Citrus Cover Cropping Impace: A Case Study on Valencia Orange Yield, External Fruit Quality and on Key Beneficial and Pest Species
by Phil A. Phillips, BCE, CCA
As more of the citrus industry moves toward the use of ground covers comprised of either cover crops or resident vegetation, it is important to understand the positive and negative effects of such a shift in horticultural practices. Improved water infiltration rates, erosion control and dust mitigation are well documented positive effects while increased risk of frost damage and interference with sprinkler irrigation patterns are known potential negative effects. However, the effect of ground covers on pests and beneficials, crop yield and quality had not been well explored prior to this study. The goals of this study were to measure ground cover effects on yield and external fruit quality and to measure any changes in key pest and beneficial species. The pest species monitored were the pocket gopher, citrus nematode, brown garden snail, Fuller rose beetle, California red scale and honeydew foraging ant species. The beneficial species monitored were Aphytis melinus, spiders and predaceous mites (primarily Euseius hibisci).
Six commercial blocks of Valencia oranges, three blocks with a clover ground cover paired with three blocks without a ground cover, were monitored throughout a three-year period from 1993 to 1996 for pests and beneficials. Leaf tissue nutrient analyses were conducted and packout and return records were maintained each year of the study.
Pests and beneficials: Although surveys for active pocket gopher mounds didn't show any difference between the two orchard types, pocket gopher pressure was significantly greater in the clover ground cover orchards, requiring considerable additional effort at trapping to maintain populations at a tolerable level. Surveys for citrus nematode resulted in higher levels of juveniles in the cover plots, probably due to increased feeder root proliferation below the tree's drip line in response to higher levels of nitrogen in this area being furnished by the clover. The clover itself is not a host to this nematode. Fuller's rose beetle (FRB) levels were three times higher in the cover plots during 1996, but were considerably lower overall relative to the previous two years. Over the entire three-year study, the total number of FRB collected were very similar between orchard types. Brown garden snail activity was nearly doubled in the cover plots, increasing in severity each of the three years of the study. In the first year of this study (1993), Argentine ant activity was actually less in the cover plots and it was mistakenly thought that the ground cover was suppressing ant activity. However, ant activity actually increased over the last two years of the study to more than twice the levels in the cover plots as in the non-cover plots. This dispelled the theory that ground cover actually helps with Argentine ant management (see Figs.1-5).
For the first two years of this study, numbers of male California red scale trapped on pheromone traps were greater in the cover crop plots (+24% 1994, +11.2% 1995). For the third and final year, there was a substantial shift to 22.9% more scale trapped in the non-cover plots. Aphytis and other parasites were consistently trapped at greater levels in the non-cover plots, just the opposite of what one might expect if a flowering cover crop such as clover were to actually enhance parasite and predator activity (see Figs.6-7).
Predaceous mite surveys for Euseius hibisci, a facultative predator of citrus red mite which can also feed and reproduce on pollens, showed slightly higher activity in the non-cover plots. Additional levels of pollen from the flowering clover cover crop did not enhance predaceous mites in this study. No phytophagus citrus red mites were detected in either orchard type. Anystis agilis (whirlygig) predator mite levels were also slightly higher in the non-cover plots. Over the three-years of the study, spiders were only slightly higher (8%) in the non-cover plots. Out of the ten families inhabiting coastal citrus tree canopies, five families predominated: Anyphaenidae, Clubionidae, Aranaeidae, Salticidae, and Therididae.
Nutrient analysis: Leaf nutrient analyses did not show any significant differences in macro or micro-nutrients between orchard types during any of the three years of the study. The two pair of older orchards were a little high in zinc, but not excessively so and were similar across orchard types. Each of the non-cover orchards had slightly larger trees than its paired cover crop orchard, and thus had slightly higher yields for each year of this study.
Packout and returns: On average the cover plots packed considerably more premium grade fruit (SK 66.7% vs. 56.1%) and slightly more choice grade fruit (CH 11.8% vs. 11.0%) than the non-cover plots in the third year of this study. The cover plots also packed more #72 and #88 sized fruits in both the premium grade (SK 26.8% vs. 17.3%) and the choice grade of fruit (CH 5.3% vs. 4.1%). Prior to planting the clover ground cover, these same plots had nearly equivalent packouts in the years of 1989 and 1990.
Over this three year study the cover plots consistently averaged more fruit in premium grade and provided from $0.07 to $0.50 per carton additional return to the grower. Gophers and snails were the major problems associated with the cover crop plots while there were no measured pest management benefits (as detailed above) other than the obvious dust mitigation. None of the other pests or beneficials were consistently different between the orchard types over the three year study. Since there was no improvement in pest management in the cover plots, their increased performance was likely due to improved water infiltration during irrigation cycles.
This research was funded by the Citrus Research Board of California