Pathogens on Monarchs by Phil Nixon

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Pathogens & Parasitoids Reduction in Monarch Butterfly Rearing

1.It is difficult to avoid all parasitoids and diseases as long as caterpillars and their food are collected from the wild.

2.Totally captive-reared insects tend to lose some survival abilities after several generations and adult monarchs would be difficult to maintain. You are better off collecting wild eggs or larvae to rear realizing that some diseases and parasitism will come with them. Rearing them greatly reduces their mortality from predation and parasitism that would occur in the wild. However, mass-rearing (in collective cages) can result in one infected individual transmitting a pathogen to the entire cage.

3.If you collect eggs, subject them to a wash of 3% bleach (not concentrated bleach) and 97% water and then rinse them with water. This reduces the likelihood of the hatching caterpillar consuming chlorine when it eats its egg shell.

4.Collected milkweed leavesfor caterpillar food can be rinsed with the above 3% bleach solution followed with a water rinse to remove NPV and other pathogens.

5.Make sure that leaves remain fresh. Older leaves that have started to dry out may still be eaten by the caterpillars, but the caterpillars may not receive enough water in the process. Dehydrated caterpillars may die or not form complete pupae. Heavy disease or parasite load can also result in incomplete pupation.

6.Anything that interrupts pupa formation can cause the pupa to form incompletely, killing the monarch. Avoid disturbing pupating caterpillars with excess light, camera flashes, or movement of the caterpillar, cage, or those observing the process.

7.NPV infested caterpillars will liquefy. The body and liquid contains large numbers of infective NPV.

8.Infection with the protozoan OE can be detected in the pupal stage as shown at These pupae should be destroyed as the adult males will be weak and the adult females are carriers of the disease. OE is notorious for spreading in a colony on wing scales.

9.Tachinid fly larvae, pupae, and adults can be destroyed as they emerge. They kill the host as they emerge, but it is not easy to recognize an infected caterpillar before emergence. Infected caterpillars commonly don't eat well and have a blotchy appearance, but healthy caterpillars can sometimes have this appearance.

10.Diseases and parasites are density dependent organisms, meaning that they become more numerous in the wild as the monarchs become more numerous. Increased prevalence in wild monarchs is generally an indication of more monarchs. However, they can build to high numbers that devastate mass-rearings.

11.Soak rearing containers in a 3-5% bleach solution for 15 to 20 minutes, then rinse thoroughly and dry before using to help reduce disease incidence. Make sure containers are not soft plastic, to which chlorine can cling.

August 2015 by Phil Nixon and Leellen Solter, University of Illinois