Question
I have some new flytraps and have them in a terrarium. they are in the office and receive lighting from morning sun, then office fluorescent light.
The traps are starting to turn yellow and not looking as healthy as before.
what causes the plant to turn yellow and are they getting sick?

Answer
The usual cause of Venus Flytraps turning yellow and brown is the natural cycle of life and death of each leaf. The oldest leaves, around the outside of the growing rosette, die first. The traps turn yellow, then brown and black. The discoloration continues to the bottom of the leaf as the entire leaf slowly dies. Any material that is already black and fairly dry can be trimmed off; but leave the good green growth because it continues to supply food to the plant through photosynthesis.

The second most frequent cause of yellowing and blackening of leaves, including newer leaves, is the use of water or growing medium ("soil") that is much too high in soluble minerals and other material. Tap water and potting soil, and even Miracle-Gro brand sphagnum peat moss and perlite, both common ingredients of growing medium for carnivirous plants, will kill the plants because they contain too many nutrients, too many mineral salts and other soluble material.

The third most frequent cause of yellowing/browning is fungal infection, the likelihood of which is strongly increased when Venus Flytraps are grown in terrariums where the usual lack of air movement tends to concentrate fungal spores and the very high humidity makes it easier for the fungi to grow.

The truth is, Venus Flytraps do not need excessively high humidity and can grow very healthily, once adapted, in arid or desertlike conditions so long as their growing medium is kept moist, but not wet all the time. Venus Flytraps generally grow much better outside a terrarium. Growing them in a terrarium is more challenging and requires a greater expertise. The aforementioned lack of air movement, as well as the usual lack of drainage, are both usually detrimental to the plants' health. The lack of drainage can cause a lethal buildup of TDS (total dissolved solids) in the water and medium, and can stagnate and if saturated the growing medium can have anaerobic areas that can cause bacterial rot of the roots and "bulb" of the Venus Flytrap.

So, the most important things are: use only distilled water, rain water, or water from reverse osmosis, use only sphagnum peat moss and perlite (or silica sand, composed of silicon dioxide (quartz)) that has absolutely nothing added to it--no plant food, etc. (avoid all Miracle-Gro products for this reason), and grow them in containers in fresh, moving air, not in terrariums, keeping the growing medium moist, not wet all the time.

For more tips and techniques, and for a variety of comments and responses to all of your questions regarding Venus Flytraps and other carnivorous plants, consider joining a forum such as the FlytrapCare.com Forum:

http://FlytrapCare.com/phpBB3/

Best wishes,
Steve