Drivers of mosquito presence and abundance in urban and garden ponds in a European city

A study of urban and garden ponds in Budapest reveals that fish presence is the primary factor suppressing mosquito abundance, while urbanization levels and invasive species occurrence play minimal roles in driving mosquito populations.

The Gist

Imagine the city of Budapest as a giant, bustling neighborhood. Scattered throughout this neighborhood are hundreds of small water features: some are official city ponds in parks, and others are tiny, decorative plastic pools sitting in people's backyards.

For years, people have worried that these little pools are like mosquito "all-you-can-eat buffets," breeding grounds where disease-carrying insects multiply and swarm the city. This study went out to investigate that fear, acting like a team of mosquito detectives trying to solve the mystery: Are these urban ponds actually dangerous breeding factories, or is the fear overblown?

Here is what they found, broken down into simple concepts:

1. The Two Detective Tools

To catch the mosquitoes, the researchers used two different methods, like using both a fishing net and a high-tech metal detector:

• The Dip-Net (The Fishing Net): They physically scooped the water to catch mosquito larvae (baby mosquitoes). This is the old-school, hands-on way.
• The eDNA (The Metal Detector): They took water samples and looked for tiny traces of mosquito DNA left behind in the water. This is a modern, high-tech way to see if mosquitoes were there, even if they didn't catch them physically.

The Verdict: The fishing net was much better at finding the bugs. The metal detector (eDNA) was good at finding a few extra ones the net missed, but it also missed quite a few that the net caught. It turns out, the "metal detector" needs a better database of mosquito DNA to work perfectly.

2. The Big Surprise: The City Isn't the Problem

The researchers expected to find that the more "concrete jungle" a pond was surrounded by, the more mosquitoes it would have. They thought the city itself was the villain.

They were wrong.
The level of urbanization (how many buildings and roads were nearby) didn't really matter. Whether the pond was in the middle of a busy downtown square or a quiet green suburb, the mosquito numbers were roughly the same. The city isn't the main driver here.

3. The Real Hero: The Fish

If the city isn't the problem, what is? The study found a single, powerful factor that acted like a mosquito vacuum: Fish.

• No Fish = Mosquito Party: If a pond had no fish, mosquitoes were much more likely to be there.
• Fish Present = Mosquito Silence: If a pond had fish, the mosquitoes were almost nowhere to be found.

Think of fish as the security guards of the pond. They eat the baby mosquitoes before they can grow up and become annoying adults. In fact, the presence of fish was the strongest factor the researchers found, far more important than water quality, pond size, or even the type of pond.

4. The "Bad Guys" (Disease Carriers)

The team was also looking for the "bad guys"—invasive mosquitoes or those that carry diseases like malaria.

• They found a few, including Aedes koreicus (an invasive species) and some Anopheles species (potential malaria carriers).
• However, they were very rare. They were like finding a needle in a haystack. Most of the mosquitoes they found were harmless garden varieties. The ponds in Budapest are not currently acting as major factories for dangerous disease vectors.

5. The "Fish Paradox" (A Warning)

Here is the tricky part. While fish are great at killing mosquitoes, the authors warn us not to just dump fish into every pond.

Imagine a pond as a small ecosystem. If you bring in a fish to eat the mosquitoes, the fish might also eat the good guys (like dragonfly larvae and other insects that naturally control pests) and the amphibians (like frogs). The fish might turn the pond into a "turbid, algae-filled soup" where only the fish survive.
So, while fish stop mosquitoes, they might ruin the pond's natural balance. It's a trade-off: Do you want fewer mosquitoes, or do you want a healthy, diverse pond?

The Bottom Line

• Mosquitoes in Budapest's ponds are actually quite rare. They aren't the massive breeding grounds people fear.
• The city itself isn't to blame.
• Fish are the ultimate mosquito killers. If you have fish, you likely won't have mosquitoes.
• But be careful: Putting fish in a pond to kill mosquitoes might hurt other wildlife living there.

The Takeaway: Instead of panicking about every garden pond, we should focus on managing them wisely. If you have a pond, having fish might keep mosquitoes away, but it's a decision that affects the whole little underwater world, not just the bugs.

Technical Summary

1. Problem Statement

Urbanization significantly alters biodiversity, often creating fragmented landscapes with reduced species richness. However, urban "blue infrastructure" (ponds, waterways) can support freshwater biodiversity but also serve as breeding grounds for mosquitoes, which are vectors for diseases and a nuisance to citizens. There is conflicting evidence in the literature regarding the relationship between urbanization and mosquito abundance: some studies suggest urbanization increases mosquito populations due to artificial breeding sites, while others suggest a decline due to habitat loss. Furthermore, the specific drivers of mosquito presence in small, artificial water bodies like garden ponds and urban ponds in Central Europe remain poorly understood. The study aims to:

• Determine if urban and garden ponds in Budapest host high occurrences of mosquitoes, including invasive species and potential disease vectors.
• Identify the abiotic (water quality, morphology), biotic (predators, fish), and landscape-level (urbanization) drivers of mosquito presence and abundance.
• Compare the efficacy of traditional dip-net sampling versus environmental DNA (eDNA) metabarcoding for detection.

