A Brief Introduction to the Cactus Moth (Cactoblastis cactorum) and its Threat to the local Prickly Pear (Opuntia) Cactus Species By Philip Rose Natural History Division Institute of Jamaica Plants belonging to the genus Opuntia (Tuna or Prickly Pear Cactus) are cacti that are characterized by having succulent, flattened, jointed stems (cladodes or pads) which are branched from the base of the plant or developing trunk. The leaves, if present, are very small and easily fall off. Otherwise the pads bear spines, short barbed or simple hairs, glands, or flowers. The flowers are bell-shaped and sometimes have the appearance of being one with the pad (i.e. sessile). The fruits are edible pear-shaped berries that may possess spines themselves. In some cultures it is an important food crop both for human and livestock consumption. In Mexico, for example, approximately 75,000 ha of land is used for Opuntia fruit and vegetable production; an industry responsible for supporting over 20,500 cactus farmers.
Figure 1: Photo of Opuntia cochenillifera (Cochineal or Smooth Pear) with bloom The Cactus Moth (Cactoblastis Cactoblastis cactorum cactorum, family: Pyralidae)) is an invasive insect species with larvae that feed primarily on the succulent pads of Opuntia cacti. The adult is tan in colour with faint dark dots and lines on the wings, and has an elongated hhead. It normally rests with its wings wrapped around its body but at full extension the moth has a wingspan of about 2 cm. The larvae are orange-brown orange with black bands between each segment. Figure 2: Adult Cactoblastis: wings folded Cactoblastis: wings extended Figure 3: Adult
Figure 4: Cactoblastis larvae showing black-banded segments (Photo from www.cactoblastis.org) The Cactus Moth is originally from South America but was introduced to Australia in 1926 in an attempt to control the O. stricta (Prickly Pear) cactus population which had rendered approximately 12.5 million ha of arable land non-productive. By 1930 all the original O. stricta stands were destroyed to ground level and by 1940 Cactoblastis had brought the cactus population under complete control. After the initial success of Cactoblastis in Australia they were introduced from there in 1933 to South Africa which had a similarly severe Opuntia population and then to Hawaii in 1950. They were later introduced from South Africa to the Caribbean, namely Nevis, Antigua, Montserrat and the Cayman Islands in phases over a period spanning 1957 1970. Figure 5: Pad of an Opuntia sp. showing early signs of infestation. (Photo from www.cactoblastis.org
Figure 6: Complete destruction of Opuntia sp. by Cactoblastis. (Photo from www.cactoblastis.org) Since its introduction to the Caribbean it has spread to several islands such as Haiti, the Dominican Republic, the Bahamas and Cuba (mainly via the legitimate and illegitimate transport of live cactus material): causing damage to the local Opuntia populations in each case. In 1990 Cactoblastis was discovered to have reached the US mainland in Florida. From here it has spread north to south-eastern Georgia and South Carolina. The population is projected to move through the southern US and reach northern Mexico by 2007 as the rate of spread is estimated at 160 km/yr. The absence of a natural predator is the major factor behind the success of the cactus moth in these intended and unintended areas of introduction. The adult, female Cactus Moth lays her eggs in a stick-like arrangement upon the succulent regions and pads of the potential host Opuntia sp. The larvae hatch after 40 days and chew a communal hole through the cactus pad at the region of the egg-stick. Within a day, the larvae enter here and feed as a colony, tunnelling as they progress through the pad and eventually consume the entire interior except for the fibrous vascular tissues. After all the edible material has been consumed in one cladode the larvae either burrow into an adjacent cladode or the entire colony relocates: leaving the foraged cladode to find another suitable pad.
Figure 7: Egg stick laid by female Cactoblastis Within 60 days after hatching, the mature larvae fall to the ground and spin white, silk cocoons under or within the rotting detritus of the former host plant. After approximately 28 days the adult moths emerge. The adults do not feed and must carry out their part in the reproductive cycle within their nine-day life span. Indicators of Cactoblastis infestationn include the following: Early infestation may be noticed by the yellowing of cactus pads Conspicuous hollowing of cactus pads Noticeable slimy-green mucilage and frass (Cactoblastis faeces) on affected segments and on the ground Clustering of the larvae on the surface of pads during temperature extremes Taller, woody cacti species such as Opuntia ficus-indica (Indian Fig) and O. spinosissima (the Prickly Pear Tree) tend to be not as susceptible to attack as the more succulent species (e.g. O. cochenillifer Smooth Pear). However, the pads and the plants themselves are most susceptible when young and before woody tissues develop. In August, 2005 Dr. Helmuth Zimmerman led a team of scientists to Jamaica to investigate the spread of the cactus moth in the Caribbean. He is a consultant for the Mexican and U.S. governments with the task of slowing or eliminating the spread of this pest. Unfortunately, his findings show that the cactus moth does occur here; feeding on small patchy populations of Opuntia within the parishes of Trelawny and Kingston (specifically along the dunes of the Palisados, Port Royal). The Cactus Moth is virtually impossible to control in the wild due to the scattered and less than accessible locations of naturally occurring Opuntia cactus populations and introducing another bio-control agent could prove to have similar deleterious effects, incapable of being checked. The recovery of Opuntia populations that are widely distributed show limited success, however, possibly the most effective control method conceived is the release of irradiated, sterile Cactoblastis moths into the wild. They will in turn produce an
autocidal (self eliminating) effect in successive Cactoblastis generations as some sterile adults will mate with fertile adults producing no offspring. Research by Dr. Zimmerman into this methodology continues and it promises to be biodiversity safe as it targets the Cactus Moth alone and no other organism. Mr. Philip Rose PhD Candidate Department of Life Sciences University of the West Indies Mona Kingston 7, Jamaica, West Indies Tel: (876) 995-0007 Email: philip.rose@uwimona.edu.jm