In most instances, symbiotic relationships between fungi and plants are considered with reference to the opposite categories of ‘antagonists’ and ‘mutualists’. The ‘neutral’ condition, to which endophytic fungi are often referred, merely derives by the absence of indications enabling their circumstantial ascription. However, the increasing evidence that many fungal pathogens are able to spread endophytically in unrelated plant species introduces the possibility that they can actually shift from one category to the other depending on a series of ecological factors. In the case of fungi producing bioactive secondary metabolites, this adaptation could be related to the toxic or phagodeterrent effects on pests possibly induced by these products. Sphaeropsidin A (SphA), a pimarane diterpene produced by several Diplodia species (Dothideomycetes, Botryosphaeriaceae), was extracted from culture filtrates of a strain of D. corticola isolated from Quercus suber in Algeria. Despite this fungus is known as a pathogen, it is able to spread endophytically to other hosts in forest contexts. Following previous evidence of insecticidal properties of SphA, we investigated effects of this compound against chewing insects using larvae of the polyphagous lepidopteran Spodoptera littoralis as a model. Bioassays were carried out through both contact application and oral exposure. More in detail, the topical application assay consisted in treating two groups respectively of 2^nd and 5^th instar larvae which were directly sprayed with a solution containing SphA (pre-dissolved in ethanol) at the concentration of 8000 ppm, while the oral administration assay was realized by directly pouring 2 µl of the experimental solution into the foregut lumen of anesthetized 5^th instar larvae on three consecutive days, for a total amount of 0.12 µg SphA/larva. Control larvae were identically treated with a solution without SphA. The compound showed no lethal effect when directly sprayed both on 2^nd or 5^th instar larvae, while it produced an evident toxic effect on 5^th instar larvae which were orally injected with the experimental solution; in fact, these larvae showed a significantly lower survival rate and a reduced weight compared with the control. The larval mortality started from the last administration of SphA (day 3) and increased over the time until pupation. SphA-treated larvae showed a survival rate of 31%, while 98% of the control larvae survived. The orally treated larvae showed a modified bodily appearance: they were smaller and their weight before pupation was significantly lower compared to the control; however, no alteration of the development time was recorded. The pupae obtained from SphA-treated larvae also showed a lower weight with a survival rate of about 67%, significantly lower compared with the control pupae (100%). No differences were recorded for the pupal development, and for the adult fertility and longevity. Based on these results, we can speculate that SphA might play a defensive role against lepidopteran insects in plants harboring the producing fungus, depending on the extent at which the endophytic strains of D. corticola are able to perform synthesis of this and eventually other bioactive metabolites in vivo.