2. Methodology

Study Area and Sampling Design:

• Location: Budapest, Hungary (Central European city).
• Sites: A total of 93 ponds were sampled between April 4 and May 17, 2022.
  • 53 Urban Ponds: Public or institutional ponds (natural, semi-natural, ornamental, stormwater) within the metropolitan area.
  • 40 Garden Ponds: Private ornamental ponds selected via an online survey ("MyPond" project), covering a gradient of urbanization from the city center to suburbs.
• Sampling Techniques:
  1. Dip-netting: Active sampling using nets (500 µm mesh) to collect macroinvertebrates and mosquito larvae. Effort was adjusted to pond size.
  2. eDNA: Water samples collected from 10 points per pond, filtered (0.45 µm), and analyzed using a Culicidae-specific COI metabarcoding assay.

Data Collection:

• Abiotic Variables: Surface area, open water percentage, pond age, physico-chemical parameters (pH, conductivity, chlorophyll-a), nutrients (TP, TN, TOC), and heavy metals (Al, As, Ba, Cr, Cu, Fe, Li, Mn, Ni, Zn). Heavy metals were reduced to a single principal component (PC1) via PCA.
• Biotic Variables: Presence/absence of fish and amphibians; abundance of predatory macroinvertebrates (e.g., Dytiscidae, Odonata).
• Landscape Variables: Urbanization level calculated as the proportion of "Urban" land cover within a 1 km radius using the Ecosystem Map of Hungary. Spatial autocorrelation was addressed using Moran's Eigenvector Maps (MEMs).

Statistical Analysis:

• Variation Partitioning: Used to determine the unique and shared contributions of local abiotic, local biotic, and landscape predictors to mosquito community composition.
• Regression Modeling:
  • Linear Models (LM) for mosquito abundance.
  • Generalized Linear Models (GLM) with binomial distribution for mosquito presence (separately for dip-net and eDNA data).
• Model Selection: Stepwise selection (AIC) was used to identify significant predictors.

3. Key Contributions

• Comparative Detection: The study provides a rare direct comparison of dip-netting and eDNA for mosquito monitoring in urban ponds, highlighting their complementary nature.
• Driver Identification: It isolates fish presence as the dominant negative driver of mosquito abundance in urban settings, outweighing the effects of urbanization or water quality.
• Vector Surveillance: It documents the presence of specific vectors of public health concern (Anopheles maculipennis complex, An. claviger, An. plumbeus, and the invasive Aedes koreicus) in urban garden and public ponds.
• Management Implications: It challenges the assumption that urbanization universally drives mosquito abundance and suggests that local biotic interactions (predation) are more critical than landscape-scale urbanization.

4. Results

Mosquito Occurrence and Diversity:

• Mosquitoes were detected in only 21 of the 93 ponds (approx. 22.6% by dip-net, slightly lower by eDNA).
• 14 species from 5 genera were identified.
• Invasive Species: Only one invasive species, Aedes koreicus, was found (in 2 urban ponds).
• Vectors: The Anopheles maculipennis complex was the most widespread potential malaria vector, found in 6 ponds (5 urban, 1 garden). An. claviger and An. plumbeus were also detected.
• Habitat Preference: Culex species were dominant in garden ponds, while Aedes and Ochlerotatus were more common in urban ponds.

Detection Method Comparison:

• Dip-netting detected more taxa overall (13 exclusive taxa) and was more effective in urban ponds.
• eDNA detected 2 exclusive taxa (including Aedes vexans) and was comparable to dip-netting in garden ponds.
• The methods were complementary; they only agreed on mosquito presence in 3 ponds.

Drivers of Presence and Abundance:

• Fish Presence: Had a significant negative effect on both mosquito abundance and presence (p < 0.01). This was the strongest predictor across all models.
• Urbanization: Did not play a major role in explaining mosquito presence or abundance.
• Spatial Patterns: Spatial variables (MEMs) were marginally significant, suggesting a trend of higher mosquito presence in suburban/peripheral zones compared to the city center.
• Abiotic Factors: Heavy metals (metalsPC1), nutrients, and pond area did not show significant effects on overall abundance or presence, though pond area influenced community composition.
• Predatory Macroinvertebrates: While variance partitioning showed a relationship, regression analysis did not find a significant negative effect on mosquito abundance, likely due to alternative prey or low predator diversity in artificial ponds.

Variance Partitioning:

• Local abiotic factors explained 7.8% of the variance.
• Local biotic factors explained 4.9%.
• Landscape factors explained 3.6%.
• Shared fractions were also significant, indicating complex interactions between drivers.

5. Significance and Conclusion

The study concludes that urban ponds in Budapest are not major mosquito breeding hotspots, contrary to public perception. The low occurrence of mosquitoes is likely regulated by local biotic factors, specifically the presence of fish, which exert strong top-down control.

Key Implications:

1. Public Health: While the overall risk is low, the presence of Anopheles and Aedes vectors warrants continued monitoring, particularly in suburban areas where mosquito presence was slightly higher.
2. Management: Stocking ponds with fish is an effective method for mosquito control but must be weighed against ecological costs, as fish can decimate amphibian populations and alter pond ecosystems.
3. Monitoring: eDNA is a viable, non-invasive tool for mosquito monitoring, particularly in garden ponds, though dip-netting remains superior for comprehensive species inventories in larger urban water bodies.
4. Urban Planning: Urbanization itself is not a primary driver of mosquito abundance; instead, local habitat management (e.g., vegetation, fish stocking) is the critical factor.

The authors note limitations, including the early sampling season (spring) which may have underestimated peak abundance, and the need for future studies incorporating seasonal dynamics and broader geographic comparisons.