Slurry toxicity experiment

Good findings.

I fight with Mozi's all the time...

I didn't know the squigler's ate brine shrimp.

Details for anyone interested. The study design is very flawed, but it did answer the basic question of "is rock tumbling slurry toxic to mosquito larvae?"

Methods
Mosquito larvae (most likely Culex sp., possibly C. quinquefasciatus but potentially also Aedes sp. because they were identified by eye, not microscope) were collected from a puddle of rainwater and added to one of three treatments:
- control (no substrate): no substrate added
- control (sand): one tablespoon of sand used as a substrate
- rock slurry: one tablespoon of dewatered slurry from coarse grinding agates/rhyolite in a rotary tumbler used as a substrate.

Two replicates were used for each treatment. A replicate consisted of a clear plastic cup with substrate added, followed by aged rainwater and organic matter (from where the larvae were found) and topped up with dechlorinated tap water. Ten mosquito larvae were added to each replicate. The larvae were of different sizes (indicating different ages/instars), so these were randomly collected and assigned to replicates to avoid the influence of larval stage on any results. The cups were placed in a sheltered, partially shaded, low light position outside and exposed to ambient air temperatures throughout the experimental period (autumn). The larvae were checked on occasion. On the second day of the experiment, all larvae were alive (as evidenced by their presence at the water surface, which is a typical behaviour of mosquito larvae), and were fed dried vegetable powder (brine shrimp food).

The experiment was ended a month later when it was observed that all larvae but one were dead in the slurry treatments.

Water chemistry was tested at the end of the experiment using aquarium test sticks (yes, not fancy, but that's all I've got  )


Results


Compared with the two controls, the rock tumbling slurry had adverse effects on mosquito larvae. Almost all larvae were dead. In contrast, there was evidence of successful development and adult emergence in both controls. The control with no substrate performed the best (this was the closest to natural conditions), while overall survival in the sand control was marginally lower.



Average overall survival (standard deviation)

Control (no substrate): 9 (1.41)

Control (sand): 7.5 (2.12)

Slurry: 0.5 (0.71)



Emergence of adult mosquitoes, as evidenced by pupal exuviae, was higher in the control (no substrate treatment) than the control (sand treatment), and no larvae survived to adulthood in the slurry treatment.


Average adult emergence (standard deviation)

Control (no substrate): 5 (0)

Control (sand): 3 (1.41)

Slurry: 0



The one surviving larva from the slurry treatment was lethargic and only moved when prodded. In contrast, larvae from both control treatments were active. They tended to sit at the water surface and dived to the bottom of the cups when approached.


Water quality

The general hardness of the control (sand) and slurry treatments were hard (180 ppm), while the general hardness of the control (no substrate) treatment was slight to moderately hard (60 ppm).



The KH of the slurry was low (0 ppm), while it was 40 ppm in both control groups (which is still low).

The pH was 6.5 in the slurry, 6.5–7.0 in the control (no substrate), and 7.0–7.5 in the control (sand) treatments, respectively.

NO2- was not detected in any treatment, and NO3- levels were also low (0–20 ppm in the slurry and control (sand) treatment, and even then I’m not sure I trust my eyes. The test strips aren’t great).

The water in the slurry treatments was noticeably turbid despite not being disturbed for days. This was not the case in either control group.

Some algal growth on the bottom of the cups was observed in the control (no substrate) group, and it was more extensive in the control (sand) group, while no algal growth was observed in the slurry treatment.

Conclusions and thoughts


The rock tumbling slurry adversely affected mosquito larvae, causing high levels of mortality. As the larvae don’t directly interact with the substrate, any toxicity caused by the slurry would presumably be from contaminants leaching into the water column. In addition, the lower pH of the water caused by the slurry would increase the bioavailability of any metals present in the slurry or rainwater (although the pH difference wasn’t that great). This may have contributed to increased stress for the larvae.


I suspect it wasn’t toxicity that killed the larvae, but an indirect effect of the slurry (though a direct toxic effect of the slurry may have been involved as the presence of toxic compounds in the slurry cannot be ruled out; I don’t definitively know what exactly was in the rocks I was tumbling). The high turbidity of the slurry potentially restricted the growth of an important food source (algae) for the larvae, resulting in poor growth and ultimately death. In one slurry replicate, I found ten dead larvae that were all relatively large (late instars). This is interesting because the larvae initially added to the cup were of different sizes (ages), but rather than the oldest larvae pupating and emerging as adults, they seemed to remain stuck at that developmental stage, while the smaller larvae grew larger before dying. Pupation is a sensitive time for insects. Insufficient nutrition preventing pupation and/or increased toxicant/stressor sensitivity in the fourth instar larvae may have prevented pupation and caused instar-specific mortality.


There may have been differences in the dissolved oxygen that affected the larvae, but given how cold it was I suspect the dissolved oxygen concentration should have been sufficient, plus the larvae breathe at the surface.



Environmental implications

None. Unless someone likes dumping slurry in their fish pond. Mosquito larvae and other aquatic organisms are unlikely to be exposed to rock tumbling slurry like this, even if it runs off from someone dumping it in their yard.


Limitations
Controls were possibly inappropriate and experimental conditions not optimised for these larvae. I'm pretty sure I underfed them.
Exposure scenario not environmentally relevant (it would be more relevant to test this with soil organisms. Maybe in the future).
Larvae should have been the same age.
A full life cycle test would have been better and more informative.
I ended the experiment too early. For it to actually be valid, 80% of the larvae in the controls needed to have successfully pupated and emerged as adult mosquitoes.
Insufficient replicates, so forget running stats on it.

It's so true. They kill a lot of people with their injections.

I've read in the past that some people throw their old tumbling slurry into their gardens with the idea that it provides stuff plants like. I've been a little suspicious about that and wonder what you think about it if they are tumbling agates and jaspers with silicon carbide as grit. Maybe vegasjames could add perspective also.

That's what I think too. Better safe than sorry.

Interesting experiment, I think you are correct about the turbidity, and possibly the silica content of the water with slurry. If any of the tumbled stones had been saw cut in oil I would also suspect that, a relatively thin "sheen" of oil on the surface will clog up the mosquito larvae's breathing tube, an old and mostly discontinued practice to control their population.
When it comes to historically deadly critters, seems the smaller they are the more dangerous, the single celled ones once were the worst, maybe still are.

I forgot to mention that in this study (buried in the study) they found significant vegetable production increases in addition to the insect damage reduction from the application of "granite rock dust" into the garden soil directly. 

Of course, I am sure some rock grinding slurry should not be put in the garden and could obtain much different results from what was measured in this study. 

Here are a few additional experimental groups should be added in the future if there is a second trial: Fresh/unused grit as substrate [is the grit itself an issue], some form of worn down grits as a substrate [testing for particle size], slurry made from running rocks without grit [opposite of the previous two grit questions, and is the rock the toxic part], various slurries from different types of rocks [which rocks are toxic, if any], water from the tumbler after the slurry has settled [if there is a toxic component, is it soluble in water], and then an entire extra set of these experiments where the water has been sterilized first .

Regardless of my pedantry of experimental design, I still found this post entertaining and raised more questions in my own head. Biologist to biologist, this was a fun read!