Quibans 29: Slugs

This is an edited excerpt from an article in the Daily Mail (with thanks to @jamesbakermaths for the suggestion).

Prepare for a slime wave! _____ slugs are expected to invade our gardens over the coming weeks as mild weather continues

The mild weather has created perfect conditions for the slimy pests, which have not been encouraged to hibernate because the weather hasn’t been cold enough.  As a result, the so-called 'sleepless' slugs have been eating and breeding non-stop - causing their numbers to explode.

According to the charity BugLife, the average British garden contains around 20,000 slugs, with each one laying between 20 and 100 eggs.  Combined with last year's wet summer, BugLife reckons the general slug population could be up 10 per cent this year.

‘The impact of super-sized, sleepless slugs could spell devastation for our gardens this summer.’
Slugs will not go into hibernation unless the temperature drops below 5°C - but the average temperature during the winter was a balmy 6.7°C.


Questions:

1. Estimate how many slugs there are in British gardens in total.
2. Is it reasonable to think there are 20,000 slugs in a garden?
3. How many slug eggs are there in total?
4. What was the ‘general slug population’ last year?
5. What is wrong with the final sentence of the article?

Answers:

1. From the article (before it was edited):  According to the charity BugLife, the average British garden contains around 20,000 slugs, with each one laying between 20 and 100 eggs. And with 21 million gardens in the UK, that means up to 420 billion slugs could be waiting to set upon the nation's plants.
2. If we assume a garden is the size of the classroom, that might give 6 metres by 6 metres. 600cm by 600cm is 360,000 sq cm, so there would be one slug per 18 sq cm. If the average garden is a square of side 10m then that becomes one slug per 50 sq cm. Seems like a lot!
3. Should we assume that on average each slug lays 60 eggs? (Do male slugs lay eggs too, or just female ones?) 420 billion x 60 = 25 trillion eggs.
4. To undo a 10% increase we divide by 1.1 - this is about 380 billion slugs.
5. Slugs hibernate when the temperature dips below 5°C. An average temp of 6.7°C doesn’t mean the temp was above 5 degrees the whole time. Presumably it was below 5 at some point, so the slugs will have slept then.

Source:
http://www.dailymail.co.uk/sciencetech/article-3565939/Prepare-slime-wave-420-billion-slugs-expected-invade-gardens-mild-weather-continues.html