Hello madeleenr! Arum lillies do like a lot of moisture, and will die off if they are too dry, becoming deciduous, and losing their leaf. Wikipedia has a good section on care.

Madeleenr, are the temperatures going much above 35º C, there? If so, they are probably going to sleep for the summer. They only grow all summer in cooler climates, such as the UK, or very close (<30 km) to the coast of the Cape. If the temps are less than 32º C, they may be suffering water problems, but both too much and too little show pretty much the same symptoms. Exactly which kind of watering problem depends on a number of factors:
Are they in pots, or the ground?
What kind of soil do you have?
What is the temperature?
How often do you water?
Do they wilt between soakings, and then recover?
Or does the soil stay constantly wet?
Exactly which kind of Arum Lily do you have. It is nearly imposssible to overwater the common White Arum Lily (Zantedeschia aethiopica), since it is a marsh plant, but the other species need much better drainage to thrive.
I hope that this helps, Madeleenr!

Tugbrethil
Thanks for the feedback. I have a few in containers and some in the ground. We have been experiencing temperatures of over 30 degrees for the past few weeks. We have clay soil here and those in the garden are under a hawthorn tree and mostly in shade. I have white and yellow lilies there. In the one pot I have some white Arum lilies and in the other container I have a Green/white lily planted with clivias. These are planted in potting spoil mixed with some of the garden soil. Clivias do not like too much water so I water that container about twice weekly when I see the soil is dry on top and the other one every two or three days. This container is well drained. The garden gets watered about every two or three days. The garden lily leaves are not yellow but have not flowered this year and not many came up in the spring. We have frost in winter which caused the leaves to die down.

Avkg47

Thanks for the response. I will check out Wikipedia. I Googled and came upon this website.

Madeleenr, 30º+ temps would trigger dormancy in Z. albomaculata and Z. elliotiana (two species of white and yellow arum lilies) but Z. aethiopica should hang in there up to 35º. Z. aethiopica can take very mild frost, but it will also go dormant if the temps go much below -1.7º C, while the other species don't like it even close to freezing. Lakeside Callas (the American name for arum lilies) has a good page on how to grow them:

http://www.lakesidecallas.com/Growing-guide.htm

Thanks Tugbrethil...this link has given my a lot of information I can use. Probably I will need to replant some of my lilies as they are not really getting enough sun under the Hawthorne tree for them to flower.

Glad that I could help! : )