Parasites attack Monarchs but one in particular called Ophyocystis elektroscirrha is significant. This parasitic neogregarine ( protozoan ) is known only to exist on Monarchs and the closely related Queen butterfly ( Danaus gilippus ). During egg laying an infected female inadvertently showers her eggs and the food-plant with spores. The larvae then ingest these spores which develop within and cause an infestation. The larva thus grows into an infected adult. The spores can then be passed on during copulation or again through egg laying.  

Infected adults are weak and do not live as long as their healthy relatives and are very unlikely to survive hibernation. As the milkweeds also die back during the winter , thus cleansing the plants, it poses a question as to how the parasite continues to exist. In 1994 students on an Experimental Field Ecology course run by Dr. Orley R " Chip " Taylor, Professor of Entomology at The University of Kansas examined the possibility of passive spore transmission. Backed up by laboratory experiments it was found that spores could rub off from infested adults onto healthy ones during the close proximity encountered in the mass roosts. More work is needed to find out if this is the only way the parasite ensures its winter survival.

Environmental conditions can greatly influence Monarch populations, determining high numbers in good summers to devastating losses from bad weather during the winter. Ever since the discovery of the roosts, concern for the butterflies' safety has been expressed. Research on the populations at these sites shows that losses may be as high as 40% due to natural enemies, disturbance from other lepidoptera invading the trees or unfavourable climatic conditions, thus the roost sites have been described as the Monarchs' Achilles' heel. Once the Monarchs leave the roost sites it is important that they move north as fast as the climate allows. Many eggs and larvae in Texas fall victim to Fire Ants ( Soenopsis invicta) and this can have an adverse effect on the summer population.

There is a weather system known as an "El Nino event" that can have a large effect on the Monarch population. El Nino, sometimes referred to as "the earth's heartbeat"  is Spanish for " The Boy Child " as the effect occurs around Christmas time, and  has an effect on the entire Globe. El Nino moves a vast volume of warm water Eastward across the Pacific ocean. This displaces the normally cool water along the Western American coast and can be seen clearly with satellite photography. There is evidence that ancient civilisations knew of this mysterious event, as it drives all of the fish away from what would be fertile waters and even today still starves fishermen of their catch. This normally lasts a few weeks, however in certain years El Nino grows ever bigger and lasts for months causing massive climatic changes. With the warm water comes rain to a normally dry season. It has the power to cause floods from California to China and turn deserts green on one side of the Globe, whilst causing drought on the othe. According to Professor William Gray, a climate forecaster, El Nino can cause the winds that form the Atlantic hurricanes to reverse causing their destruction. For the Monarchs, this event causes damp, rain and even snow to fall on the normally dry mountain slopes. This happened in 1992 and left 70% of the total population dead. During the following summer the Monarchs made a good recovery, but in early January of 1996 El Nino struck again. First reports indicated that losses would be as high as in 1992. Conflicting reports from government sources put one estimate of snow mortality at 5% while conservation organisations claimed up to 30%. A team of U.S. biologists lead by Lincoln Brower however put the mortality from snow at between 6 and 7%. This is very different from the 1992 losses. Although both the 1992 and 1996 snow storms were significant, the earlier storm was preceded by generally damp weather. This gradually soaked the Monarchs, leaving them more vulnerable to the frost and snow that killed them.The more favourable conditions before the 1996 snow storm meant the Monarchs were relatively dry. Many were knocked from their roosts and assumed dead, but subsequently revived once the storms subsided.

As a species, many generations of Monarchs must have witnessed such natural dangers, but man's influence has posed the greatest threat, both directly and by causing naturally occurring hazards to have more influence. If global weather is becoming more unstable, Monarch losses on such a cataclysmic scale may be inevitable, however a more immediate and local problem may be to blame. Illegal logging within the reserves and also removal of trees in the peripheral areas of the forests has opened up the once dense tree canopy that provided shelter for the butterflies against adverse weather conditions. Lincoln Brower, who described the migration as " An endangered phenomenon " , said "If the thinning of the forests continues, even in the peripheral areas of the sanctuaries, I predict that the Monarch will become extinct in Eastern North America within 20 years." The Eastern Monarchs comprise 90% of the total  population. Biologists and the Mexican environmental organisation "Group of 100" have called on the governent to increase the size of the reserves and to reduce the pressures people place on the forests.

It is not just in Mexico that the Monarchs are threatened. The main sites in California are protected by law with heavy fines for anyone who causes disturbance or threat.  In other areas throughout the region there is no protection, particularly from building development. Monarch roosts formed on Eucalyptus trees are particularly at risk. These trees are not indigenous and have been declared a fire- hazard because of their high oil content, and so are not protected. Steps have also been taken to eradicate the Milkweeds, especially from pasture and roadsides as this plant causes death in cattle if eaten.  Loss of larval food-plant combined with pesticides takes its toll on the butterflies throughout their entire migration .

Although the outlook appears bleak for the long-term survival of the Monarch, it is not too late to help. If governments ensured the protection and conservation of the environment they would be helping wildlife in general, as well as improving the quality of everyone's life. The Californian Monarchs generate revenue and employment for the local people, giving greater incentive to preserve the environment. In Mexico, however, the people in the forest areas are generally poor.  With the threat of starvation looming for whole families, and the logging industry ever hungry for timber, the poaching of trees is inevitable regardless of protective laws.  Ecotourism in this area may generate much-needed cash, and ease poaching.  Angangueo is a small mining town with a population of approximately 4600 people, and along with the tiny village of El Rosario now promotes butterfly trips, to the El Rosario Reserve, the only one open to the public. Visitors can watch parades in honour of the Monarchs, visit the reserves and buy souvenirs, helping support local craft industries. The locals may need help and education in these new business ventures, to avoid exploitation. The materials used to build visitors' accommodation must come from properly managed sources.  It would not do the new industry or visitors any credit to find that their log cabin had been made from trees within the reserve. Research will need to be carried out into how much tourism is permissible without adversely disturbing the butterflies or damaging the environment.

The problem of eradication of food-plant may be partly overcome by gardeners.  There are many different species of milkweeds in the US and Canada. Asclepias tuberosa, A. incarnata and A. curassavica are often found in ornamental flowerbeds and could serve as Monarch caterpillar food as well as a good nectar source for the adults. It should be noted that although these plants grow wild, they are poisonous and care should be taken, as with any toxic plant, that children are made aware of this fact.  Once children are taught to treat the plants with caution, they may find it interesting to rear Monarchs and watch their lifecycle progress. Monarchs are ideal butterflies to study, particularly for children as they are easy to keep and provide quick results. This has been made use of by an organisation now called Monarch Watch. Their goals are to further science education, particularly in schools, promote conservation, and to involve thousands of people in the co-operative study of the Monarch's autumnal migration The project is directed by "Chip" Taylor, Karen Oberhauser and Brad Williamson, with many regional and local coordinators. In 1995, Monarch Watch had an estimated 30,000 students covering 30 states who sent out 80,000 tags to enable monitoring of the southerly journey.  They also produce kits consisting of larval Monarchs to allow anyone interested to rear and release them. This stock is screened for the parasitic neogregarine to ensure the stock is healthy, and to avoid artificially increasing the number of parasites in the wild population.