Utilization of a Neem Product in a Reduced Synthetic Chemical Insecticide Management Program for Colorado Potato Beetle

Effects on Egg-Laying (Kenney's Farm): Among the neem treatment regimes, reductions in egg-laying rates (numbers of egg masses/adult) from prespray rates were proportional to the amount of neem applied (Fig. 2). The egg-laying rate in the NR treatment regime (2 neem applications), dropped from 4.1 egg masses/adult before the first treatment to 1.2 egg masses/adult after the second treatment--a 70% reduction. The NM treatment (1.5 neem applications) showed a 49% reduction from prespray egg-laying rates. In contrast, the egg-laying rate doubled in the M treatment (0.5 neem applications). The I treatment also reduced egg-laying by 53%, though estimates of adult densities (Fig. 3a) indicate that reduced egg-laying was due, in part, to toxicity ultimately resulting in mortality of adult beetles. Egg densities (egg masses per plant) did not differ among any of the treated or untreated blocks until after the first larvicidal spray, when the two blocks which did not receive the adult spray (M and R) showed significantly higher egg-mass densities compared with the adult-treated blocks (Fig. 3b).

These results are consistent with numerous reports of neem-caused interference with egg-laying activity, including our own tests conducted at a commercial farm in central Maine in 1995 in which we showed a 50% reduction in egg-laying rate and 51% reduction in egg-mass density in a 1-acre neem-treated field compared with an adjacent 1-acre field treated with a conventional insecticide (unpublished data).

Effects on adult and larval CPB densities: Kenney's Farm: There were no significant differences among treatments in weekly densities of adults (Fig 3a), although seasonal density (which reflects density over the entire season) showed densities of colonizing adults (adults present in early summer) were significantly lower in the I, NM, and M blocks, compared with the R and NR blocks, indicating the effectiveness of imidicloprid, even at half rate, against CPB adults (Fig. 3d). Seasonal incidence of summer generation adults was very low in all treatments.

Weekly densities of larvae remained below treatment thresholds except for the two rotation treatments (NR and R) which did not provide adequate control of CPB larvae (Fig 3c). However, in the NR treatment larvae were primarily small larvae; the density of large larvae (the most damaging stage) remained below threshold (=1.5 large larvae/plant). Accumulation of larvae in early stages of development is often seen with applications of neem, due to its effect as an insect growth-regulator (Schmutterer 1995) which points to the drawbacks of relying on conventional treatment thresholds when using non-traditional pest management materials.

When treatments were adjusted for among-block differences in prespray densities of colonizing adults, the treatment regimes most effective in reducing seasonal densities of large larvae were NM (0.33 large larvae produced/colonizing adult/plant) and M (0.68 large larvae/colonizing adult/plant). These two regimes each used three times less conventional insecticide compared with the conventional treatment regime (I). Treatment NR was intermediate (1 large larva/adult/plant) and R and I were least effective (2.15 and 2.28 large larvae/adult/plant).

Bondeson's Farm: Weekly densities of larvae were higher in the M treatment (neem + Bt) compared with the Bt and the N (neem alone) treatments, but CPB densities remained well below thresholds throughout the season in all blocks (Fig. 5a). There were no significant differences among treatments in seasonal densities of CPB except for summer generation adults which were less than half as numerous in the Bt block compared with the N and M blocks.

Effects of Treatments on Tuber Yields. Yield estimates at Bondeson's farm indicated no significant differences among treatments (Fig. 5e), however, the actual yield from the Bt treatment block was 10% higher than the M treatment block and 3% higher than the N treatment block. Miscommunication prevented us from obtaining yield data from Kenney's farm, but Gary Kenney's observation was that there were no differences among treatments. Average yields at Kenney's farm was 168 barrels/acre.

Effects on Non-targets. Beneficials. Densities of beneficial arthropods (ladybugs and spiders) were low across all treatments at Kenney's farm and there was no evidence direct treatment-related effects (Fig. 4a). No ladybugs or spiders were found in either the M or the R blocks, however, there also were no aphids in these blocks suggesting that lack of prey may have been a factor in the low numbers of ladybugs in those blocks. At Bondeson's, there was a significant neem-related reduction in ladybug densities, which were about four times less abundant in the N and M blocks compared with the Bt block (Fig. 5c).

