UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE BERKELEY Agricultural Experiment Station BENJ. IDE WHEELER, President THOMAS FORSYTH HUNT, Dean and Director H. E. VAN NORMAN, Vioe-Director and Dean University Farm School CIRCULAR No. 125 January, 1915 APHIDS ON GRAIN AND CANTALOUPES* BY C. W. WOODWOETH The two most serious insect pests in the Imperial Valley are the grain aphis and the cantaloupe aphis. Both of these insects are well known in other regions and both have a long list of other plant foods. The life histories run as follows : The young are produced alive, sometimes as many as eight a day and they become full grown and Anal plates and honey tubes of the grain and cantaloupe aphids, the latter on the right. begin to reproduce within two weeks, often considerably less. Most of the individuals are wingless, but occasionally a winged insect appears. The rate of reproduction given by the U. S. Department of Agriculture (Bulletin 112) shows that if favorable conditions con- tinued and all the descendants could live, the progeny of a single individual grain aphis in six months would make a mass sufficient to cover the whole State of California a foot thick. * The two aphids are Aphis avenae and Aphis gossypii Glov. and are generally called the oat aphis and the cotton aphis, as indicated by their Latin names. The common wingless forms are readily distinguishable by the hairing of the anal plate and the shape of the honey tubes as shown in the figure. Such figures aid us in understanding how, when favorable condi- tions occur, the aphids can increase to countless millions and destroy a crop. Fortunately the aphids develop very slowly in the colder period in the spring and very frequently the weather becomes too severe long before harvest, leaving but a very short period during which rapid development is possible. Most of the time these insects lead precarious existences but now and then, and with unpleasant regularity in the Imperial Valley, the weather conditions and food supply permit the reproduction of the insect to approach its maximum rate. The cantaloupe aphis is almost annihilated when the vines die in the early summer, but a few survive on alfalfa or various weeds, particularly pepper grass, and the orange is fairly favorable while the young shoots are growing. The grain aphis, during the unfavorable periods, often penetrates the ground for shelter and finds food on the roots of plants, chiefly grasses, and in fall or spring it often goes to the leaves of the apple and passes one or more generations there. Sexual forms occur in the autumn in all northern sections and eggs are produced to carry the insect over the winter. This occurs only in the generation produced on the apple. We have not noticed this wintering egg form in the Imperial Valley and the cantaloupe aphis has never been observed producing eggs. Parasites and predaceous insects have often been given the credit of controlling aphids, but a knowledge of the enormous rate of repro- duction in these insects will show that when conditions are right for increase those insects that prey upon them can make very little impression. When an aphis is on the down grade, however, the attacks of other insects seem very efficient since the aphids cannot in any manner defend themselves or avoid wholesale destruction. The experiment of importing ladybirds into the Imperial Valley has been carried out on a larger scale than anywhere in the world, but the growers in that valley know that the aphids still collect toll in no diminishing ratio. Sprays are very effectively used against aphids. They are perhaps the easiest of all insects to kill in this way. AVhile these aphids are quite as defenseless against sprays as against ladybirds they cannot usually be controlled by a single treatment because of their great reproductive powers. The descendants of a single individual that escaped contact with the spray material would be able to seriously reinfest a plant in an incredibly brief space of time, thus requiring repeated treatments as long as the conditions for rapid reproduction persist. Whether sprayed or not plants often suddenly become free from aphids when a change of weather produces a less favorable condition. The frequent recurrence of such weather changes is accountable for most of the mistakes made by careless observers who do not notice that the conditions are general and not limited to particular fields where ladybirds or other inefficient means of control have been experimented with. In most districts the aphids on any particular crop give trouble only during occasional seasons and the remainder of the time are negligible factors. Gardeners do not consider aphids difficult insects to combat because under the worst conditions the necessary spraying is neither difficult nor expensive, even though requiring many repetitions of the treatment. The formula most frequently used consists of one part of 40% nicotine and one part of soap to 900 parts of water. This is cheap and effective. In field crops the cost of treatment, though one of the cheapest methods known for killing insects, is generally considered prohibitive. It would seem that the value per acre of the cantaloupe crop should justify the treatment for the aphis but here we have a peculiar con- dition prevailing. In the cantaloupe fields in the Imperial Valley the aphids do not, as a rule, become generally abundant enough to seriously affect pro- duction till after the more profitable portion of the crop has been harvested. The grower will have to decide whether the continuation of the season, by spraying the plants and thus keeping them pro- ductive, is profitable with the decreasing market value of the melons which is always experienced at that time of the year. The question before the grower is not primarily entomological, having to do with the method of killing the aphid, but almost entirely a question of farm management, whether it pays to invest the additional cost of produc- tion to increase the volume of the least profitable shipments. Those who have tried spraying are far from convinced that they have sufficient returns for the money expended and we would recom- mend that any further experimenting be done on a small scale, rather than by the treatment of whole fields. There is, however, one practice which has developed in the Imperial Valley which seems to be justified by the experience of the growers. When the vines first become infested in the spring it is usual to find the insect limited to a very small number of vines upon which they become exceedingly abundant before developing wings and spreading generally over the field. When this is the case the practice of many growers is to sprinkle gasolene on each of these early infested vines and set fire to it, thus killing all the aphids as well as the plant. This practice does not prevent the final general infestation of the field, but delays the time of infestation very appreciably. Where this aphis gives trouble in other parts of the State it would be well to adopt this method of controlling the insect, provided the winged spring migrants are as few as is the rule in the Imperial Valley, with the result that they are able to infest only a very small proportion of the vines. If the early infestation is general this method is of no value. Regarding the grain aphis it is clear that spraying does not pay and it will have to be considered as one of the risks in growing the crop, belonging in the same class as the direct effects of damaging weather conditions. It has never been considered profitable in any country to apply treatment for these insects on grain and the con- ditions in the Imperial Valley do not offer any reason for expecting it to be practical there.