Hi Brian,
Manfred's point was that if you're used to an 18-55mm, on whatever sensor size the Sony A58 is (it seems to be "DX" in Nikon terminology), then a 105mm will get you twice as close on your camera as you're used to; '105' is about twice '55'. I fear it may confuse the discussion if we start considering the full frame equivalent focal lengths in this thread - you are used to what your lens, at 55mm, achieves and how close you can get to the subject and still be able to focus. Although; I guess it may help you compare to what the Fuji S4200 takes.
I have never been all that precise on the difference between close up and macro.
"Macro" (as a term) means 1:1 - e.g. a 24mm long subject is projected on to your sensor (which is 18 x 24mm) as frame filling (side-to-side).
"Close up" (as a term) usually means 'more magnification than what you're used to', but not as much as Macro. So; a 48mm subject might be full width at 1:2 (= 0.5x), or 72mm at 1:3 magnification (= 0.33x), which is about what you get with your current lens (0.34x) when used at closest focus distance and the zoom setting for largest magnification.
However, you may find "close up lenses" advertised which are actually just a filter-like thing that goes on to the front of a lens to allow closer focusing. These should not be confused with a Macro lens, which is a proper lens which fits on to the camera. 'Close up lenses' are liable to suffer image quality issues that increase towards the edge of frame, so possibly OK for some subjects, but will reveal their weaknesses on others. I've not used one, but when fitted, they'll probably prevent you grabbing a shot at infinity, since they'll stop the lens focusing at those distances.
The problems you face with lens choice (apart from suppliers who will ship to your shipping address), are:
a) If you buy a 'normal' zoom (i.e. not "macro" tagged) telephoto lens, you will not be able to focus close enough for some subjects you would wish to capture - at least, not unless also using extension tubes - more cost, darker images and potentially a tripod mounting problem.
b) If you buy a "macro" tagged zoom telephoto lens, you won't be able to magnify your bug shots as far as you might wish - in fact, it may find it to be little improvement on what you achieve with the 18-55mm, so the only advantage will be a greater working distance (more space between end of lens and subject). Although again, this will be improved if you use extension tubes, but with those same disadvantages - more cost, darker images and potentially a tripod mounting problem. You should be able to find out the (non-tube) magnification factors in advance (from the specs or reviews) to judge though.
c) If you buy a true (fixed focal length) macro lens, you'll have no problems with focusing close enough, nor with really light loss and likely less issues with mounting camera on tripod with lens on front (although they are heavy). However, since it is a fixed focal length, you have to control magnification and framing by physically moving the lens (+ camera + tripod) closer to, or further from, the subject. This might be a problem, only you can really know. If you cannot get close enough, cropping in PP is the obvious solution, but as you already know, there are limits before image quality suffers too much.
To expand upon the issue of tripod mounting - if (any) telephoto lens does not have a tripod collar and bush, you'll have to mount the camera on tripod and I fear the weight of the telephoto lens at the increased distance from body due to the extension tubes may be an issue, either for aiming or drooping if tilt lock doesn't tighten enough. Conceivably even the lens mount on the A58 body if a particularly heavy lens is used in combination with several tubes and if the A58 lens mount is not metal - but I have no experience of Sony cameras, so I could be scaremongering here - but better that than the alternative.
As I see it, the issues you face are:
For the birds, with the 18-55mm lens; these are probably just too far away, so a longer focal length is needed, any telephoto zoom will help, for garden use, up to 300mm may be sufficient, unless you get ambitious with small birds at the far end.
For the blooming flowers, with the 18-55mm lens; these are probably less of an issue, unless they're a bit too far away for 55mm, as close as you can get with your tripod, so anything that gives a longer focal length will help, either a macro lens of (say) 105mm, or a zoom telephoto of (say) 70/75-300mm.
For the smaller bugs (and tiny flowers, buds or detailed parts of larger blooms), with the 18-55mm lens; you have a limited range of shooting distances at which you can capture these sufficiently large enough in the frame. This limitation is based upon them either being too close for the focus distance of the 18-55mm or too far away and hence too small in the frame.
A Macro lens will help with getting closer to any subject and a longer focal length will help bring distant things closer than the 55mm does. Longer focal length macro lenses (IF available for Sony mount) allow the 1:1 capture of small things at a greater distance and obviously are more useful as a telephoto lens for other subjects.
If you are able to get close enough to a subject, a set of extension tubes to use with the 18-55mm will help expand your options, but a macro lens will be easier to use, those of 90 or 105mm are a good compromise.
Cheers, Dave