Aphids. At Kenney's, aphids were highest in the NM block and low in all other blocks (Fig. 4b). The location of the NM block at the southern end of the field may explain the high densities of aphids, which often colonize from the edge of a field. Since aphids did not arrive until four weeks after the last treatment, we did not expect to see any direct treatment-related effects on aphids. Aphid densities at Bondeson's were almost zero in all blocks.

Flea beetles. Flea beetles were significantly lowest in the M block, intermediate in the NR and R blocks and highest in the NM and I blocks at Kenney's (Fig. 4b). There are no logical treatment-related explanations for these differences. Flea beetle densities at Bondeson's were almost zero in all blocks.

Black and Red Stinkbug (Cosmopepla bimaculata). This plant sap-feeding insect, which is not a common potato pest, was abundant before treatment in a number of fields at Bondeson's, although the amount of feeding damage it inflicted on the potato plants is difficult to determine. Neem appeared to have a significant effect in reducing the density of this pest, as densities were lowest in the N, intermediate in the M and highest in the Bt blocks (though M and Bt were not significantly different) (Fig. 5d). The significant neem-related reductions in ladybugs and the stink-bug are consistent with reports of the somewhat broad-spectrum activity of this material against insects (Schmutterer 1995).

Laboratory Bioassays: The preliminary results of the laboratory bioassays are shown in Figs. 6 and 7. These early results show none of the neem plus imidacloprid treatment combinations resulted in mortality greater than the additive mortality from each single treatment (Fig. 6). Thus, there was no evidence of synergism between neem and imidacloprid when the two compounds were mixed or applied sequentially. This is consistent with our field results. It must be noted, however, that these bioassays are not yet completed. The final results will be added to this report by March 1998 as an addendum.

The results of the neem and Bt bioassays suggest there may be synergism between neem and Bt when the two compounds are mixed together and applied against newly hatched larvae. However, when applied against 48 hr-old larvae, the mixture was much less effective than the additive effectiveness of Bt alone and neem alone. In a natural population, where eggs do not hatch synchronously, we would expect the Bt + neem mixture to be ineffective. In fact, our field results showed that neem +Bt mixture was somewhat less effective than Bt alone. These laboratory results indicate that Bt is most effective against second stage larvae, while neem is more effective against newly-hatched first stage larvae. This is supported by the responses in the two sequential treatments. That is, the treatment sequence is much more effective against CPB larvae when neem is applied first, followed by Bt. The opposite sequence (Bt followed by neem) was about as effective as neem applied alone on Day 3. In the field, insects are of varying stages of development, therefore, synergistic effects are less likely to be seen in practical use. However, in earlier unpublished studies we have found that neem applied to reduce egg-laying, followed by Bt to kill larvae was a very effective strategy for protecting potatoes from CPB damage.

Conclusions

The results indicate that when neem is applied for control of CPB larvae, its efficacy is enhanced when neem is first applied to deter egg-laying. While earlier studies had indicated enhanced efficacy when neem was applied in rotation with conventional insecticides, in the study reported here, we did not find that neem applied in rotation or mixed with imidicloprid was more effective than imidicloprid applied alone. Based on these findings, it appears that CPB can be effectively managed by substituting neem for conventional insecticides, but not by mixing the two materials together. Neem applied to deter egg-laying followed by neem, Bt, or conventional insecticides when needed for control of CPB larvae provided acceptable levels of CPB control in these tests. When used as an alternative to Bt, neem provides acceptable control of CPB, but may be somewhat deleterious to some beneficial arthropods. Neem applied in mixture with Bt appeared to be less effective against CPB than either material applied alone. At present, the high cost of neem products is prohibitive, however, with increased demand and competition, and improved extraction efficiency it is hopeful that the price will be more affordable in the near future.