The effects of selected pesticide application regimes on pest predator populations as well as pests were investigated in tomato (Lycopersicon esculentum L.) agroecosystems. The treatments included: (i) the full spectrum of recommended pesticides, including insecticides (carbaryl, endosulfan, and esfenvalerate), a fungicide (chlorothalonil), and herbicides (trifluralin and paraquat); (ii) only insecticides, using the same insecticides and doses as in the full-spectrum pesticide treatment; (iii) only fungicides and herbicides; using the same fungicides and herbicides and doses as in the full-spectrum pesticide use treatment; (iv) control plots which received no pesticide applications. In 1994, the insecticides controlled aphids and flea beetles and reduced their populations by 85% and 72%, respectively. However, aphid populations were 125% greater in the insecticide-treated plots than in the control plots in 1995. The fungicides and herbicides caused increases in the numbers of aphids by 33% in 1994 and by 39.8% in 1995 and those of flea beetles by 55% in 1994 and 17% in 1995. All the full-spectrum pesticide treatments had some degree of detrimental effects on populations of predatory arthropods. The different pesticide applications reduced coccinellid beetle Coleomegilla maculata (Col: Coccinellidae), populations by 6.6% to 35.5% in 1994 but only slightly in 1995; Anthocoridae (Heteroptera) numbers by 26.2% to 55.8% in 1994 and 13.5% to 38.8% in 1995; spider (Araneae) populations by 44.6% to 70.9% in 1994 and 37.0% to 91.4% in 1995. Five hypotheses are proposed to explain these results: (1) the fungicide and herbicide applications reduced the populations of predatory arthropods which in turn resulted in higher pest populations; (2) the fungicide and herbicide applications suppressed the fungal parasites of the pests; (3) the applications increased the fecundity of the pests and resulted in more offspring of the pests and hence higher populations, (4) the applications caused some physiological changes in the tomato plants that attracted more pests or stimulated their reproduction and; (5) the fungicide applications provided more nutritious and suitable habitats for the pests by suppressing the disease of the tomatoes, (C) 1998 Elsevier Science B.V. All rights reserved.