These pesky creatures have scurried into shopping malls, crawled into food and flown into homes.
They have grabbed headlines by poisoning food, and causing illness and even death.
The rat, cockroach and mosquito are three of the most dreaded pests in Singapore. Despite strenuous efforts by the authorities and pest control companies, they continue to flourish.
Last year, the authorities went on the warpath, destroying 39,000 rat burrows in public areas, eradicating 16,000 mosquito-breeding sites and taking enforcement action 420 times against operators for cockroach infestations following 148,500 inspections by the National Environment Agency (NEA).
The rats returned, three active Zika clusters have been discovered in the Hougang-Serangoon vicinity and, last week, the Jack's Place outlet in Paya Lebar was suspended for harbouring cockroaches.
Why are these pests so hard to stamp out?
CHANGE IN ENVIRONMENT
The urbanised environment in Singapore provides easy access to the three basic necessities of life, namely food, water and shelter, said pest control experts.
39,000 Number of rat burrows found and destroyed last year.
16,000 Number of mosquito-breeding sites found and eradicated last year.
148,500 Number of inspections done on food establishments by the National Environment Agency last year.
In the wild, a pair of rats may give birth to eight pups but half would be killed by a snake, bird or cat.
But if they are hiding within the hollow structures of a mall, a pair could turn into 40 in just four months, said Mr Bernard Chan, general manager of Star Pest Control.
The gestation period of a rat is about three weeks and each litter can have eight to 12 pups. They reach sexual maturity within two months.
Singapore's population has also grown by over a million in the last 10 years, which has resulted in more buildings and more eateries. This, in turn, has provided the pests with more hideouts and food.
The development of once vacant or forested lands has also encroached upon their natural habitat, so it is no wonder that they are getting closer and closer to humans, said the experts.
REACTIVE PEST CONTROL
Mr Thomas Fernandez, chief executive of Pestbusters, said that while the NEA requires premises such as malls, food shops and supermarkets to be free of pests like cockroaches and rats, thorough routine pest control checks and treatment are not always done by owners.
"The price is always the biggest concern," said Mr Fernandez, adding that 40 per cent of calls he receives are from owners of premises sussing out prices.
Pest experts said that, on average, owners and food shop operators have their premises inspected once a month, but there are some who engage them only once every few months.
Mr J. Ashokkumar, pest control manager at Major's Pest Management Services, said keeping the pests at bay requires frequent monitoring even if the initial infestation has been put under control.
"The mosquito's life cycle, for example, is about seven days, so even if you get a pest control team in once a month, you are allowing three weeks of vulnerability," he said.
Closer monitoring, such as the use of sensors in malls to track rat movements 24/7, requires money and many are not willing to pay.
ELUSIVE AND INTELLIGENT
But even with the best technology, these pests can be hard to catch.
They hide in dark places: the cockroaches behind ovens and fridges, rats within the columns of buildings, and the mosquitoes under tables and dim corners. Rats, especially, are highly adaptive.
Said Star Pest Control's Mr Chan: "You think you have caught them all but, in fact, they have learnt to avoid coming out during the times when pest control is usually done."
That is why pest companies have started to use infra-red cameras and sensors to track the movements of rats in order to locate their nests.
To eradicate these pests, a coordinated effort is needed.
Even if one stall owner has regular checks, nothing is stopping an infestation from a neighbouring stall.
Waste collectors also play an equally important role. About 82 per cent of rat burrows found in housing estates were near rubbish chutes and bin centres.
In the fight against dengue and Zika, residents are critical in ensuring that Singapore is free of mosquito-breeding sites. Last year, more than 4,100 households were fined for mosquito breeding by the NEA.
Mr G. Surajan, managing director of Major's Pest Management Services, said: "It has got to be a concerted effort and not a sporadic one where each group does its own little thing. It is like a military operation. You have to plan how you encompass the whole problem and take it on. If this is not done, then you are fire-fighting all the time."