Thunberg, who has taken a year off school to advocate action on climate change, hit back on Twitter, saying it did not take a degree to know the world was not meeting its climate targets.
Independent candidate Arnold Cassola has asked the police to investigate Infrastructure Malta CEO Frederick Azzopardi for breaching environmental laws.
Leadership hopefuls Fearne and Abela are no different from the previous leadership if they fail to denounce the ongoing works, Partit Nazzjonalista said.
The bushfires in Australia have killed 24 people, destroyed more than 1,500 homes and burnt through more than 5.25 million hectares (13 million acres) since September.
Locusts have already destroyed 70,000 hectares (175,000 acres) of farmland in Somalia and Ethiopia, threatening food supplies in both countries in the worst locust invasion in 70 years, according to the Food and Agriculture Organisation (FAO).
"I'm not the kind of person who celebrates birthdays," Greta Thunberg said as she turned 17 on Friday.
The death toll in Jakarta and surrounding areas rose to 43 as of Friday, the country's disaster mitigation agency said, while tens of thousands of people have been displaced.