Encouraging natural defenses in pecan orchards
Williamson, Joey Robert
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The pecan aphids- yellow pecan aphid (Monelliopsis pecanis Bissell), blackmargined aphid (Monellia caryella (Fitch)), and black pecan aphid (Melanocallis caryaefoliae (Davis))- are major pests in improved pecan (Carya illinoinensis (Wangenheim) K. Koch) orchards. It is believed that understory plants can enhance biological control of pecan aphids and other insect pests, as well as enhance soils. Previous studies confirm increased insect diversity in the understory and soil enhancement. However, evidence of aphidophaga migrating into trees to control pecan aphids is lacking. Since these studies were conducted, the multicolored Asian lady beetle (Harmonia axyridis Pallas) established in pecan and is credited with greatly increasing biological control. Our study looks further into soil enhancement and insect dynamics with understory plants in pecan since this introduction. First, we conducted a feeding preference bioassay with multicolored Asian lady beetle larvae, the three pecan aphids, and two aphid species common in understory plantings- crape myrtle aphid (Sarucallis kahawaluokalani (Kirkaldy)) and cowpea aphid (Aphis craccivora (Koch))- to determine if the beetles may be drawn out of pecan trees. Each beetle instar was offered one of three combinations of aphids in an arena: all of the same aphid species, the three pecan aphids, and all five aphid species together. Ladybeetle larvae showed several significant differences and a slight trend in feeding rates for the three pecan aphids and against cowpea aphid, when offered each species alone, but no significant differences with the three pecan aphids or all five aphid species together. With no observed preferences for understory aphids, we conducted our field study with eight different combinations of the following understory plants- mowed sod, crimson clover/hairy vetch, sesbania/hairy indigo, buckwheat, and crape myrtle- and observed pecan leaf nitrogen, soil organic matter, soil compaction, and population dynamics with the pecan aphids, coccinellid beetles, and parasitized pecan aphids. Results were highly variable, mostly insignificant, and inconclusive for all understory treatments in this two year study, for insect dynamics and soils. This study took place during the transitional stage of understory plantings in pecan orchards. We believe better results would occur beyond this transition stage.