a larger amount of insecticide available to be picked up by the scorpion.

Substrates (tile and concrete) treated with the combination of Demand CS and Demon Max resulted in a lower mortality level when compared to Demand CS alone. These results suggest that Demon Max might interfere with the insecticidal action of Demand CS or cause a stronger avoidance behavior response not documented in this study. More research is needed to confirm the nature of the interactions between insecticide formulations and thus determine the efficacy of mixtures of various insecticide formulations. More is not necessarily better, as was demonstrated here.

Additional research is also needed to confirm the amount of AI picked up per scorpion. Demand CS may be the primary insecticide responsible for mortality in the "mixture" treatment, especially in the concrete-treated assays. We hypothesize that Demand CS may have been more available for pickup on the porous surface, due to the microencapsulated formula, while Demon Max moved into the porous surface. It is also worth noting that rates used in this study were low-label rates and short exposure times relative to real-world scenarios. We would expect higher mortality at the high-label rate usually used by PMPs in the field.

Among the Centruroides group, the Arizona bark scorpion (Centruroides sculpturatus) is the most medically important scorpion in the United States.

Another aspect that we evaluated was whether scorpions would choose to hide in insecticide-treated harborages or remain exposed to bright light. This is a typical scenario in scorpion control, where the insecticide application is targeted to potential scorpion harborages. In all the insecticide treatments, most of the scorpions were found outside of the treated harborage after 12 hours, despite a strong tendency to avoid exposure to bright light. In contrast, 100% of scorpions remained in the harborage in the untreated controls. Scorpions with insecticide-treated shelters were found wandering in the experimental arena, some of which displayed obvious signs of intoxication. In fact, scorpions from assays with Demand CS or Demand CS + Demon Max displayed neurotoxic symptoms compatible with pyrethroid toxicity. This was a clear indication that the insecticide deposited in the harborages exerted a noxious and lethal effect on the scorpions in a relatively short amount of time (12 hours). We conclude that crack and crevice treatments in areas known to harbor scorpion populations should be highly effective at reducing populations. Weep-screed crevices at foundation bases and gaps in concrete block walls are just two examples of well-known harborage areas that should receive these targeted treatments in effective management programs.

Scorpions exhibit cryptic, nocturnal behavior, and an ability to reduce contact with insecticide deposits through behaviors including stilting and avoidance movements. However, effective pesticide formulations, a knowledge of harboring behavior and targeted applications to those harborages can result in high scorpion mortality based on our experiments.

Alvaro Romero is associate professor at the New Mexico State University, Las Cruces, N.M. John Agnew and Brittny Blakely are students at the same institution. Eric Paysen is technical service manager at Syngenta.

This study was funded by Syngenta.