What is eating the leaves on my orange trees and why are some of the leaves turning brown?

~could it be too hot or too dry and if it is outside you can expect slugs snails and caterpillars who love the taste of orange leaves to have a nibble! I have problems with all of the above taking chunks out of mine which is a citrus mitis.
see this info from the RHS

The citrus family includes such familiar fruit as oranges, lemons and limes; while small plants are not difficult to grow in pots, given the right conditions, overwintering them successfully is difficult without a conservatory or glasshouse.

Citrus are not demanding plants, but a conservatory or frost-free glasshouse is a must for anyone wanting to grow them well. They do not make happy house plants in winter, when light levels and humidity are usually too low, and centrally-heated rooms indoors are usually too hot.

Equally, citrus are not well suited to year-round outdoor culture, as the combination of winter wet and cold readily kills young citrus plants in the UK. Examples of outdoor success are largely limited to a handful of more-mature plants in sheltered London or south-coast gardens. For healthy plants, provide a winter minimum of 13°C (55°F) for calamondin orange, 10°C (50°F) for lemons and 5–6°C (41-43°F) for many other citrus.
Citrus x meyeri 'Meyer'.

Citrus is a wonderfully diverse genus, from breakfastbowl-sized grapefruits to diminutive kumquats, and from pretty limes to bizarrely-shaped Citrus medica var. digitata (Buddha’s hand).

Popular choices include:

Citrus sinensis (sweet orange): a slow-growing citrus. Try juicy ‘Valencia’ or virtually seedless ‘Washington’.
Citrus aurantium (bitter orange): not as popular as sweet oranges but does have attractive foliage. ‘Bouquet de Fleurs’ has particularly highly-scented flowers.
x Citrofortunella microcarpa (calamondin orange): dwarf, hybrid plant with fruit suitable for preserves and marmalades.
Citrus reticulata (mandarin orange): one of the few spineless citrus. Matures quickly but needs warm conditions (18°C/65°F) in spring to induce flowering.
Citrus limon (lemon): C. x meyeri ‘Meyer’ is still one of the most popular and hardy lemon hybrids, with prolific cropping; C. limon ‘Garey’s Eureka’ is similarly reliable. For a lemon-lime hybrid, grow C. ‘La Valette’, while ‘Variegata’ has highly ornamental green and cream foliage and stripy young fruits.
Citrus aurantiifolia (lime) and C. hystrix (kaffir lime): key lime is compact with small fruit; kaffir lime leaves are used in Thai food.
Citrus medica (citron): an old ‘heirloom’ citrus from which candied peel was made.
Citrus x paradisi (grapefruit or pomelo): quite tender, especially when young, and may take a few years to begin cropping.
Fortunella japonica and F. margarita (kumquat): close relatives to citrus and fairly hardy, kumquats produce thin-skinned fruit eaten whole, either raw or candied.
When first brought indoors, citrus often respond by shedding a large proportion of their leaves. Image: Tim Sandall
The yellowing of leaves is encouraged by over wet or too dry roots. Image: Tim Sandall
Growing citrus

During summer, stand citrus plants outdoors in a sunny, sheltered spot such as a south-facing patio. Water regularly and feed weekly with a summer citrus fertiliser. As the weather cools in autumn, bring the pots into a conservatory or glasshouse, reduce watering and stop feeding. A few leaves may yellow and drop from the initial change of environment. Excessive leaf drop suggests overwatering and either too high or too low a temperature. If in doubt, let the compost dry slightly between waterings. Repot only in spring, using a John Innes No 2 potting compost with extra grit or sharp sand. If the roots are not congested, simply topdress by replacing the top 5cm (2in) of compost.
Pollination

Fragrant white flowers are borne on one-year-old wood from December to February. Warmth and

humidity encourage good flowering. Flowers are bisexual and self-pollinating, so don't require artificial pollination. Fruits take almost a year to develop fully. Thin fruits on younger plants.
Pruning

Remove congested growth in early spring, and pinch out shoot growing tips in summer. If renovation is required prune back by two-thirds in early spring.
Problems

Flower fall before fruit set: dry roots or lack of humidity
Flower failure: poor light, poor nutrition, erratic watering or cold
Leaf yellowing: excessively wet or dry roots, draughts, cold or poor nutrition
Leaf fall: cold, draughts, high winter temperatures or over-watering

Read more on citrus problems
Further information

National Collection Holder - www.readsnursery.co.uk
Specialist nursery - www.citruscentre.co.uk