160 L. M. BLACK 



althougli the adults may fly great distances, they must be treated Uke 

 obligate parasites, a fact which severely restricts the experimental procedures 

 that can be used. In order to keep the insects alive during a test for virus 

 increase they must be maintained upon suitable host plants. At the same 

 time it is necessary to ensure that they are not obtaining increments of virus 

 from these plants. This can be accomphshed by using plants insusceptible 

 to the virus or by using susceptible plants under special conditions. Inoculated 

 plants undergo an incubation period of at least 2 days in some cases to almost 

 a month in others (Kunkel, 1948, p. 42). The leaf hoppers cannot acquire virus 

 from plants until shortly before the symptoms develop so that daily transfers 

 or frequent transfers may prevent acquisition of virus from susceptible plants 

 (Maramorosch, 1953b, p. 799). Another special condition for using susceptible 

 plants is provided by the long incubation period of the virus in the vector. 

 This is the period between the time of acquisition of virus and the time the 

 insect is able to inoculate plants with it. In cases in which this is long, for 

 example, two weeks, it may be possible to measure virus increase while the 

 insects are maintained on very susceptible plants, because the subsequent 

 failure of any of the plants to develop disease assures that no virus could have 

 been secured from them (Black, 1941). The elimination of weeds from the 

 cages is, of course, an elementary precaution. 



The reproductive habits of leaf hoppers also have a bearing upon the design 

 of experiments. Sometime after mating, the female inserts her eggs into the 

 leaf or stem of a plant by means of her ovipositer; about a week or more 

 elapses before the eggs hatch and the small wingless nymphs emerge. At times, 

 as the nymph grows, it sloughs off its entire hard chitinous skin or exoskeleton, 

 including the lining of the mouth parts, and fore- and hind-gut. This process 

 usually occurs five times before it reaches the adult or winged stage. Wing 

 pads and the external male and female genitalia are apparent before the 

 insects become adults. The fact that the eggs imdergo development to the 

 nymphal stage while inserted inside and protected by plant tissues means 

 that, unless precautions are taken, yoimg vector insects may emerge in the 

 greenhouse from eggs inserted into plants by inoculating insects, may acquire 

 virus, and may subsequently spread it to other plants. Such secondary, 

 uncontrolled spread must be avoided. This can be accomplished by using only 

 males in the tests or by separating the sexes in the nymphal stage. If the 

 incubation period in the insect is more than a week long, it is simpler to use 

 both males and females indiscriminately and, by fumigation once a week, to 

 kiU hatched nymphs before they are able to inoculate plants. 



Besides being transmitted specifically in nature by one or a number of 

 related leafhopper vectors, the propagative viruses also have their peculiar 

 and rather exasperating traits. Most of these viruses are very unstable outside 

 the living ceU. With one or two exceptions none of them can be successfully 



