In their natural environment aloes suffer very little damage from insect pests. Their main herbivore predators are mammals and include baboons (Figure 17.1), elephants and cattle.
However, when grown under high densities in glasshouses or in fields they can be susceptible to damage from scale insects, beetles and mealy bugs. Although species of Aloe in cultivation or grown as part of a botanical collection can be damaged by pests it is uncommon for them to be totally destroyed.
Very little has been reported on any specific aloe-insect interactions. It is known that insects, such as solitary bees, can be involved in their pollination but it is not know whether there is any specificity in these interactions. Nectar of some aloes are rich in phenolic compounds and these could deter insects. For example, a phenolic-rich extract from the nectar of A. littoralis Baker deterred the honeybee, Apis mellifera L., from feeding (Hagler and Buchmann, 1993).
The majority of the estimated 420 species of Aloe are not grown commercially and thus have not been grown in conditions that might expose them to horticultural pests.
In fact, many horticulturists looking after botanical collections that can harbour a range of pests say that they rarely have to use insecticides to control pests on the aloes. This suggests that the plants must be resistant to many forms of pests. Whether this resistance is associated with the diversity of compounds in their leaves or to their morphology has not been established.
Due to the observed lack of herbivory on aloes, researchers have investigated whether aloe-derived extracts could be used to control insect pests. For example, extracts from Aloe barbadensis Miller, A. ferox Miller and A. succotrina Lam. have general anti-insect activity, whereas extracts from A. striata Haw. were reported to attract fleas (Grainge and Ahmed, 1988). Extracts from A. barbadensis deterred feeding of Athalia proxima Klug and repelled Popillia japonica Newman (Jacobson, 1990). Ash from A. marlothii A.Berger was able to control the maize weevil, Sitophilus zeamais Motschulsky (Achiano etal., 1999). Mosquito coils containing A. vera (L.) Burm.f. had moderate knock-down activity against mosquitoes after a 24-hour period but caused very little mortality. When D-trans-allethrin was added to the coil, mosquito mortality increased (Jantan etal., 1999). In these studies, the compounds in the aloes associated with the respective activities were not identified.
Some aloes, particularly the spotted group (Saponariae), are prone to rust (Uromyces aloes) (Figure 17.2), especially in cultivation where it is damp and shady (Jeppe, 1969). Recovery is slow even after treatment with fungicides (Van Wyk and Smith, 1996).
Black leaf spots can be caused by a number of fungi and can be controlled by fungicides (Jeppe, 1969). Bacterial infections show up as rots, wilts and blights and sometimes crown galls (Jeppe, 1969).
Was this article helpful?