## 2 Materials and Methods  
### Biological Material: Aphid Colonies, Plants and Fungal Isolates  
A virus-free laboratory population of _A. gossypii_ was provided by the Institute of Agricultural Sciences (ICA) CSIC (Madrid, Spain) for use in the assays. The aphids were then maintained in rearing cages on melon plants (_Cucumis melo_ L. cv. Siglo) for several generations in an environmental growth chamber under controlled conditions: 25 \(\pm\) 2 \({}^{\circ}\)C, 16:8 h light:dark regime, and 70% RH. For each experiment, newly emerged apterous adult females (24-72 h after last mout) were collected from the rearing cages using a camel-hair brush and used immediately in experiments. Certified endophyte-free melon seeds were surface sterilized in 2% NaOCI (Sigma-Aldrich, MO, USA) for 2 min, rinsed twice with sterile Mili-Q water and dried under a sterile airflow. The soil substrate in which they were to be grown was sterilized in an autoclave for 20 min at 121 \({}^{\circ}\)C; this was done twice with a 24 h interval between each sterilization. Seeds were germinated in 9x9 cm pots using a mixture of equal parts of vermiculite (No. 3, Asfaltex S.A., Barcelona, Spain) and sterilized soil substrate (Floragard, Oldenburg, Germany) and maintained in an environmental chamber under controlled conditions: 25 \(\pm\) 2 \({}^{\circ}\)C, 16:8 h light:dark regime. Plants were watered three times a week and a nutrient complex of 20-20-20 (N:P:K) Nutrichem 60 fertilizer (Miller Chemical & Fertilizer Corp., Hanover, PA, USA) was added to the irrigation water in a proportion of 1 g/L. Two isolates of _Beaurevia bassiana_ and one isolate of _Metahizium brunneum_ were used in the assays; all isolates had verified endophytic activity when inoculated onto melon plants [9; 10]. These isolates are deposited at the University of Cordoba Entomopathogenic Fungi Collection, Cordoba, Spain and full details can be found in Table 1. All isolates were grown over cellophane film in Petri dishes of PotatoDextrose Agar (PDA) (Becton Dickinson Franklin Lakes, NJ, USA); the cellophane between the agar and the fungus was used to prevent nutrients entering the conidial suspensions at harvest. Cultures were incubated for 15 days at 25 \({}^{\circ}\)C in darkness and conidial suspensions were prepared by scraping the fungus from the cellophane into a sterile aqueous solution of 0.01% Tween 80. The resulting suspension was filtered through several layers of sterile cheesecloth to remove mycel, and sonicated for 5 min to homogenize the inoculum. Concentration of viable conidia used for inoculation was determined using a haemocytometer and appropriate dilutions were made in 0.01% Tween 80 to obtain a conidial concentration of 10\({}^{8}\) conidia/ml. Prior to experimentation, conidial viability was determined on liquid Czapek-Dox broth plus 1% (w/v) yeast extract medium and only suspensions with \(>\)97.0% germination after 24 hours were used. Inoculation of Melon Plants with Entomopathogenice Fungi and Verification of Endophytic Colonization  
For each experiment and each fungal isolate, replicate groups of four-leaf-stage melon plants were treated. Two leaves per plant were sprayed with 2 mL of the fungal suspension with an aerograph 27085 (piston compressor of 23 L/min, 15-50 PSI and a 0.3 mm nozzle diameter, Artesania Latina S.A., Lamadrid, Spain). One millilitre of 10\({}^{8}\)-conidial suspension was sprayed over each leaf, resulting in a spray deposition at the level of the target surface of approximately 0.1 \(\upmu\)L/mm\({}^{2}\). Using the CFU method, we estimated that the suspension produced a deposition of 10,000 viable conidia/mm\({}^{2}\). During application, the remaining two leaves per plant were protected from the spray with a transparent plastic sheet and remained uninoculated. After inoculation, all plants were covered by another plastic sheet to promote fungal growth for 24 h. Control plants were treated in the same way but only sprayed with sterile water with 0.01% Tween 80.